JP6912287B2 - Polyimide film and its manufacturing method - Google Patents

Description

The present invention relates to a polyimide film and a method for producing the same.

In recent years, in the field of touch panel materials such as transparent electrode films, the use of plastic films as substrates has been studied from the viewpoint of weight reduction and thinning, instead of glass.

Further, bendable flexible devices such as flexible displays and devices having curved surfaces such as organic EL lighting and organic EL displays are being studied. In the above device, it is considered to use a foldable film instead of a hard substrate as a substrate for forming a surface protective layer, a color filter, a touch panel, a TFT, and the like.

As the foldable film, for example, the adoption of a polyethylene terephthalate film (PET film), a polyethylene naphthalate film (PEN film), and a cycloolefin film (COP film) having excellent optical characteristics is being studied.

On the other hand, the polyimide resin generally has excellent properties such as heat oxidation resistance, heat resistance properties, heat resistance radioactivity, low temperature resistance, and chemical resistance, so that the polyimide film can be used as the above-mentioned bendable film. Adoption is also being considered.

International Publication No. 2016/158825 Japanese Unexamined Patent Publication No. 2004-231946 International Publication No. 2006/11286

A film used for an optical device that is required to be repeatedly bent, for example, a flexible display, is required not only to be bendable, but also to have excellent optical characteristics and excellent bending resistance to a plurality of bendings. Since the above-mentioned PET film and PEN film have a drawback of poor optical characteristics and poor visibility, and a COP film has a drawback of poor toughness, it is difficult to use it as a substrate for such a flexible optical device.

On the other hand, polyimide resins are generally excellent in properties such as optical properties, bending resistance, heat oxidation resistance, heat resistance properties, heat resistance radiation resistance, low temperature resistance, and chemical resistance. It was considered to adopt a polyimide film as described in 1 to 3 as a substrate of a flexible optical device. However, it has been found that the conventional polyimide film as described in Patent Documents 1 to 3 has room for improving the bending resistance to a plurality of bendings.

Therefore, one of the objects of the present invention is to provide a polyimide film having improved bending resistance to a plurality of bendings.

上記式（１）中、Ａは２価の有機基、Ｂは４価の有機基、ｎは２以上である。

上記式（Ａ−２）中、Ｘは、下記式（Ｘ−１）〜（Ｘ−３）から選択される２価の有機基である。

上記式（Ａ−３）中、ａは０又は１である。

［４］
上記ポリイミドは、上記式（１）におけるＡとして、上記式（Ａ−１）で表される構造と、下記式（Ａ−５）で表される構造とを含む、項目３に記載のポリイミドフィルム。

［５］
上記式（Ａ−１）で表される構造と、上記式（Ａ−５）で表される構造との比（式（Ａ−１）で表される構造／式（Ａ−５）で表される構造）が、モル基準で２／８〜６／４の範囲内である、項目４に記載のポリイミドフィルム。
［６］
上記式（１）におけるＢとして、下記式（Ｂ−１）〜（Ｂ−４）で表される構造のうち少なくとも一つを含む、項目３〜５のいずれか一項に記載のポリイミドフィルム。

上記式（Ｂ−１）中、Ｙは、下記式（Ｙ−１）〜（Ｙ−３）から選択されるいずれかである。

［７］
上記式（１）におけるＢとして、下記式（Ｂ−５）で表される構造を含む、項目２及び３〜６のいずれか一項に記載のポリイミドフィルム。

［８］
上記フィルムのＭＤ方向及びＴＤ方向のうち、ヤング率が高い方向におけるヤング率が３．５ＧＰａ以上である、項目１〜７のいずれか一項に記載のポリイミドフィルム。
［９］
請求項１に記載のポリイミドフィルムを製造する方法であって、上記方法は、
ポリイミド及び溶媒を含むポリイミドワニスを支持体上に塗工することと、
上記溶媒を仮乾燥させて、上記溶媒を０．１質量％〜２０質量％含む溶媒含有フィルムを形成することと、
上記溶媒含有フィルムを延伸することと、
上記溶媒含有フィルムから上記溶媒を０．１質量％未満に乾燥させることと
を含み、
上記延伸において、上記溶媒含有フィルムのＭＤ方向における延伸倍率が１．５倍以上である、方法。
［１０］
請求項１に記載のポリイミドフィルムを製造する方法であって、上記方法は、
ポリイミド及び溶媒を含むポリイミドワニスを支持体上に塗工することと、
上記溶媒を乾燥させて、上記溶媒を０．１質量％未満のフィルムを形成することと、
上記フィルムを延伸することと、
上記延伸において、上記フィルムのＭＤ方向における延伸倍率が１．５倍以上である、方法。 As a result of diligent studies to solve the above problems, the inventors of the present application have solved the above problems by setting the difference (ΔN) between the refractive index of the polyimide film in the MD direction and the refractive index in the TD direction within a specific range. We have found that it can be solved and have completed the present invention. That is, the present invention is as follows.
[1]
A polyimide film containing polyimide as a main component, wherein the difference (ΔN) between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film is 0.02 or more.
When bent 100,000 times under the conditions of bending radius 2 mm, bending angle 135 °, and load 1.25 kg / m ² , it does not break and bending marks cannot be visually confirmed.
Polyimide film.
[2]
The polyimide film according to item 1, wherein the yellowness (YI) of the film is 5.0 or less.
[3]
The polyimide contains a polyimide represented by the following formula (1), and as A in the following formula (1), a structure represented by the following formula (A-1), the following formula (A-2), and the following The polyimide film according to item 1 or 2, which comprises at least one selected from the group consisting of the structure represented by the formula (A-3) and the following formula (A-4).

In the above formula (1), A is a divalent organic group, B is a tetravalent organic group, and n is 2 or more.

In the above formula (A-2), X is a divalent organic group selected from the following formulas (X-1) to (X-3).

In the above formula (A-3), a is 0 or 1.

[4]
The polyimide film according to item 3, wherein the polyimide contains a structure represented by the above formula (A-1) and a structure represented by the following formula (A-5) as A in the above formula (1). ..

[5]
The ratio of the structure represented by the above formula (A-1) to the structure represented by the above formula (A-5) (structure represented by the formula (A-1) / represented by the formula (A-5)). The polyimide film according to item 4, wherein the structure) is in the range of 2/8 to 6/4 on a molar basis.
[6]
The polyimide film according to any one of items 3 to 5, wherein B in the above formula (1) contains at least one of the structures represented by the following formulas (B-1) to (B-4).

In the above formula (B-1), Y is any one selected from the following formulas (Y-1) to (Y-3).

[7]
The polyimide film according to any one of items 2 and 3 to 6, wherein B in the above formula (1) includes a structure represented by the following formula (B-5).

[8]
The polyimide film according to any one of items 1 to 7, wherein the Young's modulus in the direction in which the Young's modulus is high among the MD direction and the TD direction of the film is 3.5 GPa or more.
[9]
The method for producing the polyimide film according to claim 1, wherein the above method is
Applying a polyimide varnish containing polyimide and a solvent onto the support,
The solvent is temporarily dried to form a solvent-containing film containing 0.1% by mass to 20% by mass of the solvent.
Stretching the solvent-containing film and
Including drying the solvent from the solvent-containing film to less than 0.1% by mass.
A method in which the stretching ratio of the solvent-containing film in the MD direction is 1.5 times or more in the stretching.
[10]
The method for producing the polyimide film according to claim 1, wherein the above method is
Applying a polyimide varnish containing polyimide and a solvent onto the support,
The solvent is dried to form a film containing less than 0.1% by mass of the solvent.
Stretching the above film
A method in which the stretching ratio of the film in the MD direction is 1.5 times or more in the stretching.

The present invention can provide a polyimide film having improved bending resistance to multiple bendings. It should be noted that the above description shall not be deemed to disclose all the embodiments of the present invention and all the advantages relating to the present invention.

FIG. 1 is a schematic view showing an example of a bending test in the present specification. It is a schematic diagram for demonstrating the state of a film after a bending test.

Hereinafter, embodiments of the present invention (hereinafter, referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist thereof. In the specification of the present application, the upper limit value and the lower limit value of each numerical range can be arbitrarily combined.

《Polyimide film》
The polyimide film of the present embodiment is a polyimide film containing polyimide as a main component. In the specification of the present application, the term "film contains polyimide as a main component" means that the film contains 50% by mass or more of polyimide based on the total mass of the film. The polyimide film contains polyimide in an amount of preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, still more preferably 90% by mass or more, and particularly preferably 90% by mass or more, based on the total mass of the film. It is composed of a polyimide film.

<Refractive index difference>
In the polyimide film of the present embodiment, the difference (ΔN) between the refractive index of the polyimide film in the MD direction (also referred to as “mechanical direction”) and the refractive index in the TD direction (also referred to as “width direction”) is 0.02. That is all. The polyimide film of the present embodiment is excellent in bending resistance to a plurality of bendings, mainly due to this configuration. The reason is not limited to the theory, but the following can be considered, for example. The refractive index of the film correlates with the molecular orientation of the polymer in the film. When the difference between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film is 0.02 or more, it means that the polyimide molecules are preferentially oriented in the direction having the higher refractive index. .. When polyimide molecules having a high aromatic ring density are oriented in the MD or TD direction, a difference in tensile elastic modulus (hereinafter referred to as “Young's modulus”) in the film plane is generated, and the elastic modulus (Young's modulus) in the orientation direction of polyimide is generated. Rate) can be increased. Therefore, according to the present invention, it is possible to provide a polyimide film having excellent bending resistance to a plurality of bendings.

