Optical Film

Abstract

The present invention relates to an optical film having a glass transition temperature of 165° C. or higher, an optical transmittance at 350 nm of 10% or less, an in-plane retardation of 30 nm or less, and a retardation in a thickness direction of 100 nm or less.

Claims

1. An optical film having a glass transition temperature of 165° C. or higher, an optical transmittance at 350 nm of 10% or less, an in-plane retardation of 30 nm or less, and a retardation in a thickness direction of 100 nm or less.

2. The optical film according to claim 1, wherein the glass transition temperature is higher than 180° C.

3. The optical film according to claim 1, wherein the optical film has an optical transmittance at 500 nm of 90% or more.

4. The optical film according to claim 1, wherein the optical film has a tensile strength of more than 86 MPa.

5. The optical film according to claim 1, wherein the optical film has a film thickness of 10 to 100 μm.

6. The optical film according to claim 1, wherein the optical film has a solvent content of 3.0% by mass or less based on a mass of the optical film.

7. The optical film according to claim 1, comprising a polyimide-based resin having a constitutional unit represented by Formula (1) ##STR00011## wherein X represents a divalent organic group, Y represents a tetravalent organic group, and * represents a bonding hand.

8. The optical film according to claim 7, wherein the constitutional unit represented by Formula (1) comprises a divalent aliphatic group as X.

9. The optical film according to claim 7, wherein the constitutional unit represented by Formula (1) comprises, as Y, a structure represented by Formula (2) ##STR00012## wherein R.sup.2 to R.sup.7 each independently represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an aryl group having 6 to 12 carbon atoms, the hydrogen atoms contained in R.sup.2 to R.sup.7 are each independently optionally substituted by a halogen atom, V represents a single bond, —O—, —CH.sub.2—, —CH.sub.2—CH.sub.2—, —CH(CH.sub.3)—, —C(CH.sub.3).sub.2—, —C(CF.sub.3).sub.2—, —SO.sub.2—, —S—, —CO—, or —N(R.sup.8)—, R.sup.8 represents a hydrogen atom or a monovalent hydrocarbon group having 1 to 12 carbon atoms which is optionally substituted with a halogen atom, and * represents a bonding hand.

10. The optical film according to claim 7, wherein the polyimide-based resin contains a fluorine atom.

11. The optical film according to claim 7, wherein the polyimide-based resin has a weight average molecular weight (Mw) of more than 250,000.

12. A flexible display device comprising the optical film according to claim 1.

13. The flexible display device according to claim 12, further comprising a polarizer.

14. The flexible display device according to claim 12, further comprising a touch sensor.

Description

EXAMPLES

[0109] Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples. First, measurement methods will be described.

[0110] <Measurement of Glass Transition Temperature>

[0111] The glass transition temperature (Tg) by DSC (differential scanning calorimetry) of each of the optical films obtained in Examples and Comparative Examples was measured with a thermal analyzer (“DSC Q200”, manufactured by TA Instruments). The measurement conditions were as follows: measurement sample amount: 5 mg, temperature range: from room temperature to 400° C., and temperature raising rate: 10° C./min.

[0112] <Measurement of Retardation>

[0113] Rth and R0 of each of the optical films obtained in Examples and Comparative Examples were measured using a retardation measuring device (trade name: KOBRA) manufactured by Oji Scientific Instruments Co., Ltd. The retardation in the thickness direction Rth of the optical film is calculated by equation (A), where the refractive index in one direction in the film plane is Nx, the refractive index in a direction orthogonal to Nx is Ny, the refractive index in the thickness direction of the film is Nz, and the thickness of the optical film is d (nm). Here, Nx is a refractive index in the slow axis direction, Ny is a refractive index in the fast axis direction, and these satisfy Nx>Ny.

Rth={(Nx+Ny)/2−Nz}×d(nm)  (A)

[0114] The in-plane retardation R0 of an optical film is calculated by equation (B) when the refractive index in one direction in the film plane is Nx, the refractive index in a direction orthogonal to Nx is Ny, and the thickness of the optical film is d (nm). Here, Nx is a refractive index in the slow axis direction, Ny is a refractive index in the fast axis direction, and these satisfy Nx>Ny.

R0=(Nx−Ny)×d(nm)  (B)

[0115] <Optical Transmittances at 350 nm and 500 nm>

[0116] The optical transmittances at 350 nm and 500 nm in each of the optical films obtained in Examples and Comparative Examples were obtained by measuring the optical transmittance of light of 200 to 800 nm using a UV-Visible/NIR spectrophotometer V-670 manufactured by JASCO Corporation.

[0117] <Measurement of Tensile Strength>

[0118] The tensile strength of each of the optical films obtained in Examples and Comparative Examples was measured as follows using a precision universal tester (“Autograph AG-IS”, manufactured by Shimadzu Corporation).

[0119] The optical film was cut into a width of 10 mm and a length of 100 mm to prepare a strip-shaped test piece. Next, using the precision universal tester, a tensile test was carried out under the conditions of a chuck distance of 50 mm and a tensile speed of 20 mm/min, and the tensile strength of the optical film was measured.

