KR20190008766A - Preparation method for transparent plastic film - Google Patents

Abstract

The present invention relates to a preparation method for transparent plastic film, which comprises: reacting alicyclic or aromatic tetracarboxylic acid anhydrides and aromatic dicarboxylic acids or derivatives thereof with an aromatic diamine compound at -30 to 120°C to form a partially imidized poly (amide-imide) copolymer; and coating the partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature above 80. The present invention is to provide the transparent plastic film having high transparency with mechanical properties such as high scratch resistance and hardness.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a transparent plastic film,

The present invention relates to a method for producing a transparent plastic film having a high hardness.

The aromatic polyimide resin is a polymer having mostly an amorphous structure and exhibits excellent heat resistance, chemical resistance, electrical properties, and dimensional stability due to its rigid chain structure. Such a polyimide resin is widely used as an electric / electronic material. However, the polyimide resin has many limitations in its use because it has a dark brown limit due to the formation of a charge transfer complex (CTC) of π electrons present in the imide chain.

A method in which a strong electron attracting group such as a trifluoroalkyl group is introduced in order to solve the above limitation and obtain a colorless transparent polyimide resin, thereby restricting the movement of? Electrons; A method of reducing the formation of CTC by introducing a sulfone (-SO 2 -) group, an ether (-O-) group, or the like into the main chain to form a bent structure; Or introducing an aliphatic cyclic compound to inhibit the formation of resonance structures of? Electrons have been proposed.

However, the polyimide resin according to the above proposals is difficult to exhibit sufficient heat resistance by a bending structure or an aliphatic cyclic compound, and a film produced using the polyimide resin still has a limit to exhibit poor mechanical properties.

On the other hand, in recent years, poly (amide-imide) copolymers having a polyamide unit structure introduced to improve the scratch resistance of polyimide have been developed.

However, the poly (amide-imide) copolymer shows a tendency to easily haze when a film is formed due to its high crystallinity. This haze characteristic of the poly (amide-imide) film is severely expressed as the thickness of the film is thicker and affects the yellow index (YI).

However, copolymerized polyamide-imide films which simultaneously satisfy optical and mechanical properties as well as optical properties required in the market have not yet been provided. Therefore, the development of a copolymer polyamide-imide capable of producing a transparent film having excellent thermal, mechanical and optical properties is required.

The present invention is to provide a transparent plastic film having high transparency with mechanical properties such as high scratch resistance and hardness.

In the present specification, an alicyclic or aromatic tetracarboxylic acid anhydride and an aromatic dicarboxylic acid or a derivative thereof are reacted with an aromatic diamine compound at -30 ° C to 120 ° C to form a partially imidized poly (amide-imide) copolymer ; And coating the partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature of 80 or more.

Hereinafter, a method of manufacturing a transparent plastic film according to a specific embodiment of the present invention will be described in detail.

As described above, according to one embodiment of the present invention, an alicyclic or aromatic tetracarboxylic acid anhydride and an aromatic dicarboxylic acid or a derivative thereof are reacted with an aromatic diamine compound at -30 ° C to 120 ° C to obtain a partially imidized Forming a poly (amide-imide) copolymer; And coating the partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature of 80 or more.

In accordance with the development trend of flexible or foldable devices, it is required to realize a display that is lightweight, unbreakable, and foldable or tangible. In addition, there is a demand for a colorless transparent film having high hardness and good bendability for realizing such a display. Poly (amide-imide) copolymers obtained by copolymerizing an aromatic amide and an aromatic imide as a material for simultaneously satisfying high hardness and repeated bending properties are being developed. However, poly (amide-imide) copolymers obtained by copolymerizing an aromatic amide and an aromatic imide have limitations that are unsuitable for use in the display field due to their inherent hue.

Accordingly, the present inventors have continued research on the production of a high-hardness transparent plastic film using a poly (amide-imide) copolymer, and have succeeded in studying an aromatic aliphatic or aromatic tetracarboxylic acid anhydride and an aromatic dicarboxylic acid, (Amide-imide) copolymer with a diamine compound at -30 캜 to 120 캜 to form a partially imidized poly (amide-imide) copolymer, and the partially imidized poly (amide- imide) copolymer is separately imidized It is possible to produce a transparent plastic film having high transparency as well as mechanical properties such as high scratch resistance and hardness when coated on a base material in an untreated state.