The lower limit of the difference (ΔN) between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film is preferably 0.02 or more, more preferably 0.023 or more, and further preferably 0. It is 026 or more. From the viewpoint of transparency, the upper limit of the difference in refractive index is preferably 0.073 or less, more preferably 0.061 or less, still more preferably 0.048 or less. As a method of causing a difference in the refractive index between the MD direction and the TD direction, as will be described later, there is a difference between the draw ratio of the polyimide film in the MD direction and the draw ratio in the TD direction. The refractive index may be higher in the MD direction than in the TD direction and may be higher in the TD direction than in the MD direction, but the refractive index in the MD direction is easy to manufacture by a so-called roll-to-roll process. Is preferably higher than the refractive index in the TD direction.

The refractive index of the polyimide film of the present embodiment in one of the MD direction and the TD direction may be higher than the refractive index of the unstretched polyimide film in the direction. The refractive index of the polyimide film of the present embodiment in the other direction may be substantially the same as or lower than the refractive index of the unstretched polyimide film in the direction. For example, the refractive index of the polyimide film of the present embodiment in the MD direction may be preferably 0.011 or more higher, more preferably 0.013 or more higher than the refractive index of the unstretched polyimide film in the MD direction. .. The refractive index of the polyimide film of the present embodiment in the TD direction may be substantially the same as the refractive index of the unstretched polyimide film in the TD direction, preferably 0.005 or more lower, and more preferably 0.01 or more lower. May be good.

Of the MD direction and the TD direction of the polyimide film of the present embodiment, the lower limit of the refractive index in the direction having a high refractive index is preferably 1.580 or more, more preferably 1.600 or more, still more preferably 1.650 or more. The upper limit is preferably 1.740 or less, more preferably 1.730 or less, and even more preferably 1.720 or less. Of the MD direction and the TD direction of the polyimide film of the present embodiment, the lower limit of the refractive index in the direction of low refractive index is preferably 1.540 or more, more preferably 1.590 or more, still more preferably 1.610 or more. The upper limit is preferably 1.700 or less, more preferably 1.690 or less, still more preferably 1.680 or less.

<Young's modulus>
By causing a difference between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film, it is possible to make a difference between the Young's modulus in the MD direction and the Young's modulus in the TD direction. In the polyimide film of the present embodiment, the lower limit of the Young's modulus in the direction in which the Young's modulus is high in the MD direction and the TD direction is preferably 3.5 GPa or more, more preferably 3.55 GPa or more, still more preferably 3.60 GPa or more. The upper limit is preferably 6.00 GPa or less, more preferably 5.50 GPa or less, and further preferably 5.00 GPa or less. The lower limit of Young's modulus in the direction of lower Young's modulus is preferably 2.50 GPa or more, more preferably 2.80 GPa or more, still more preferably 3.00 GPa or more, and the upper limit is preferably 5.50 GPa or less, more preferably. It is 5.00 GPa or less, more preferably 4.50 GPa or less. The Young's modulus may be higher in the MD direction than in the TD direction and may be higher in the TD direction than in the MD direction, but the Young's modulus in the MD direction is easy to manufacture by a so-called roll-to-roll process. Is preferably higher than the Young's modulus in the TD direction.

The Young's modulus in one of the MD direction and the TD direction of the polyimide film of the present embodiment may be higher than the Young's modulus in the direction of the unstretched polyimide film. The Young's modulus of the polyimide film of the present embodiment in the other direction may be substantially the same as or lower than the Young's modulus of the unstretched polyimide film in the direction. For example, the Young's modulus of the polyamide film of the present embodiment in the MD direction may be preferably 0.10 GPa or more higher, more preferably 0.20 GPa or more, higher than the Young's modulus of the unstretched polyimide film in the MD direction. The Young's modulus of the polyamide film of the present embodiment in the TD direction may be substantially the same as the Young's modulus of the unstretched polyimide film in the TD direction, preferably 0.1 GPa or more lower, and more preferably 0.2 GPa or more lower. May be good.

<Shrinkage rate and expansion rate>
When the polyimide film of the present embodiment is heated above the glass transition temperature (Tg) of the polyimide, it may shrink in the direction of high refractive index and expand in the direction of low refractive index among the MD direction and the TD direction. .. This is because the oriented polyimide molecular chain is loosened by heating and loses its orientation, so that it contracts in the orientation direction and expands in the non-aligned direction. Preferably, when the polyimide film is heated to Tg or more of the polyimide, it shrinks in the MD direction and expands in the TD direction. When the polyimide film is heated to Tg or more of polyimide, it shrinks preferably 5% or more in the direction of high refractive index, more preferably 7% or more, further preferably 10% or more, and the direction of low refractive index. May be preferably expanded by 5% or more, more preferably expanded by 7% or more, and further preferably expanded by 10% or more. The shrinkage and expansion coefficient of the polyimide film can be measured by thermomechanical analysis using a polyimide film cut into a size of 3 mm × 20 mm as a test piece. For example, a measuring device manufactured by Seiko Instruments Inc. (EXSTAR6000) is used, and the temperature is 50 to 350 ° C. (Tg or more) under the conditions of a tensile load of 49 mN, a heating rate of 10 ° C./min, and a nitrogen stream (flow rate of 100 ml / min). ), Measure the elongation of the test piece. The length of the sample after the measurement can be measured, and the contraction rate and the expansion rate can be obtained from the day of the length of the sample before the measurement.

<Bending resistance>
The polyimide film of the present embodiment does not break when bent 100,000 times under the conditions of a bending radius of 2 mm, a bending angle of 135 °, and a load of 1.25 kg / m ^{2, and bending marks cannot be visually confirmed.} Since the polyimide film of the present embodiment has this configuration, it can be suitably used as a substrate for a flexible optical device.

Such bending resistance can be tested by any method as long as the above conditions can be realized, and can be performed as shown in FIG. 1, for example. The size of the film (1) to be tested can be 10 to 20 mm in width, 110 mm in length, and 5 to 20 μm in thickness. As shown in FIG. 1, the film (1) to be tested is sandwiched between chucks (4) attached to a plunger (3) arranged above the rotary table (2), and the rotary table (2) is held. Let it hang down through the center of the film. The film (1) is sandwiched between the two clamps (5) so that the two clamps (5) attached to the rotary table (2) are in contact with the film (1) at the center of rotation of the rotary table (2). The tip of the clamp has a curved surface with a radius of 2 mm, which allows the film (1) to be bent with a bending radius of 2 mm. A weight can be set at the tip of the plunger (3), and ^{a load of 1.25 kg / m 2} can be applied to the film (1) by changing the weight of the weight. The rotary table (2) can repeatedly rotate by switching between left rotation and right rotation. The position of the lower end of the film in the starting state is set as the starting point (0 °), and the rotary table (2) is rotated left (or right). As the rotary table (2) rotates, the lower end of the film is lifted upward while being clamped by the clamp (5), and the film (1) bends while being in contact with the clamp (5). When the bending angle (6) reaches 135 °, the rotation direction of the rotary table (2) is switched in the opposite direction. When the film (1) passes the starting point and the bending angle (6) reaches 135 ° again in the opposite direction, the rotation direction of the rotary table (2) is switched again. As described above, the operation from the start point of the film (1) to the right (or left) of 135 °, the start point, the left (or right) of 135 °, and the return to the start point is defined as one bending. do.

FIG. 2 is a schematic view for explaining the state of the film after the bending test. Even if the above-mentioned bending is repeated 100,000 times, the polyimide film of the present embodiment does not break and no bending marks can be visually confirmed as shown in FIG. 2A. For example, when the haze of the bent portion of the film is measured after the bending test, it is preferably 5.0 or less, more preferably 3.0 or less, and more preferably 1.0 or less. FIG. 2B shows an example of a conventional polyimide film that is broken when bent about 29,000 times. FIG. 2C shows an example of a PET film in which bending marks could be visually confirmed after bending about 1,000 times.

<Polyimide>
The polyimide film according to this embodiment preferably contains a polyimide represented by the following formula (1).

ここで上記式（１）中、Ａは２価の有機基、Ｂは４価の有機基、ｎは２以上である。

Here, in the above formula (1), A is a divalent organic group, B is a tetravalent organic group, and n is 2 or more.

A in equation (1)
The polyimide contained in the polyimide film can be produced from acid dianhydride and diamine as raw materials. A of formula (1) can be obtained from diamine.
Further, in the present embodiment, as A in the formula (1), a structure represented by the following formula (A-1) (hereinafter, also referred to as “structure A1”), the following formula (A-2), and the following formula are used. (A-3) and any one or more of the structures represented by the following formula (A-4) (hereinafter, also referred to as “structure A2”) and more (hereinafter, “first polyimide film”. ”) Is preferable.

上記式（Ａ−２）中、Ｘは、下記式（Ｘ−１）〜（Ｘ−３）から選ばれる２価の有機基である。

In the above formula (A-2), X is a divalent organic group selected from the following formulas (X-1) to (X-3).

上記式（Ａ−３）中、ａは０または１である。

In the above formula (A-3), a is 0 or 1.