[0120] <Bending Resistance Test>

[0121] In accordance with ASTM D 2176-16, the number of folds of each of the optical films obtained in Examples and Comparative Examples was determined as follows. The optical film was cut into a strip shape having a width of 10 mm and a length of 120 mm using a dumbbell cutter. The cut optical film was set in the main body of an MIT type folding endurance tester (“Model 0530”, manufactured by Toyo Seiki Seisaku-sho, Ltd.), and under the conditions of a test speed of 175 cpm, a folding angle of 135°, a load of 0.75 kf, and a folding radius R of a folding clamp=1 mm, the number of reciprocated folds in the back and front directions until the optical film was broken was measured, and this was defined as the number of folds.

[0122] <Method for Measuring Residual Solvent Amount>

[0123] (Thermogravimetry-Differential Thermal Analysis (TG-DTA) Measurement) The residual solvent amount of each of the optical films obtained in Examples and Comparative Examples was measured using a TG-DTA measuring apparatus (“TG/DTA 6300”, manufactured by Hitachi High-Tech Science Corporation).

[0124] About 20 mg of sample was obtained from the optical film. The sample was heated from room temperature to 120° C. at a temperature raising rate of 10° C./min and held at 120° C. for 5 minutes, and then the mass change of the sample was measured while raising the temperature (heating) to 400° C. at a temperature raising rate of 10° C./min.

[0125] From the results of TG-DTA measurement, the mass loss ratio S (% by mass) from 120° C. to 250° C. was calculated according to the following equation (1).

S(% by mass)=100−(W1/W0)×100  (1)

[In equation (1), W0 is the mass of the sample after holding at 120° C. for 5 minutes, and W1 is the mass of the sample at 250° C.].

[0126] The mass loss ratio S calculated was defined as a residual solvent amount S (% by mass) in the optical film.

[0127] <Thickness>

[0128] The thicknesses of each of the optical films obtained in Examples and Comparative Examples was measured three times using a contact type digital thickness meter (manufactured by Mitutoyo Corporation), and the average value of the values measured three times was taken as the thickness of the optical film.

[0129] <Viscosity>

[0130] The viscosity of each of the varnishes obtained in Examples and Comparative Examples was measured using an E-type viscometer (“HBDV-II+P CP” manufactured by Brookfield) under the conditions of 25° C. and a rotation speed of 3 rpm using 0.6 cc of the varnish as a sample.

[0131] <Weight Average Molecular Weight (Mw)>

[0132] The weight average molecular weight (Mw) of each of the polyimide-based resins obtained in Synthesis Examples was measured under the following conditions using GPC.

[0133] (GPC Conditions)

Instrument: Shimadzu LC-20A

[0134] Column: TSKgel GMHHR-M (mix column, exclusion limit molecular weight: 4,000,000)
Guard column: TSKgel guardcolumn HHR-H
Mobile phase: N-methyl-2-pyrrolidinone (NMP) with addition of 10 mM LiBr * NMP used was of a grade for HPLC, and LiBr used was an extra pure reagent (anhydride).
Flow rate: 1 mL/min
Measurement time: 20 min
Column oven: 40° C.

Detection: UV 275 nm

[0135] Washing solvent: NMP
Sample concentration: 1 mg/mL (*20 wt % reaction mass was analyzed with dilution to 5 mg/mL with a mobile phase.)
Molecular weight calibration: Polystyrene standards manufactured by Polymer Laboratories Ltd. (17 molecular weights ranging from a molecular weight of 500 to a molecular weight of 4,000,000)

[0136] <Imidization Rate>

[0137] The imidization rate of the polyimide-based resins obtained in Synthesis Examples was measured under the following conditions using NMR.

[0138] (NMR Conditions)

[0139] 10 mg of a polyimide-based resin was weighed, 0.75 ml of deuterated DMSO was added thereto, and then the resulting mixture was heated at 120° C. for 20 minutes to dissolve the resin. The solution was transferred to an NMR tube, and 1H NMR measurement was carried out at 100° C. using an AV600 instrument manufactured by Bruker Corporation. Protons derived from an imide group and protons derived from an amide group were assigned from a 1H NMR spectrum, and the imidization rate was determined using the following equation.

Imidization rate={imide group integration ratio/(imide group integration ratio+amide group integration ratio)}×100

Synthesis Example 1

[0140] A polyimide-based resin (6FDA-DAB) composed of a constitutional unit derived from 6FDA and a constitutional unit derived from 1,4-DAB was produced as follows by the method described in WO 2019/156717 A.