Previously, a copolymer obtained by reacting an aromatic amide and an aromatic imide-forming monomer was subjected to chemical imidization or the like to obtain a poly (amide-imide) copolymer having 90 mol% or more and 100 mol% (Amide-imide) copolymer obtained by the method has low transmittance and high yellowness and is not suitable for use in a display field. In particular, a film having a thickness of 10 占 퐉 or more or 30 占 퐉 or more has haze There was a limitation such as

More specifically, by reacting an alicyclic or aromatic tetracarboxylic acid anhydride and an aromatic dicarboxylic acid or a derivative thereof with an aromatic diamine compound at -30 ° C to 120 ° C, 1 mol% to 20 mol%, or 2 mol% 10 mol% The partially imidized poly (amide-imide) copolymer can be obtained by coating the partially imidized poly (amide-imide) copolymer in the form of a film or sheet and heat treatment to obtain high transparency and low yellow color A transparent plastic film having excellent mechanical properties can be produced.

The partial imidation rate of the poly (amide-imide) copolymer can be confirmed by a conventionally known method, for example, an IR spectrum. For example, the area of the imide specific peaks on the IR spectrum of the poly (amide-imide) copolymer is divided by the area (reference value, all the same value) of the characteristic peak of the aromatic ring as a standard, The imidization ratio can be calculated by relatively comparing the calculated value of completely imidized compound to 100%.

Accordingly, the method of the present invention for producing a transparent plastic film is characterized in that the partially imidized poly (amide-imide) copolymer is not subjected to thermal imidization or chemical imidization before being coated on the substrate .

As is commonly known, the thermal imidization can be performed by heating at a temperature of 150 ° C or higher, and the chemical imidization can be carried out by chemically imidizing the reaction using a dehydration catalyst. have.

Accordingly, in the method of manufacturing the transparent plastic film of the embodiment, before the partially imidized poly (amide-imide) copolymer is formed and coated on the substrate, the partially imidized poly (amide-imide) -Imide) copolymer is not reacted with the dehydration catalyst.

Examples of the dehydration catalyst include aliphatic acid anhydrides such as acetic anhydride and aromatic acid anhydrides. Aliphatic tertiary amines such as triethylamine, aromatic tertiary amines such as dimethylaniline, pyridine, isoquinoline, Picoline,? -Picoline, and 3,5-lutidine; and the like.

On the other hand, in the method of manufacturing the transparent plastic film of the embodiment, the partially imidated poly (amide-imide) copolymer is coated on the substrate and heated to a temperature of 80 or more, (Amide-imide) copolymer can be prepared in the form of a film.

In the method of producing a transparent plastic film of the embodiment, problems such as haze are generated even when the film has a thickness of 30 占 퐉 or more, 50 占 퐉 or more, or 30 占 퐉 to 200 占 퐉 or 50 占 퐉 to 150 占 퐉 .

Accordingly, the step of coating the partially imidized poly (amide-imide) copolymer on the substrate and heating to a temperature of 80 or more may be carried out so that the thickness of the final transparent plastic film is 30 탆 or more, 30 탆 to 1,000 (Amide-imide) copolymer on the substrate such that the partially imidized poly (amide-imide) copolymer is present.

The partially imidized poly (amide-imide) copolymer may be coated on the substrate in a state dissolved in an organic solvent. Thus, coating the partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature of 80 or more can be accomplished by heating the partially imidized poly (amide-imide) And coating a solution of the polymer dissolved in the solvent on the substrate to a thickness of 30 mu m to 3,000 mu m. Examples of the organic solvent usable herein are not limited. For example, N-methylpyrrolidinone (NMP), N, N-dimethylacetamide (DMAc), tetrahydrofuran The solvent is selected from the group consisting of tetrahydrofuran (THF), N, N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), cyclohexane, acetonitrile, ethyl lactate lactate, and mixtures thereof.

The partially imidized poly (amide-imide) copolymer coated on the substrate can be heated to a temperature of from 80 to 300, or from 100 to 250.