The structure represented by the formula (A-1) is derived from 3,3'-diaminodiphenyl sulfone (hereinafter, also referred to as 3,3'-DDS), formula (A-2), The structure represented by the combination of (X-1) and (X-1) (corresponding to the formula (A-5)) is 4,4'-diaminodiphenyl sulfone (4,4'-Diaminodiphenyl Sulfone: hereinafter, 4,4'-. Derived from DDS), the structure represented by the combination of formulas (A-2) and (X-2) is α, α'-bis (4-aminophenyl) -1,4-diisopropylbenzene (hereinafter, also referred to as DDS). Derived from BAPDB), the structure represented by a combination of formulas (A-2) and (X-3) is derived from 4,4'-bis (4-aminophenoxybiphenyl) (hereinafter also referred to as BAPB). The structure represented by the formula (A-3) is derived from cyclohexyldiamine (hereinafter, also referred to as CHDA) when a is 0, and 1,4-bis (aminomethyl) cyclohexane (hereinafter, 14BAC) when a is 1. The structure represented by the formula (A-4) is derived from bis (aminomethyl) norbornan (hereinafter, also referred to as BANBDA). However, the present invention is not limited to these compounds.
The formula (A-5) of the combination of the formulas (A-2) and (X-1) is shown below.

式（Ａ−２）と（Ｘ−３）の組み合わせの式（３）を以下に示す。 The formula (2) of the combination of the formulas (A-2) and (X-2) is shown below.

The formula (3) of the combination of the formulas (A-2) and (X-3) is shown below.

The polyimide in the present embodiment contains the structure represented by the formula (A-1) (derived from 3,3'-diaminodiphenylsulfone) as A of the formula (1) as an essential repeating unit, and further, the formula (A). The repeating unit to be combined with the structure of -1) includes any one or more of the structures represented by the formula (A-2), the formula (A-3), and the formula (A-4).

By including the repeating unit as described above, the polyimide in the present embodiment has a low yellowness (Yellowness Index, hereinafter also referred to as “YI”), and a film having excellent bending resistance can be obtained. It is desirable that YI is as low as possible. When the YI is low, the coloration, cloudiness, and turbidity of the polyimide film can be reduced, and the visibility of display devices such as touch panels, organic EL lighting, and flexible displays can be improved. Therefore, it is preferable to make YI as low as possible and increase the total light transmittance in visible light. Coloring of polyimide is said to be derived from the formation of charge transfer complexes (CT complexes) between polyimide molecules. It is considered that all the structures represented by the formulas (A-1) to (A-4) inhibit the formation of the CT complex between the polyimide molecules due to the bending of the main chain. Of these structure represented by the formula (A-1) and (A-5) may weaken the electron-donating N atom of the imide group with electron-withdrawing with the SO ₂ group, CT complex is formed It is considered to be difficult and is particularly preferable.

The absorption of visible light possessed by aromatic polyimides also causes the polyimide to be colored. It is considered that the alicyclic structures of the formulas (A-3) and (A-4) can reduce the absorption of visible light as compared with the aromatic polyimide.

It is considered that the solubility of the polyimide is improved by disturbing the orientation of the polyimide. It is considered that all the structures represented by the formulas (A-1) to (A-4) exhibit solubility because the orientation of the polyimide molecule is disturbed due to the bending of the main chain. Among them, the structure represented by the formula (A-1) has excellent solubility because the orientation of the polyimide molecule is remarkably disturbed by the bent structure of _{two SO groups and the bent structure due to the bond generated from the 3rd and 3'positions.} Is considered to be expressed.

As described above, the polyimide contained in the polyimide film in the present embodiment has a structure represented by the formula (A-1) as A of the formula (1), and the formulas (A-2) and (A-3). , And any one or more of the structures represented by the formula (A-4).

By copolymerizing the structure represented by the formula (A-1) and the structure represented by the formula (A-5), the molecular weight of the polyimide is increased, and the bending resistance of the film produced by using this polyimide is increased. Can be further improved. At least one type having a structure represented by the formula (A-1) and selected from the structures represented by the formulas (A-2), (A-3) and (A-4). The same effect is exhibited for the polyimide having the above structure.

In the present embodiment, it is preferable to use at least a structure represented by the formula (A-1) and a structure represented by the formula (A-5). In the following, a configuration using both 3,3'-DDS and 4,4'-DDS as diamines will be described.

The structural unit represented by the formula (A-1) can be obtained from the 3,3'-DDS component as described above. The structure represented by the formula (A-1) is a site for expressing solubility in a solvent.

The structural unit represented by the formula (A-5) can be obtained from 4,4'-DDS. The structure represented by the formula (A-5) has a glass transition temperature (Tg) of 250 to 250 in a polyimide film obtained by dissolving the polyimide of the present embodiment in a solvent and heat-drying a varnish (resin composition). It can be expressed in the range of 350 ° C.

In the present embodiment, it is preferable to contain both the structure represented by the formula (A-1) and the structure represented by the formula (A-5). The structural unit represented by the formula (A-1) is preferably introduced from the viewpoint of the solubility of polyimide. The structural unit represented by the formula (A-5) is preferable from the viewpoint of high glass transition temperature (Tg). By containing both the structure represented by the formula (A-1) and the structure represented by the formula (A-5), the solubility of polyimide and the elongation at break of the film and the elongation at break of the film, which cannot be achieved by each of them alone, A high glass transition temperature (Tg) can be obtained without impairing colorless transparency and high total light transmittance.

In the present embodiment, the composition ratio of the structure A1 and the structure A2 (structure A1 / structure A2) is 2/8 to 8/2 in terms of molar ratio from the viewpoint of further improving the bending resistance of the polyimide film. preferable. In particular, when the structure A2 has a structure represented by the formula (A-5), the composition of the structure A1 and the structure represented by the formula (A-5) (hereinafter, also referred to as “structure A21”). The ratio (structure A1 / structure A21) is preferably in the range of 2/8 to 6/4, and more preferably in the range of 3/7 to 4/6 in terms of molar ratio. That is, the structure A1 is preferably 20 mol% or more and 60% or less when the total amount of A in the formula (1) is 100 mol%. The structure A21 is preferably 40 mol% or more and 80% mol% or less when the total amount of A in the formula (1) is 100 mol%.

Further, at least one of the structural units represented by the formulas (A-2) to (A-4) as the structure A1 and the structure A2 (however, excluding the structural units represented by the above-mentioned formula (A-5)). (Hereinafter, also referred to as “structure A22”), the composition ratio (structure A1 / structure A22) of the structure A1 and the structure A22 is preferably 5/5 to 8/2 in terms of molar ratio.

The formulas (A-1) and (A-5) are in the range in which the desired elongation at break can be expressed, and more preferably in the range in which the desired glass transition temperature (Tg) can be expressed. ) Can contain a small amount of structural units other than the structural units represented by). That is, the polyimide according to the present embodiment may contain a structural unit derived from a diamine component other than 4,4'-DDS and 3,3'-DDS as long as its performance is not impaired. For example, an aromatic diamine having 6 to 30 carbon atoms is a preferred embodiment.

Specifically, 2,2'-bis (trifluoromethyl) benzidine (TFMB), 1,4-diaminobenzene, 4-aminobenzenesulfonic acid-4-aminophenyl ester, 4-aminobenzenesulfonic acid-3- Aminophenyl ester, 3-aminobenzenesulfonic acid-3-aminophenyl ester, 2-aminobenzenesulfonic acid-2-aminophenyl ester, 2,2'-dimethyl 4,4'-diaminobiphenyl, 1,3-diaminobenzene , 4-Aminophenyl 4'-Aminobenzoate, 4,4'-Diaminobenzoate, 4,4'-(or 3,4'-, 3,3'-, 2,4'-) Diaminodiphenyl Ether, 4,4 '-(Or 3,3'-) diaminodiphenyl sulfide, 4,4'-benzophenonediamine, 3,3'-benzophenonediamine, 4,4'-di (4-aminophenoxy) phenylsulphon, 4,4'- Di (3-aminophenoxy) phenylsulphon, 4,4'-bis (4-aminophenoxy) biphenyl, 1,4-bis (4-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 2,2'-bis {4- (4-aminophenoxy) phenyl} propane, 3,3', 5,5'-tetramethyl-4,4'-diaminodiphenylmethane, 2,2'-bis (4-amino) Phenyl) propane, 2,2', 6,6'-tetramethyl-4,4'-diaminobiphenyl, 2,2', 6,6'-tetratrifluoromethyl-4,4'-diaminobiphenyl, bis { (4-Aminophenyl) -2-propyl} 1,4-benzene, 9,9-bis (4-aminophenyl) fluorene, 9,9-bis (4-aminophenoxyphenyl) fluorene, 3,3'-dimethyl Bentidine, 3,3'-dimethoxybenzazine and 3,5-diaminobenzoic acid, 2,6-diaminopyridine, 2,4-diaminopyridine, bis (4-aminophenyl-2-propyl) -1,4- Benzene, 3,3'-bis (trifluoromethyl) -4,4'-diaminobiphenyl (3,3'-TFDB), 2,2'-bis [3 (3-aminophenoxy) phenyl] hexafluoropropane ( 3-BDAF), 2,2'-bis [4 (4-aminophenoxy) phenyl] hexafluoropropane (4-BDAF), 2,2'-bis (3-aminophenyl) hexafluoropropane (3,3') -6F), 2,2'-bis (4-a) Examples of structural units derived from aromatic diamine components such as minophenyl) hexafluoropropane (4,4'-6F) can be mentioned.

Also, 2,2'-bis (trifluoromethyl) benzidine (TFMB), 3,3'-bis (trifluoromethyl) -4,4'-diaminobiphenyl (3,3'-TFDB), 2,2'- Bis [3 (3-aminophenoxy) phenyl] hexafluoropropane (3-BDAF), 2,2'-bis [4 (4-aminophenoxy) phenyl] hexafluoropropane (4-BDAF), 2,2'- Bis (3-aminophenyl) hexafluoropropane (3,3'-6F) and 2,2'-bis (4-aminophenyl) hexafluoropropane (4,4'-6F) have high steric hindrance of fluorine atoms. Can be introduced to suppress the formation of CT complexes between the molecules of polyimide and to reduce the YI of the film.

The structural unit derived from 2,2'-bis (trifluoromethyl) benzidine (TFMB) is represented by the following formula (4).

B in equation (1)
Next, B of the formula (1) will be described. In B of the formula (1), the structural unit can be obtained from an acid dianhydride.

In the present embodiment, the structural unit derived from the acid dianhydride component contained in the polyimide may be the same molecule or a molecule having a different structure.

The structural unit represented by B is preferably a structural unit represented by the formulas (B-1) to (B-4).

In the present embodiment, it is preferable that B in the formula (1) includes at least one or more of the structures represented by the following formulas (B-1) to the following formulas (B-4).

上記式（Ｂ−１）中、Ｙは、下記式（Ｙ−１）ないし下記式（Ｙ−３）から選ばれる構造のうちいずれかである。

In the above formula (B-1), Y is any of the structures selected from the following formula (Y-1) to the following formula (Y-3).

The structure represented by combining the formulas (B-1) and (Y-1) (corresponding to the structure of the formula (B-5)) is 4,4'-oxydiphthalic acid dianhydride (hereinafter, also referred to as ODPA). The structure represented by the combination of the formula (B-1) and the formula (Y-2) is derived from 4,4'-(hexafluoroisopropyridene) diphthalic acid dianhydride (hereinafter, also referred to as 6FDA). Origin, The structure represented by combining the formula (B-1) and the formula (Y-3) is derived from 9,9-diphenylfluorenic dianhydride (hereinafter, also referred to as DPFLDA), the formula (B-2). The structure represented by is derived from hydroxypyromellitic dianhydride (hereinafter, also referred to as HPMDA), and the structure represented by the formula (B-3) is bicyclo [2,2,2] octo-7-ene-. Derived from 2,3,5,6-tetracarboxylic dianhydride (hereinafter, also referred to as BODA), the structure represented by the formula (B-4) is 1,3,3a, 4,5,9b-hexahydro-. It is derived from 5- (tetrahydro-2,5-dioxo-3-furanyl) naphtho [1,2-c] furan-1,3-dione (hereinafter, also referred to as TDA).

The polyimide according to the present embodiment contains a structural unit derived from an acid dianhydride component other than the structural units represented by the above formulas (B-1) to (B-4) as long as the performance is not impaired. It may be.

For example, an aromatic tetracarboxylic dianhydride having 8 to 36 carbon atoms, an aliphatic tetracarboxylic dianhydride having 6 to 50 carbon atoms, and an alicyclic tetracarboxylic dianhydride having 6 to 36 carbon atoms. It is preferably a compound selected from. The number of carbon atoms referred to here also includes the number of carbon atoms contained in the carboxyl group.

More specifically, as an aromatic tetracarboxylic dianhydride having 8 to 36 carbon atoms, 4, pyromellitic dianhydride (hereinafter, also referred to as PMDA) 1,2,3,4-benzenetetracarboxylic acid. Dianhydride, 3,3', 4,4'-bezophenone tetracarboxylic acid dianhydride, 2,2', 3,3'-benzophenone tetracarboxylic acid dianhydride, 3,3', 4,4 '-Biphenyltetracarboxylic acid dianhydride (hereinafter, also referred to as BPDA), 3,3', 4,4'-diphenylsulfonetetracarboxylic acid dianhydride, 2,2', 3,3'-biphenyltetracarboxylic Acid dianhydride, methylene-4,4'-diphthalic hydride, 1,1'-ethylidene-4,4'-diphthalic hydride, 2,2'-propylidene-4,4'-diphthalic acid Dianhydride, 1,2-ethylene-4,4'-diphthalic acid dianhydride, 1,3-trimethylen-4,4'-diphthalic acid dianhydride, 1,4-tetramethylene-4,4'- Diphthalic hydride, 1,5-pentamethylene-4,4'-diphthalic hydride, thio-4,4'-diphthalic hydride, sulfonyl-4,4'-diphthalic hydride, 1,3-bis (3,4-dicarboxyphenyl) benzene dianhydride, 1,3-bis (3,4-dicarboxyphenoxy) benzene dianhydride, 1,4-bis (3,4-dicarboxy) Phenoxy) benzene dianhydride, 1,3-bis [2- (3,4-dicarboxyphenyl) -2-propyl] benzene dianhydride, 1,4-bis [2- (3,4-dicarboxyphenyl) -2- (3,4-dicarboxyphenyl) ) -2-propyl] benzene dianhydride, bis [3- (3,4-dicarboxyphenoxy) phenyl] methane dianhydride, bis [4- (3,4-dicarboxyphenoxy) phenyl] methane dianhydride , 2,2'-bis [3- (3,4-dicarboxyphenoxy) phenyl] propane dianhydride, 2,2'-bis [4- (3,4-dicarboxyphenoxy) phenyl] propane hydride (Hereinafter also referred to as BPADA), bis (3,4-dicarboxyphenoxy) dimethylsilane dianhydride, 1,3-bis (3,4-dicarboxyphenyl) -1,1', 3,3'-tetra Methyldisiloxane dianhydride, 2,3,6,7-naphthalenetetracarboxylic acid dianhydride, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride, 1,2,5,6-naphthalenetetracarboxylic Acid dianhydride, 3,4,9,10-perylenetetracarboxylic hydride, 2,3,6 , 7-Anthracene tetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride and the like can be mentioned.

Examples of the aliphatic tetracarboxylic dianhydride having 6 to 50 carbon atoms include ethylenetetracarboxylic dianhydride and 1,2,3,4-butanetetracarboxylic dianhydride.

As the alicyclic tetracarboxylic acid dianhydride having 6 to 36 carbon atoms, 1,2,3,4-cyclobutanetetracarboxylic acid dianhydride (hereinafter, also referred to as CBDA), cyclopentanetetracarboxylic acid dianhydride, Cyclohexane-1,2,3,4-tetracarboxylic dianhydride, 3,3', 4,4'-bicyclohexyltetracarboxylic dianhydride, carbonyl-4,4'-bis (cyclohexane-1,2,2) -Dicarboxylic acid) dianhydride, methylene-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, 1,2-ethylene-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) Acid) dianhydride, 1,1'-ethylidene-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, 2,2'-propylidene-4,4'-bis (cyclohexane-1) , 2-Dicarboxylic acid) dianhydride, oxy-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, thio-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) Dianhydride, sulfonyl-4,4'-bis (cyclohexane-1,2-dicarboxylic acid) dianhydride, rel- [1S, 5R, 6R] -3-oxabicyclo [3,2,1] octane-2 , 4-Dione-6-Spiro-3'-( tetrahydrofuran-2', 5'-Dione), 4- (2,5-Dioxotetra-3-yl) -1,2,3,4-tetrahydronaphthalene -1,2-dicarboxylic acid dianhydride, ethylene glycol-bis- (3,4-dicarboxylic acid dianhydride phenyl) ether, 4,4'-biphenylbis (trimellitic acid monoesteric dianhydride), etc. Can be mentioned.

In the above formula (B-1), the formulas (Y-1) and (Y-2) are preferable from the viewpoint of the solubility of the polyimide in the solvent and the reduction of the yellowness when the polyimide film is formed. Further, since the formula (Y-3) has a negative intrinsic birefringence, it is preferable from the viewpoint of reducing the yellowness when the polyimide film is formed and improving the glass transition temperature (Tg).

The formulas (B-2) to (B-4) are preferable from the viewpoint of solubility of the polyimide in a solvent and reduction of yellowness when the polyimide film is formed.

Among them, B in the formula (1) is a component derived from ODPA from the viewpoints of solubility of polyimide in a solvent, high total light transmittance when made into a polyimide film, low yellowness, high elasticity, and high elongation at break. It is particularly preferable to use the structure represented by the following formula (B-5), and among the structural units B derived from acid dianhydride in the polyimide represented by the formula (1), the formula (B). −5) is preferably 50 mol% or more, more preferably 80 mol% or more, and may be 100 mol% with respect to the total acid dianhydride.

The polyimide according to the present embodiment mainly includes a unit 1 represented by the following formula (5) and a unit 2 represented by the following formula (6).

In the present embodiment, when the units other than the unit 1 and the unit 2 are further included, the content of the units other than the unit 1 and the unit 2 is preferably smaller than the contents of the unit 1 and the unit 2. These units may be bonded alternately or in a permutation in the polymer chain, or these units may be bonded randomly.