Further, the step of coating the partially imidized poly (amide-imide) copolymer on the substrate to a thickness of 30 탆 or more and heating to a temperature of 80 or more may be performed by partially coating And heating the imidated poly (amide-imide) copolymer at a temperature of 200 or more for 1 minute or more.

Thus, as the partially imidized poly (amide-imide) copolymer is coated on the substrate, as it is heated to 80 or higher, 80 to 300, or 100 to 250 or higher, the partially imidized The polymer structure or arrangement within the poly (amide-imide) copolymer can be oriented or vertically oriented in the direction of the coating surface, and the imidization rate of the partially imidized poly (amide-imide) It can be gradually increased.

The material usable as any support for coating a solution containing the poly (amide-imide) copolymer is not particularly limited. For example, various polymer substrates or plastic substrates, glass substrates, metal substrates such as copper Can be used.

In addition, in the step of applying the solution containing the poly (amide-imide) copolymer on the support, a commonly used application method and apparatus can be used without limitation, and examples thereof include a spray method, Roll coating, spin coating, slit coating, extrusion coating, curtain coating, die coating, wire bar coating, or knife coating may be used.

Also, in the step of producing the plastic film, the step of drying the solution containing the applied poly (amide-imide) copolymer may be included. The polymeric resin solution applied on the support may be dried at a temperature lower than the boiling point of the solvent used, for example, from 60 to 200 for 30 seconds to 30 minutes. In the drying step, an arch-type oven or a floating-type oven may be used.

On the other hand, the examples of the alicyclic or aromatic tetracarboxylic acid anhydrides and the aromatic dicarboxylic acid or derivatives thereof used for forming the partially imidized poly (amide-imide) copolymer are respectively limited to the aromatic diamine compounds no.

However, considering the optical properties and mechanical properties of the finally produced plastic film, the alicyclic or aromatic tetracarboxylic acid anhydride has an alicyclic tetravalent functional group having 4 to 10 carbon atoms or an aromatic tetravalent functional group having 6 to 20 carbon atoms And the aromatic dicarboxylic acid or its derivative may include a dicarboxylic acid or a derivative thereof containing an aromatic divalent functional group having 6 to 20 carbon atoms, And a diamine compound containing a functional group of the formula (1).

[Chemical Formula 1]

L ₁ is a single bond, -O-, -CO-, -S-, -SO ₂ -, -C (CH ₃ ) ₂ -, -C (CF ₃ ) ₂ -, -CONH-, _{-COO-, - (CH 2) n1} -, -O (CH 2) and _{n2 O-, -OCH 2 -C (CH} 3) 2 -CH 2 O- , or -OCO (CH _{2) n3} OCO-, wherein n1, n2 and n3 are each an integer of 1 to 10, and R ₁ and R ₂ may be the same or different and each is a perfluoroalkyl group having 1 to 5 carbon atoms, to be.

The poly (amide-imide) copolymer may have a weight average molecular weight of 5,000 to 500,000, or a weight average molecular weight of 30,000 to 300,000. The weight average molecular weight may be a weight average molecular weight in terms of polystyrene measured by a GPC method.

There is no great limitation on the method of synthesizing the poly (amide-imide) copolymer, and synthesizing apparatuses and methods conventionally known using the above-mentioned monomers can be used. For example, after the above-mentioned monomer is dissolved in a commonly known organic solvent or the like, the polymerization reaction can be carried out at a temperature range of -30 ° C to 120 ° C, or -10 to 100 ° C. .

Examples of the organic solvent that can be used in the polymerization process are not limited. Examples of the organic solvent include N-methylpyrrolidinone (NMP), N, N-dimethylacetamide ), Tetrahydrofuran (THF), N, N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), cyclohexane, acetonitrile, ethyl Ethyl lactate, and mixtures thereof, may be used.

The poly (amide-imide) copolymer may be finally obtained through a conventional polymer recovery process such as precipitation, filtration, and drying after the formation of the copolymer.

The prepared transparent plastic film can have excellent weather resistance with high mechanical properties such as high scratch resistance and high transparency. Accordingly, the plastic film formed from the poly (amide-imide) copolymer can be used as a material of various molded articles requiring high scratch resistance and excellent weather resistance together with colorless transparency. For example, the transparent plastic film may be applied to a substrate for a display, a protective film for a display, a touch panel, or the like.