The weight average molecular weight (Mw) of the polyimide is preferably 10,000 or more, more preferably 25,000 or more, and more preferably 30,000 or more, from the viewpoint of obtaining a high elongation at break in the polyimide film. Is particularly preferable. The weight average molecular weight (Mw) of the polyimide is preferably 1,000,000 or less, more preferably 500,000 or less, and particularly preferably 250,000 or less. When the weight average molecular weight is 1,000,000 or less, the solubility in a solvent is also good, and coating can be performed with a desired film thickness during processing such as coating without bleeding. In particular, from the viewpoint of obtaining high elongation at break in the polyimide film, the weight average molecular weight is preferably 30,000 or more. Here, the weight average molecular weight refers to the molecular weight measured by gel permeation chromatography using polystyrene having a known number average molecular weight as a standard.

<Polyimide varnish>
The above-mentioned polyimide can be used as a raw material for producing the polyimide film of the present embodiment as a polyimide varnish (polyimide resin composition, hereinafter also simply referred to as "varnish") in which the polyimide varnish is dissolved in a solvent.

It has been proved in the experiment described later that the polyimide in this embodiment has excellent solubility in a solvent. Therefore, by using the polyimide of the present embodiment, a varnish having desired characteristics can be obtained by a simple process. According to the polyimide varnish of the present embodiment, since the polyimide is appropriately dissolved, when the varnish is applied on the coated surface, it is possible to form a film having excellent smoothness without forming lumps. Therefore, a resin layer having a uniform thickness can be formed and high bending resistance can be obtained.

In the polyimide varnish, for example, an acid dianhydride component and a diamine component are dissolved in a solvent, for example, an organic solvent, a co-boiling solvent such as toluene is added, and water generated during imidization is removed from the system. , Polyimide and solvent can be produced as a polyimide solution. Here, the conditions at the time of reaction are not particularly limited, but for example, the reaction temperature is 0 ° C. to 200 ° C. and the reaction time is 3 to 72 hours. In order to sufficiently proceed with the reaction with the sulfone group-containing diamines, it is preferable to carry out a heating reaction at 180 ° C. for about 12 hours. Further, during the reaction, an inert atmosphere such as argon or nitrogen is preferable. Further, a basic compound such as pyridine may be added as an imidization catalyst as appropriate.

The solvent is not particularly limited as long as it is a solvent that dissolves polyimide. As known reaction solvents, for example, m-cresol as a phenol-based solvent, for example, N-methyl-2-pyrrolidone (NMP), N, N-dimethylformamide (DMF), N, N-dimethylacetamide as an amide-based solvent ( DMAc), as a lactone-based solvent, for example, γ-butyrolactone (GBL), δ-valerolactone, ε-caprolactone, γ-crotonolactone, γ-hexanolactone, α-methyl-γ-butyrolactone, γ-valerolactone, α-Acetyl-γ-butyrolactone, δ-hexanolactone, as a sulfoxide solvent, for example, N, N-dimethylsulfoxide (DMSO), as a ketone solvent, for example, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, as an ester solvent. For example, one or more polar solvents selected from methyl acetate, ethyl acetate, butyl acetate, dimethyl carbonate, and the like are useful. Of these, NMP and GBL are preferable from the viewpoint of solubility. Further, GBL is preferable from the viewpoint of reducing the YI of the film.

Additives may be appropriately added to the polyimide varnish in the present embodiment. As an additive, a substance exhibiting a negative birefringence may be added. Examples thereof include inorganic particles such as strontium carbonate and organic compounds such as polystyrene, polyvinylnaphthalene, polymethylmethacrylate, cellulose triacetate and fluorene derivatives.

Examples of the additive include a leveling agent for improving the coatability of the film, a dispersant and a surfactant, and a surfactant and an adhesion for adjusting the peelability and adhesiveness of the film from the support. Auxiliary agent, flame retardant for imparting flame retardancy to film, etc. Other additives include, for example, antioxidants, UV inhibitors, light stabilizers, plasticizers, waxes, fillers, pigments, dyes, foaming agents, defoamers, dehydrating agents, antistatic agents, antibacterial agents. , Antifungal agent and the like.

The additive added to the polyimide varnish may be contained in the polyimide film as it is.
The polyimide film according to the present embodiment may be, for example, a polyimide film formed on the surface of a support, or a supportable film (self-supporting film) without a support. The polyimide film of the present embodiment is preferably a supportable film (self-supporting film) without a support from the viewpoint of maintaining the strength as a film substrate. In the present specification, a supportive film means a film having a breaking elongation of 5% or more. A film having a support corresponds to a film having a support when the peeled film has a breaking elongation of 5% or more.

As a method for measuring the breaking elongation, the method described later (evaluation of breaking elongation and breaking strength) can be used.

The polyimide film can be obtained by forming a polyimide varnish (resin composition) containing a polyimide and a solvent on the surface of a support by coating or the like, and then heating the polyimide varnish to evaporate the solvent. .. More specifically, as described above, a polyimide solution obtained by dissolving an acid dianhydride component and a diamine component in an organic solvent and reacting them can be formed on a support.

A polyimide varnish (resin composition) is formed on the surface of the support by coating or the like, and temporarily dried until the film is no longer sticky to form a solvent-containing film containing 0.1% by mass to 20% by mass of a solvent. do. It can also be obtained by peeling the film from the support and then further heating the film to evaporate the solvent to less than 0.1% by weight. In this case, the film has supportability even without a support (self-supporting film).

Here, the support includes, for example, an alkali glass substrate, a non-alkali glass substrate (Eagle XG (registered trademark), manufactured by Corning Co., Ltd.), a metal substrate such as a copper substrate, an aluminum substrate, and a SUS substrate, and a polyimide (registered trademark). ) Film (manufactured by Ube Kosan), Kapton (registered trademark) film (manufactured by Toray DuPont), polycarbonate film, PET film, and other plastic films, and copper foil, aluminum foil, SUS foil, and other metal foils. , The heating and drying of the polyimide varnish can be carried out without a support, and the type of the support is not particularly limited. Here, the substrate as a support means a substrate having high rigidity and not suitable for bending, and the film or a film substrate means a substrate which is flexible and can be bent.

In the present embodiment, the solubility of polyimide in a solvent is good, and even if a polyimide varnish is applied onto a support, temporary drying is performed, and the support is removed, the polyimide film can maintain its independence. Therefore, a polyimide film after temporary drying containing 0.1% by mass to 20% by mass of a solvent in a free state not supported by a support (also referred to as "temporary dry film" and "solvent-containing film" in the present specification). A polyimide film having a solvent content of less than 0.1% by mass can be obtained in a state where the orientation of the polymer is small.

By applying a temperature of 150 to 350 ° C. to the polyimide varnish in an inert gas atmosphere, the solvent can be removed to form a polyimide film, but drying can also be carried out in an air atmosphere. , Not particularly limited.

As described above, the solvent is, for example, m-cresol, NMP, DMF, DMAc, GBL, DMSO, acetone, diethylacetamide, etc. Of these, GBL is used as the solvent to ensure low YI of the polyimide film. be able to. Although the solvent is almost removed by heating and drying the polyimide varnish as described above, the content of GBL in the polyimide film is not impaired from the viewpoint of obtaining desired YI and bending resistance and the like. Is preferably less than 3% by mass, more preferably less than 1% by mass, and even more preferably 0.5% by mass or less. At least 0.01% by mass of GBL is left as the remaining amount.

Here, examples of the method for forming the polyimide varnish include known coating methods such as spin coating, slit coating, slot die coating, and blade coating.

The polyimide film in this embodiment has excellent bending resistance. When both the structures represented by the formulas (A-1) and (A-5) are included, compared with the case where only one of the structures represented by the formulas (A-1) and (A-5) is included. Therefore, higher breaking elongation and breaking strength can be obtained. As described above, when the diamine component is derived only from the 4,4'-DDS component (structure represented by the formula (A-5)), the molecular weight of the polyimide is lowered and the bending resistance of the film is lowered. Therefore, in the present embodiment, the isomer is derived from the 4,4'-DDS component, and is derived from the 3,3'-DDS component, which has a structure in which the monomer skeleton is bent when viewed from the 4,4'-DDS component (formula). (Structure represented by (A-1)) is contained, and at this time, the molecular weight is preferably reduced by adding less from the 3,3'-DDS component than from the 4,4'-DDS component. Bending resistance can be improved while increasing.

<Other features>
The yellowness (YI) of the polyimide film according to the present embodiment is preferably 5.0 or less, more preferably 4.5 or less, still more preferably 4.0 or less. At this time, the film thickness of the polyimide film is preferably in the range of 0.1 μm to 30 μm, and more preferably in the range of 1 μm to 20 μm.

When used as a film substrate for a flexible device, the film thickness of the polyimide film is preferably in the range of 1 μm to 10 μm, more preferably in the range of 1 μm to 5 μm, from the viewpoint of improving bending resistance by thinning the device. Inside.

A film having a thickness of less than 10 μm can be produced, for example, by stretching a polyimide film having a film thickness of 10 μm or more. A polyimide varnish is applied on the support, and temporary drying is performed until the polyimide film can have self-supporting property even if the support is removed. As the support, the above-mentioned PET film, polyimide film such as Kapton (registered trademark of Toray DuPont), Upilex (registered trademark of Ube Industries), metal leaf and the like can be used. The amount of solvent remaining in the solvent-containing film during the stretching treatment is preferably 0.1 to 20% by mass from the viewpoint of the self-supporting property and stretchability processability of the film.

The temporary dry film is biaxially stretched while being heated to 150 ° C. to 250 ° C. with the support attached or peeled off from the support, from 1.5 times in the MD direction and / or the TD direction. It can be produced by stretching it 5 times. The temporarily dried film after stretching is then subjected to main drying and dried until the residual solvent is less than 0.1% by mass.

In this embodiment, the yellowness (YI) can be adjusted to 2.0 or less. As described above, in the present embodiment, the yellowness can be suppressed to a low level, that is, a colorless and transparent polyimide film can be obtained. The term "colorless and transparent" in the present embodiment means a state in which the total light transmittance of the film is 80% or more, the haze is 2 or less, and the yellowness (YI) is 5.0 or less. Therefore, the polyimide film of the present embodiment can be suitably used for optical applications such as touch panels and displays. For example, when the polyimide resin according to the present embodiment is used as a substrate film for a transparent electrode film, a touch panel element is formed on at least one of the upper and lower surfaces of the substrate film, and the surface of the substrate film or the surface of the substrate film. Even when the side facing the screen is the visual plane, the color of the screen does not adversely affect the brightness.

The polyimide film of the present embodiment can be used as a substitute for glass like the PET film and the COP film, and further, since the polyimide film of the present embodiment has excellent bending resistance, it follows a foldable display body or a curved surface. It can be used for the display body.

<< Laminated body >>
In the present embodiment, a transparent electrode layer can be provided on the surface of the polyimide film to produce a laminate.

The laminate can be obtained by forming a transparent electrode layer on the surface of the polyimide film with a sputtering device or the like. The polyimide film may have a support or may be a single layer having no support. The laminate may have transparent electrode layers on both sides of the polyimide film. At this time, it is preferable to have at least one or more transparent electrode layers on both sides of the polyimide film. Further, between the transparent electrode layer and the polyimide film, an undercoat layer for imparting smoothness, a hardcoat layer for imparting surface hardness, an index matching layer for improving visibility, and a gas barrier property are provided. It may have another layer such as a gas barrier layer for the purpose. The hard coat layer for imparting surface hardness and the index matching layer for improving visibility may be laminated on the transparent electrode layer and the polyimide film. The laminate of this embodiment is suitable for use in a touch panel material such as a transparent electrode film.

When forming a transparent electrode film, the step of forming the transparent electrode layer on the surface of the polyimide film is carried out in a low temperature range of, for example, 80 to 100 ° C. It is preferable to perform sputtering at a high temperature to form a transparent electrode layer having a low specific resistance. The transparent electrode layer can be formed on both sides of the polyimide film. Thereby, for example, the touch panel elements can be arranged on both sides.

At this time, when the temperature at which the transparent electrode layer is formed is high, the glass transition temperature (Tg) of the polyimide film constituting the film-forming surface is high, problems such as shrinkage and breakage of the polyimide film occur in a high temperature region. Generally, when a transparent electrode layer is formed on a PET film, sputtering is performed at about 80 ° C., which is lower than the glass transition temperature (Tg) of the PET film, which is about 100 ° C. On the other hand, the polyimide film according to the present embodiment has a high glass transition temperature (Tg) of about 250 ° C. or higher (based on a film thickness of 15 μm) and is excellent in heat resistance. That is, high bending resistance can be maintained even when exposed to a high temperature of 200 ° C. or higher. Therefore, the surface of the polyimide film 10 of the present embodiment can be sputtered at, for example, about 150 to 250 ° C. to form a transparent electrode layer having a low resistivity.

Further, in the present embodiment, the polyimide preferably has a breaking strength of 100 MPa or more based on the thickness of the polyimide film of 15 μm from the viewpoint of improving the yield when the transparent electrode layer 21 is formed.

Further, from the viewpoint of improving the performance of the transparent electrode film, the polyimide film according to the present embodiment has a glass transition temperature (Tg) of 250 ° C. or higher based on a film thickness of 15 μm as described above. preferable.

<< Manufacturing method of polyimide film >>
The method for producing a polyimide film in the present embodiment is: coating a polyimide varnish containing a polyimide and a solvent on a support; temporarily drying the solvent and containing a solvent containing 0.1% by mass to 20% by mass of the solvent. It involves forming a film; stretching the solvent-containing film; and drying the solvent from the solvent-containing film to less than 0.1% by weight. Further, in stretching, the stretching ratio of the solvent-containing film in the MD direction is 1.5 times or more. Thereby, it is easier to adjust the difference (ΔN) between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film to 0.02 or more.

In another embodiment, the method for producing a polyimide film is: coating a polyimide varnish containing a polyimide and a solvent onto a support; drying the solvent to form a film containing less than 0.1% by mass of the solvent. And; including stretching the film. Further, in stretching, the stretching ratio of the film in the MD direction is 1.5 times or more. Thereby, it is easier to adjust the difference (ΔN) between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film to 0.02 or more.

The temperature in the temporary drying is generally 100 ° C. to 350 ° C., preferably 100 ° C. to 300 ° C., and more preferably 100 ° C. to 250 ° C. The temperature in stretching is generally 200 ° C. to 400 ° C., preferably 200 ° C. to 350 ° C., and more preferably 250 ° C. to 350 ° C. The temperature in the main drying after stretching is generally 200 ° C. to 400 ° C., preferably 200 ° C. to 350 ° C., and more preferably 250 ° C. to 350 ° C.

In stretching, the lower limit of the difference between the stretching ratio in the MD direction and the stretching ratio in the TD direction of the solvent-containing film is preferably 0.1 or more, more preferably 0.5 or more, still more preferably 1.0 or more. The upper limit is preferably 4.0 or less, more preferably 3.5 or less, still more preferably 3.0 or less. The lower limit of the draw ratio of the solvent-containing film in the MD direction is preferably 1.1 times or more, more preferably 1.3 times or more, still more preferably 1.5 times or more, and the upper limit is preferably 5.0 times or more. Below, it is more preferably 4.5 times or less, still more preferably 4.0 times or less. The lower limit of the draw ratio of the solvent-containing film in the TD direction is preferably 1.1 times or more, more preferably 1.2 times or more, still more preferably 1.3 times or more, and the upper limit is preferably 4.0 times or more. Below, it is more preferably 3.0 times or less, still more preferably 2.0 times or less. Further, from the viewpoint of more easily adjusting the difference in refractive index to 0.02 or more, the difference in refractive index is preferably 1.5 times or more, more preferably 2.0 times or more in the MD direction without stretching in the TD direction, and further. It may be stretched preferably 2.5 times or more, preferably 5.0 times or less, more preferably 4.5 times or less, still more preferably 4.0 times or less.

The method for producing a polyimide film of the present embodiment includes coating a polyimide varnish on a support. In the present embodiment, since the film is formed from a polyimide solution that has been imidized in advance, the support can be removed after temporary drying to obtain a polyimide self-supporting film. Therefore, it is easier to adjust the difference between the refractive index in the MD direction and the refractive index in the TD direction to 0.02 or more by performing stretching and heating in a free state not supported by the support.

In the method of heating and imidizing the support with a varnish containing a polyimide precursor coated on the support, the residual strain of the polyimide film tends to be large due to the strain due to the expansion difference with the support. Further, since the polyimide film imidized from the polyamic acid film which is a polyimide precursor is inferior in strength to the polyamic acid film, a support is required, and it is difficult to obtain a self-supporting film after temporary drying. Further, since distortion occurs due to dehydration shrinkage, the residual strain of the polyimide film tends to increase.

The polyimide film of the present embodiment can be used as a substitute for glass like the PET film and the COP film, and has excellent bending resistance. Therefore, for example, the polyimide film of the present embodiment can be used as a foldable display body or a display body following a curved surface. Even if the polyimide film of No. 1 is used, the film is less likely to be damaged and is easy to use.

Hereinafter, the present invention will be described in more detail based on examples, but these are described for the sake of explanation, and the scope of the present invention is not limited to the following examples. Various evaluations in the example were performed as follows.

<< Measurement and evaluation method >>
<Evaluation of refractive index>
The refractive index of the polyimide film is determined by adjusting the humidity of the film at 25 ° C. and 50% relative humidity for 24 hours, and then using a prism coupler (MODEL2010 Prism Copper, manufactured by Metrocon) to use a light source having wavelengths of 532 nm, 632.8 nm, and 838 nm. The refractive index (N) represented by the following (a) was determined using this. From this value of the average refractive index, the average refractive index (N ₅₈₉ ) at a wavelength of 589 nm was obtained.
Equation (a): N = (N _TE × 2 + N _TM ) / 3
[In the formula, _NTE is the refractive index measured by changing the plane direction of the film, and _NTM is the refractive index measured by polarization in the normal direction of the film. ]
Subsequently, the humidity-controlled film was measured with a light source of 589 nm by a parallel Nicol rotation method using a birefringence measuring device (KOBRA-WR, manufactured by Oji Measuring Instruments Co., Ltd.), and the average refractive index at a wavelength of 589 nm ( Nx and Ny were obtained from the values of N ₅₈₉ ), and the refractive index difference (ΔN) represented by the following (b) was obtained.
Equation (b): ΔN = Nx-Ny
[In the formula, Nx is the refractive index in the slow phase axis (x) direction in the film plane, and Ny is the refractive index in the new phase axis (y) direction of the film. ]

<Evaluation of flexion resistance>
The bending resistance of the polyimide film was evaluated by adjusting the humidity of the film at 25 ° C. and 50% relative humidity for 24 hours, and then using a MIT type repeated bending tester (MIT-DA, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to obtain a width of 15 mm. A test piece with a length of 100 mm and a thickness of 7 to 20 μm is hung ^{with a weight having a load of 1.25 kg / mm 2,} and the bending radius is R2 mm, the bending angle is 135 °, and the speed is 90 times / minute. A reciprocating bending test was performed. After the test, the sample was removed from the apparatus, and those without bending marks were marked with ◯, those with broken films, those with visually confirmed bending marks, or those with a haze of 5.0 or more were marked with x.