The transparent plastic film may have a pencil hardness of H grade or higher measured according to ASTM D3363 at a thickness of 50 +/- 2 mu m. In addition, the plastic film may have a light transmittance of 85% or more in a wavelength range of 400 nm to 600 nm.

The plastic film can be produced by various methods and apparatuses for producing various plastic films, which are commonly known, except that the above-mentioned poly (amide-imide) copolymer is used. For example, the plastic film can be obtained by coating a solution containing the poly (amide-imide) copolymer on an arbitrary support to form a film, and drying the film from the film by evaporating the solvent. The stretching and heat treatment for the plastic film may be performed.

According to the present invention, it is possible to provide a poly (amide-imide) copolymer and a plastic film containing the poly (amide-imide) copolymer having mechanical properties such as high scratch resistance and high weatherability with high transparency.

The invention will be described in more detail in the following examples. However, the following examples are illustrative of the present invention, and the present invention is not limited by the following examples.

[ Example  And Comparative Example : Poly (Amide-imide) copolymer and production of a plastic film]

Example 1

(One) Poly (Amide-imide) Copolymer Synthesis

The reactor was charged with 138 g of dimethylacetamide (DMAC) while slowly bubbling nitrogen into a 0.5 L reactor charged with a stirrer, a nitrogen injector, a dropping funnel and a temperature controller. The temperature of the reactor was adjusted to 25, bis (trifluoromethyl) -4,4-diaminobiphenyl) 11.973 (0.037 mol) was kept at 25 in the charged and dissolved state.

3.3 g (0.011 mol) of BPDA (3,3 ', 4,4'-biphenyltetracarboxylic dianhydride) was added to the reactor, followed by dissolving and reacting for 1 hour. The temperature of the reaction solution was maintained at 25 ° C.

After the temperature of the reaction solution was cooled to -10, 5.313 g (0.026 mol) of terephthaloyl chloride (TPC) was added and stirred to obtain a polymer solution having a solid content of 13 wt%. DMAC was added to the polymer solution to dilute the solution to a solid content of 5% or less and precipitate it with 10 L of methanol. The precipitated solids were filtered and dried in a vacuum of 100 to 10 torr for 6 hours to obtain a copolymerized polyamide- I got an imide. At this time, the imidization ratio of the copolymerized polyamide-imide is about 5 mol%.

The imidization rate of the copolymerized polyamide-imide is measured and calculated as follows (the imidization rate is calculated in the same manner in the following examples and comparative examples): Fourier transform infrared spectroscopy (FT-IR) poly (amide-imide) ^- the area of the (1370 cm ¹⁾ measuring the IR spectrum of the copolymer, and the carbonyl group of the imide ring of the characteristic (-C = O) peak (1720 cm -1) and the CN stretch characteristic peak (Calculated value 1) by the area of the absorption peak near 1500 cm ^-1 representing the aromatic moiety of the internal standard peak. Then, the imidization rate was calculated by a relative comparison of the calculated values 1 and 2 by setting the calculated value for the fully cured poly (amide-imide) copolymer to 100% (calculated value 2).

(2) Production of transparent plastic film

The solid poly (amide-imide) copolymer was dissolved in dimethylacetamide to have a solid content of 15% by weight, and then coated on a glass plate to a final thickness of 50 탆.

After the glass plate was coated, the film coated with the poly (amide-imide) copolymer solution was fixed on a glass plate and placed in a curing oven in which nitrogen was purged, (250/30 min) for 2 hours to 250, then slowly cooled and separated from the glass plate to obtain a transparent polyamideimide film (thickness 50 탆).

Example 2

A poly (amide-imide) copolymer and a plastic film were prepared in the same manner as in Example 1, except that the amounts of BPDA and TPC were varied as shown in Table 1 below.

Comparative Example  One

The reactor was charged with 138 g of dimethylacetamide (DMAC) while slowly bubbling nitrogen into a 0.5 L reactor charged with a stirrer, a nitrogen injector, a dropping funnel and a temperature controller. The temperature of the reactor was adjusted to 25, bis (trifluoromethyl) -4,4-diaminobiphenyl) 11.973 (0.037 mol) was kept at 25 in the charged and dissolved state.