<Measurement of weight average molecular weight (Mw) and number average molecular weight (Mn)>
The weight average molecular weight (Mw) and the number average molecular weight (Mn) were measured by gel permeation chromatography (GPC) under the following conditions. As a solvent, N, N-dimethylformamide (manufactured by Wako Pure Chemical Industries, Ltd., for high performance liquid chromatograph) was used, and 24.8 mol / L lithium bromide monohydrate (manufactured by Wako Pure Chemical Industries, Ltd.) was used before measurement. , Purity 99.5%) and 63.2 mol / L phosphoric acid (manufactured by Wako Pure Chemical Industries, Ltd., for high performance liquid chromatograph) were added. The calibration curve for calculating the weight average molecular weight was prepared using standard polystyrene (manufactured by Tosoh Corporation).
Column: TSK-GEL SUPER HM-H
Flow velocity: 0.5 mL / min Column temperature: 40 ° C
Pump: PU-2080 (manufactured by JASCO)
Detector: RI-2031Plus (RI: differential refractometer, manufactured by JASCO Corporation)
UV-2075Plus (UV-Vis: UV-Visible Absorber, manufactured by JASCO Corporation)

<Evaluation of Young's modulus, breaking elongation and breaking strength>
A dry sample length of 3 × 50 mm, a polyimide film was pulled at a speed of 100 mm / min using a tensile tester (manufactured by A & D Co., Ltd .: RTG-1210), and Young's modulus, breaking elongation, and breaking strength were measured.

<Evaluation of yellowness (YI) and haze>
The polyimide film was measured for yellowness (YI value), haze, and total light transmittance using a spectrophotometer (CM3600A) manufactured by Konica Minolta Co., Ltd. using a D65 light source. Unless otherwise specified, measurements were made on a film having a film thickness of 20 ± 1 μm as a sample.

<Evaluation of glass transition temperature (Tg)>
The glass transition temperature (Tg) in the temperature range of 50 to 350 ° C. was measured by thermomechanical analysis using a polyimide film cut into a size of 3 mm × 20 mm as a test piece. A test piece in the temperature range of 50 to 350 ° C. using a measuring device manufactured by Seiko Instruments Inc. (EXSTAR6000) under the conditions of a tensile load of 49 mN, a heating rate of 10 ° C./min, and a nitrogen stream (flow rate of 100 ml / min). Elongation was measured. The inflection point of the obtained chart was determined as the glass transition temperature.

Subsequently, the conditions for synthesizing polyimide and the conditions for producing a polyimide film will be specifically described.

<< Synthesis example of polyimide varnish >>
[Synthesis Example 1]
22.19 g (69.30 mmol) of 2,2'-bis (trifluoromethyl) benzidine (TFMB) while introducing nitrogen gas into a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top. , Γ-Butyrolactone (GBL) 50.00 g was added. Subsequently, 31.09 g (70.00 mmol) of 4,4'-(hexafluoroisopropyridene) diphthalic acid dianhydride (6FDA), 22.28 g of GBL, and 26.02 g of toluene were added at room temperature, and then the internal temperature reached 160 ° C. The temperature was raised, and the mixture was heated under reflux at 160 ° C. for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 2]
22.19 g (69.30 mmol) of 2,2'-bis (trifluoromethyl) benzidine (TFMB) while introducing nitrogen gas into a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top. , GBL 50.00 g was added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 22.28 g of GBL, and 26.02 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 3]
4,4'-DDS 13.77 g (55.44 mmol), 3,3'-DDS 3.44 g (55.44 mmol) while introducing nitrogen gas into a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top. 13.86 mmol), 50.00 g of NMP was added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 22.28 g of NMP, and 26.02 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and NMP was added so that the solid content had a concentration of 20% by mass to obtain a polyimide NMP solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 4]
In a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, while introducing nitrogen gas, add 4,4'-DDS to 12.05 g (48.51 mmol) and 3,3'-DDS. To 5.16 g (20.79 mmol) and 50.00 g of GBL were added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 22.28 g of GBL, and 26.02 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 5]
Except that 4,4'-DDS was changed to 10.32 g (41.58 mmol) and 3,3'-DDS was changed to 6.90 g (27.72 mmol), and the reaction time at 180 ° C. was set to 7 hours. A polyimide varnish was obtained in the same manner as in Synthesis Example 4.

[Synthesis Example 6]
A polyimide varnish was obtained in the same manner as in Synthesis Example 4, except that 4,4'-DDS was changed to 8.61 g (34.65 mmol) and 3,3'-DDS was changed to 8.61 g (34.65 mmol).

[Synthesis Example 7]
A polyimide varnish was obtained in the same manner as in Synthesis Example 4, except that 4,4'-DDS was changed to 6.89 g (27.72 mmol) and 3,3'-DDS was changed to 10.34 g (41.58 mmol).

[Synthesis Example 8]
A polyimide varnish was obtained in the same manner as in Synthesis Example 6 except that 4,4'-ODPA was changed to 15.27 g (70.00 mmol) of pyromellitic acid dianhydride (PMDA) and the solvent was changed from GBL to NMP.

[Synthesis Example 9]
Synthesis example except that 4,4'-ODPA was changed to 3,3', 4,4'-biphenyltetracarboxylic dianhydride (BPDA) 20.59 g (70.00 mmol), and the solvent was changed from GBL to NMP. A polyimide varnish was obtained in the same manner as in 6.

[Synthesis Example 10]
A polyimide varnish was obtained in the same manner as in Synthesis Example 6, except that 4,4'-ODPA was changed to 31,09 g (70.00 mmol) of 4,4'-(hexafluoroisopropyridene) diphthalic acid dianhydride (6FDA). rice field.

[Synthesis Example 11]
A polyimide varnish was obtained in the same manner as in Synthesis Example 10, except that 4,4'-DDS was changed to 13.77 g (55.44 mmol) and 3,3'-DDS was changed to 3.44 g (13.86 mmol).

[Synthesis Example 12]
A polyimide varnish was obtained in the same manner as in Synthesis Example 10, except that 4,4'-DDS was changed to 6.89 g (27.72 mmol) and 3,3'-DDS was changed to 10.34 g (41.58 mmol).

[Synthesis Example 13]
Trans-1,4-cyclohexylamine (CHDA) 1.81 (15.84 mmol), 3, while introducing nitrogen gas into a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top. 15.73 g (63.36 mmol) of 3-DDS and 50.00 g of NMP were added. Subsequently, 24.82 g (80.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 28.67 g of NMP, and 27.14 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 3 hours, the oil bath was removed and the temperature was returned to room temperature, and NMP was added so that the solid content had a concentration of 20% by mass to obtain a polyimide NMP solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 14]
4.93 g (34.65 mmol) of 1,4-bis (aminomethyl) cyclohexane (14BAC) while introducing nitrogen gas into a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top. , 8,61 g (34.65 mmol) of 3,3'-DDS and 50.00 g of GBL were added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 15.46 g of GBL, and 26.02 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 4 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 15]
In a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, while introducing nitrogen gas, 5.13 g (33.25 mmol) of bis (aminomethyl) norbornane (BANBDA) was added to 3,3'. 8.26 g (33.25 mmol) of −DDS and 50.00 g of GBL were added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 15.19 g of GBL, and 24.90 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 6 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 16]
5.11 g (13.86 mmol) of 4,4'-bis (4-aminophenoxybiphenyl) (BABP) while introducing nitrogen gas into a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top. ), 13.77 g (55.44 mmol) of 3,3-DDS and 50.00 g of GBL were added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 22.28 g of GBL, and 25.63 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 6 hours, the oil bath was removed and the temperature was returned to room temperature, and NMP was added so that the solid content had a concentration of 20% by mass to obtain a polyimide NMP solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 17]
In a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, α, α'-bis (4-aminophenyl) -1,4-diisopropylbenzene (BAPDB) was added while introducing nitrogen gas. To 11.94 g (34.65 mmol), 8.61 g (34.65 mmol) of 3,3'-DDS and 50.00 g of GBL were added. Subsequently, 21.71 g (70.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 28.47 g of GBL, and 26.99 g of toluene were added at room temperature, and then the temperature was raised to 160 ° C. at 160 ° C. The mixture was heated under reflux for 1 hour to imidize. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 6 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 18]
In a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, while introducing nitrogen gas, 4,4'-DDS 8.61 g (34.65 mmol) and 3,3'-DDS 8.61 g (34.65 mmol), 50.00 g of GBL was added. Subsequently, 10.86 g (35.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), 7.85 g (35.00 mmol) of hydroxypyromellitic dianhydride (HPMDA), 16.69 g of GBL, and 24.41 g of toluene. Was added at room temperature, the temperature was raised to an internal temperature of 160 ° C., and the mixture was heated and refluxed at 160 ° C. for 1 hour to carry out imidization. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 19]
In a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, while introducing nitrogen gas, 4,4'-DDS 8.61 g (34.65 mmol) and 3,3'-DDS 8.61 g (34.65 mmol), 50.00 g of GBL was added. Subsequently, 17.37 g (56.00 mmol) of 4,4′-oxydiphthalic anhydride (ODPA), 1,3,3a, 4,5,9b-hexahydro-5 (tetrahydro-2,5-dioxo-3) was added. -Flanil) Naft [1,2-c] furan-1,3-dione (TDA) 4.20 g (14.00 mmol), GBL 22.02 g, and toluene 26.07 g are added at room temperature, and then the internal temperature is up to 160 ° C. The temperature was raised, and the mixture was heated under reflux at 160 ° C. for 1 hour for imidization. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