3.3 g (0.011 mol) of BPDA (3,3 ', 4,4'-biphenyltetracarboxylic dianhydride) was added to the reactor, followed by dissolving and reacting for 1 hour. The temperature of the reaction solution was maintained at 25 ° C.

After the temperature of the reaction solution was cooled to -10, 5.313 g (0.026 mol) of terephthaloyl chloride (TPC) was added and stirred to obtain a polymer solution having a solid content of 13 wt%.

1.77 g of pyridine and 2.29 g of acetic anhydride were added to a polymer solution having a solid concentration of 13% by weight, and the mixture was stirred for 30 minutes, and further stirred at 70 for 1 hour and then cooled to room temperature.

DMAC was then added to the polymer solution to dilute to a solid content of 5% or less and precipitate it with 10 L of methanol. The precipitated solids were filtered and dried at 100 in vacuum for 6 hours to obtain a copolymerized polyamide-imide . At this time, the imidization ratio of the copolymerized polyamide-imide is about 95 mol%.

(2) Production of transparent plastic film

The solid poly (amide-imide) copolymer was dissolved in dimethylacetamide to have a solid content of 15% by weight, and then coated on a glass plate to a final thickness of 335 탆.

After the glass plate was coated, the film coated with the poly (amide-imide) copolymer solution was fixed on a glass plate and placed in a curing oven in which nitrogen was purged, (250/30 min) for 2 hours to 250, then slowly cooled and separated from the glass plate to obtain a transparent polyamideimide film (thickness 50 탆).

Comparative Example 2

A poly (amide-imide) copolymer and a plastic film were prepared in the same manner as in Comparative Example 1, except that the amounts of BPDA and TPC were varied as shown in Table 1 below.

A poly (amide-imide) copolymer and a plastic film were prepared in the same manner as in Example 1, except that the content of the monomers used was changed as shown in Table 1 below.

Composition ratio (moles basis of TFDB) Dehydration catalyst
Whether or not to use TFDB BPDA TPC Example 1 1.0 0.3 0.7 X Example 2 1.0 0.2 0.8 X Comparative Example 1 1.0 0.3 0.7 O Comparative Example 2 1.0 0.2 0.8 O

[ Experimental Example : Measurement of physical properties of plastic film]

(1) Measurement of permeability (T)

The total light transmittance of the plastic films obtained in the above Examples and Comparative Examples was measured using a UV-VIS-NIR Spectrophotometer (SolidSpec-3700, SHIMADZU), and the transmittance values of the 550 nm wavelength visible light are shown in Table 2 .

(2) Yellow Index; Y.I. .)

(Y.I.) of the plastic films obtained in the above Examples and Comparative Examples were measured according to the measurement method of ASTM D1925 using UV-2600 UV-Vis Spectrometer (SHIMADZU).

(3) Pencil Hardness

Pencil Hardness Tester The pencil hardness of the plastic film obtained in the above Examples and Comparative Examples was measured according to the measurement method of ASTM D3363. Specifically, when a pencil having various hardnesses is fixed to the tester and scratches on the film, the degree of occurrence of scratches on the film is visually observed with a microscope or microscope. When the scratches are not scratched by more than 70% of the total number of scratches, The corresponding value was evaluated by the pencil hardness of the film.

(4) Hayes  Measure

The haze of the film was specified using a spectrophotometer (instrument: COH-400) for the plastic film obtained in the above Examples and Comparative Examples. As a result of measurement and evaluation, '' when the haze of the film is 1 or more and 'X' when it is 1 or less.

(5) Flexibility  Measure

The plastic films obtained in the Examples and Comparative Examples were evaluated for the bending strength of the films by using an MIT type folding endurance tester. Specifically, a specimen (1 cm x 7 cm) of the film was loaded into an impact strength tester and tilted at an angle of 135 ° at a speed of 175 rpm on the left and right sides of the specimen, with a radius of curvature of 0.8 mm and a load of 250 g The number of reciprocating bending cycles was measured

HAZE
The presence or absence Yellow Index (Y.I.) Permeability
(550 nm) Pencil hardness Flexibility
(MIT, times) Example 1 X 2.63 89.3 4H 18500 Example 2 X 2.91 89 3H 16500 Comparative Example 1 16.5 45 H 6600 Comparative Example 2 15.8 37 F 4700

As can be seen from the above Table 2, the films prepared using the poly (amide-imide) copolymers obtained in Examples 1 and 2 had a high surface hardness of 3H or more and a high surface hardness of 15,000 or more Or more, and haze is not exhibited even when it is manufactured to a thickness of 50 mu m or more.

On the other hand, in the case of Comparative Examples 1 and 2 using a dehydration catalyst in the manufacturing process, haze was generated at a thickness of 50 탆 or more as well as a high yellow index was exhibited. Not only the transmittance and surface hardness were remarkably low, It was confirmed that it stayed at a considerably low level.

Claims (12)

(Amide-imide) copolymer by reacting an alicyclic or aromatic tetracarboxylic acid anhydride and an aromatic dicarboxylic acid or derivative thereof with an aromatic diamine compound at -30 ° C to 120 ° C to form a partially imidized poly ; And
Coating the partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature of 80 or more.
The method according to claim 1,
Wherein the partially imidized poly (amide-imide) copolymer is imidized at 1 mole% to 20 mole%.
The method according to claim 1,
Characterized in that the partially imidized poly (amide-imide) copolymer does not undergo thermal imidization or chemical imidization prior to coating on the substrate.
The method according to claim 1,
Before forming the partially imidized poly (amide-imide) copolymer and coating it on the substrate,
Characterized in that the partially imidized poly (amide-imide) copolymer is not reacted with the dehydration catalyst.
5. The method of claim 4,
Wherein the dehydration catalyst comprises at least one selected from the group consisting of an aliphatic acid anhydride, an aromatic acid anhydride, an aromatic tertiary amine, and a heterocyclic tertiary amine.
The method according to claim 1,
Coating said partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature above 80,
And coating the partially imidated poly (amide-imide) copolymer on the substrate such that the thickness of the finally produced transparent plastic film is 30 占 퐉 or more.
The method according to claim 1,
Coating said partially imidized poly (amide-imide) copolymer on a substrate and heating to a temperature above 80,
And coating a polymer solution obtained by dissolving the partially imidized poly (amide-imide) copolymer in an organic solvent on the substrate to a thickness of 30 mu m to 3,000 mu m.
The method according to claim 1,
Wherein the partially imidized poly (amide-imide) copolymer coated on the substrate is heated to a temperature of 80 to 300. < Desc / Clms Page number 20 >
The method according to claim 1,
Coating said partially imidized poly (amide-imide) copolymer to a thickness of at least 30 micrometers on a substrate and heating to a temperature above 80,
And heating the partially imidated poly (amide-imide) copolymer coated on the substrate to a thickness of 30 占 퐉 or more at a temperature of 200 or more for 1 minute or more.
The method according to claim 1,
The alicyclic or aromatic tetracarboxylic acid anhydride includes a tetracarboxylic acid having an alicyclic tetravalent functional group having 4 to 10 carbon atoms or an aromatic tetravalent functional group having 6 to 20 carbon atoms,
Wherein the aromatic dicarboxylic acid or derivative thereof comprises a dicarboxylic acid or derivative thereof containing an aromatic divalent functional group having 6 to 20 carbon atoms.
The method according to claim 1,
Wherein the aromatic diamine compound comprises a diamine compound having a functional group represented by the following Formula 1:
[Chemical Formula 1]

In Formula 1,
L ₁ is a single bond, -O-, -CO-, -S-, -SO ₂ -, -C (CH ₃ ) ₂ -, -C (CF ₃ ) ₂ -, -CONH-, -COO-, _{(CH 2) n1 -, -O} (CH 2) n2 O-, -OCH 2 -C (CH 3) 2 -CH 2 and O-, or -OCO (CH _{2) n3} OCO-, wherein n1, n2 and n3 Are each an integer of 1 to 10,
R ₁ and R _{2, which} may be the same or different, each represent a perfluoro alkyl having 1 to 5 carbon atoms, to be.
The method according to claim 1,
The transparent plastic film has a pencil hardness of H grade or higher measured according to ASTM D3363 at a thickness of 50 +/- 2 mu m,
And has a light transmittance of 85% or more in a wavelength region of 400 nm to 600 nm.