[Synthesis Example 20]
In a 500 mL separable flask with a stirring rod equipped with a Dean-Stark tube and a reflux tube at the top, while introducing nitrogen gas, 4,4'-DDS 8.61 g (34.65 mmol) and 3,3'-DDS 8.61 g (34.65 mmol), 50.00 g of GBL was added. Subsequently, 17.37 g (56.00 mmol) of 4,4'-oxydiphthalic anhydride (ODPA), bicyclo [2,2,2] octo-7-ene-2,3,5,6-tetracarboxylic dianhydride After adding 3.47 g (14.00 mmol) of substance (BODA), 20.67 g of GBL, and 25.58 g of toluene at room temperature, the temperature is raised to an internal temperature of 160 ° C., and the mixture is heated and refluxed at 160 ° C. for 1 hour to imidize. went. After the imidization was completed, the temperature was raised to 180 ° C., and the reaction was continued while extracting toluene. After the reaction for 12 hours, the oil bath was removed and the temperature was returned to room temperature, and GBL was added so that the solid content had a concentration of 20% by mass to obtain a polyimide GBL solution (hereinafter, also referred to as polyimide varnish).

The compositions of the polyimides obtained in Synthesis Examples 1 to 20 are shown in Table 1 below.

<< Production example of polyimide film >>
[Production Example A1]
The polyimide varnish was coated on polyethylene terephthalate (Cosmo Shine 100A4100) having a width of 508 mm as a support with a coating thickness of 150 μm and a coating width of 500 mm, and dried at 50 ° C. for 30 minutes. The residual solvent in the polyimide film at this time had a concentration of 15.5% by mass. The solvent-containing polyimide film is peeled off from the support, the film is conveyed into the heating furnace, and the film stretching speed is 60% / min by utilizing the difference in peripheral speed of the stretching rollers arranged at intervals in the MD direction. Under the above conditions, stretching was performed in the MD direction, or in the MD direction and the TD direction. The surface temperature of the stretching roller at this time was 290 ° C. The width of the film at the inlet of the heating furnace was 500 mm, and the width of the film at the outlet of the heating furnace after drying was 470 mm.

[Production Example A2]
The polyimide varnish was coated on upilex (manufactured by Ube Industries, Ltd., product number uplex125s) having a width of 508 mm as a supporting base material with a coating thickness of 150 μm and a coating width of 500 mm, and dried at 50 ° C. for 30 minutes. The residual solvent in the polyimide film at this time had a concentration of 16% by mass. The upilex film with the solvent-containing polyimide film was conveyed into a heating furnace and dried at 270 ° C. for 1 hour. The width of the film at the inlet of the heating furnace was 500 mm, and the width of the polyimide film at the outlet of the heating furnace after drying was 500 mm. The obtained film was peeled from upilex to obtain a polyimide film. This polyimide film was conveyed into a heating furnace, and MD stretching was performed under the condition that the stretching speed of the film was 60% / min by utilizing the difference in peripheral speeds of the stretching rollers arranged at intervals in the MD direction. The surface temperature of the stretching roller at this time was 290 ° C. The width of the film at the inlet of the heating furnace was 500 mm, and the width of the film at the outlet of the heating furnace after drying was 470 mm.

[Production Example B]
The polyimide varnish was coated on upilex (manufactured by Ube Industries, Ltd., product number uplex125s) having a width of 508 mm as a supporting base material with a coating thickness of 150 μm and a coating width of 500 mm, and dried at 50 ° C. for 30 minutes. The residual solvent in the polyimide film at this time had a concentration of 16% by mass. The upilex film with the solvent-containing polyimide film was conveyed into a heating furnace and dried at 270 ° C. for 1 hour. The width of the polyimide film at the inlet of the heating furnace was 500 mm, and the width of the polyimide film at the outlet of the heating furnace after drying was 500 mm. The obtained film was peeled from upilex to obtain a polyimide film.

<< Reference Examples 1 to 8, Examples 9 to 21, Reference Examples 22 to 23, Examples 24 to 34, and Comparative Examples 1 to 5 >>
As shown in Table 2 or 3, a polyimide film was prepared by combining any of the polyimides obtained in Synthesis Examples 1 to 20 with any of Film Production Examples A to B, and Reference Examples 1 to 1 were evaluated according to the above evaluation method. 8. The evaluation results of Examples 9 to 21, Reference Examples 22 to 23, and Examples 24 to 34 are shown in Table 2 below, and the evaluation results of Comparative Examples 1 to 5 are shown in Table 3 below. In Tables 2 and 3, the stretching ratio "1" indicates that the stretching operation was not performed in that direction.

<< Reference Examples 1 to 3 >>
According to the above evaluation method, only polyethylene terephthalate (PET) film (Cosmoshine 100A4100) was evaluated in Reference Example 1, and polyethylene naphthalate (PEN) film (Teijin DuPont Film Co., Ltd. "Theonex (registered trademark)") in Reference Example 2. Only the polyimide (PI) film (Toray DuPont Co., Ltd. "Capton (registered trademark)") was evaluated in Reference Example 3. The evaluation results are shown in Table 4 below.

The polyimide film of the present invention can be used as a surface protective film, a substrate film for color filters, TFTs, etc., and an insulating protective film. The polyimide fim and the laminate in the present embodiment are optical devices such as a display having a touch panel function, an organic EL lighting, a smartphone, and a tablet terminal; flexible optical devices such as a flexible display, a flexible solar cell, a flexible touch panel, and a folding body. It can be suitably used for smartphones and tablet terminals capable of the above; other flexible devices such as flexible batteries; and products such as organic EL lighting and organic EL displays having curved surfaces.

1 Film 2 Plunger 3 Chuck 4 Rotary table 5 Clamp 6 Bending angle

Claims (8)

A polyimide film containing polyimide as a main component, wherein the difference (ΔN) between the refractive index in the MD direction and the refractive index in the TD direction of the polyimide film is 0.02 or more.
When bent 100,000 times under the conditions of a bending radius of 2 mm, a bending angle of 135 °, and a load of 1.25 kg / m ² , it did not break and no bending marks could be visually confirmed .
The polyimide has the following formula (1):

In the formula (1), A is a divalent organic group, B is a tetravalent organic group, and n is 2 or more. As A in the formula (1), the following formula is used. Includes at least one structure selected from the group consisting of the structure represented by (A-1) and the structure represented by the following formula (A-2).

In the formula (A-2), X is a divalent organic group represented by the following formula (X-1).

Moreover, as B in the above formula (1), a structure represented by the following formula (B-1) is included.

In the formula (B-1), Y is a divalent organic group selected from the group consisting of the following formulas (Y-1) and (Y-2).

Polyimide film. The polyimide film according to claim 1, wherein the yellowness (YI) of the film is 5.0 or less. The polyimide according to claim 1 or 2 , wherein the polyimide includes a structure represented by the formula (A-1) and a structure represented by the following formula (A-5) as A in the formula (1). Polyimide film.

The ratio of the structure represented by the formula (A-1) to the structure represented by the formula (A-5) (structure represented by the formula (A-1) / represented by the formula (A-5)). The polyimide film according to claim 3 , wherein the structure is in the range of 2/8 to 6/4 on a molar basis. The polyimide film according to any one of claims 1 to 4 , wherein B in the formula (1) includes a structure represented by the following formula (B-5).

The polyimide film according to any one of claims 1 to 5 , wherein the Young's modulus in the direction in which the Young's modulus is high among the MD direction and the TD direction of the film is 3.5 GPa or more. The method for producing a polyimide film according to any one of claims 1 to 6 , wherein the method is:
Applying a polyimide varnish containing polyimide and a solvent onto the support,
The solvent is temporarily dried to form a solvent-containing film containing 0.1% by mass to 20% by mass of the solvent.
Stretching the solvent-containing film
Including drying the solvent from the solvent-containing film to less than 0.1% by weight.
A method for producing a polyimide film, wherein in the stretching, the stretching ratio of the solvent-containing film in the MD direction is 1.5 times or more. The method for producing a polyimide film according to any one of claims 1 to 6 , wherein the method is:
Applying a polyimide varnish containing polyimide and a solvent onto the support,
The solvent is dried to form a film containing less than 0.1% by mass of the solvent.
Stretching the film
A method for producing a polyimide film, wherein in the stretching, the stretching ratio of the film in the MD direction is 1.5 times or more.

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