[0141] Under a nitrogen gas atmosphere, 178.78 kg of m-cresol (manufactured by Honshu Chemical Industry Co., Ltd.), 7.940 kg of 1,4-DAB (manufactured by ThermoFisher), and 40.120 kg of 6FDA (manufactured by Hakko Tsusho Co., Ltd.) were added to a reaction vessel equipped with a stirring blade. Next, 3.428 kg of isoquinoline (manufactured by FUJIFILM Wako Pure Chemical Corporation) was added thereto, then the temperature was raised to 158° C., and the mixture was stirred for 5 hours. Thereafter, 74.49 kg of m-cresol was added thereto, and the resulting reaction liquid was cooled to 50° C. While stirring, 118.97 kg of Solmix AP-1 (manufactured by Japan Alcohol Trading Co., Ltd.) was added, and 356.91 kg of Solmix AP-1 was further added, then the mixture was filtered. The filtered precipitate was washed with Solmix AP-1 (70.62 kg), further subjected to suspension filtration with Solmix AP-1 (141.23 kg) four times, and the precipitate was dried in a dryer at 70° C. for 96 hours, affording 39.97 kg of a polyimide-based resin. The polyimide-based resin produced had a weight average molecular weight (Mw) of 252,000, and an imidization rate of 99.9%.

Synthesis Example 2

[0142] A polyimide-based resin was produced in the same manner as in Synthesis Example 1 except that the reaction conditions and the like were changed. The resulting polyimide-based resin had a weight average molecular weight (Mw) of 317,000, and an imidization rate of 99.9%.

Synthesis Example 3

[0143] Under a nitrogen gas atmosphere, 178.56 kg of m-cresol (Honshu Chemical Industry Co., Ltd.), 7.961 kg of 1,4-DAB (ThermoFisher), and 40.112 kg of 6FDA (Hakko Tsusho Co., Ltd.) were added to a reaction vessel equipped with a stirring blade. Next, 3.424 kg of isoquinoline (manufactured by FUJIFILM Wako Pure Chemical Corporation) was added thereto, then the temperature was raised to 130° C., and the mixture was stirred for 8 hours. Thereafter, the resulting reaction liquid was cooled to 50° C. While stirring, 93.38 kg of methanol (Sumitomo Chemical Co., Ltd.) was added thereto, and 242.13 kg of methanol was further added thereto, followed by filtration. The filtered precipitate was washed with methanol (70.62 kg), further subjected to suspension filtration with methanol (141.23 kg) four times, and the precipitate was dried in a dryer at 70° C. for 96 hours, affording 41.70 kg of a polyimide-based resin. The polyimide-based resin produced had a weight average molecular weight (Mw) of 396,000, and an imidization rate of 100.0%.

Example 1

[0144] The polyimide-based resin obtained in Synthesis Example 1 was dissolved in cyclohexanone such that the solid content concentration was 15% by mass, and 2 phr of Sumisorb 340 was added as an ultraviolet absorber (UVA) to prepare a varnish. The viscosity of the varnish was 26.0 Pa.Math.s. Next, the resulting varnish was applied to a glass substrate, heated at 140° C. for 10 minutes, further heated at 200° C. for 30 minutes, and peeled off from the glass substrate, thereby affording an optical film having a thickness of 25 μm. The residual solvent amount of the resulting optical film was 1.2% by mass.

Example 2

[0145] The polyimide-based resin obtained in Synthesis Example 2 was dissolved in cyclohexanone such that the solid content concentration was 13% by mass, and 2 phr of Sumisorb 340 was added as an ultraviolet absorber (UVA) to prepare a varnish. The viscosity of the varnish was 22.0 Pa.Math.s. Next, the resulting varnish was applied to a glass substrate, heated at 140° C. for 10 minutes, further heated at 200° C. for 30 minutes, and peeled off from the glass substrate, thereby affording an optical film having a thickness of 25 μm. The residual solvent amount of the resulting optical film was 1.4% by mass.

Example 3

[0146] The polyimide-based resin obtained in Synthesis Example 3 was dissolved in cyclohexanone such that the solid content concentration was 11% by mass, and 2 phr of Sumisorb 340 was added as an ultraviolet absorber (UVA) to prepare a varnish. The viscosity of the varnish was 17.5 Pa.Math.s. Next, the resulting varnish was applied to a glass substrate, heated at 140° C. for 10 minutes, further heated at 200° C. for 30 minutes, and peeled off from the glass substrate, thereby affording an optical film having a thickness of 25 μm. The residual solvent amount of the resulting optical film was 1.5% by mass.

Comparative Example 1

[0147] A cyclic olefin copolymer (COC) film (“ZF16”, manufactured by Zeon Corporation) was used.

Comparative Example 2

[0148] The polyimide-based resin obtained in Synthesis Example 1 was dissolved in cyclohexanone such that the solid content concentration was 15% by mass, and 2 phr of Sumisorb 340 was added as an ultraviolet absorber (UVA) to prepare a varnish. The viscosity of the varnish was 26 Pa.Math.s. Next, the resulting varnish was applied to a glass substrate, heated at 140° C. for 10 minutes, further heated at 180° C. for 20 minutes, and peeled off from the glass substrate, thereby affording an optical film having a thickness of 25 μm. The residual solvent amount of the resulting optical film was 4.9% by mass.

[0149] The results of measuring the glass transition temperature (Tg), the in-plane retardation (nm), the retardation in the thickness direction (nm), the optical transmittance (%) at 350 nm, the optical transmittance (%) at 500 nm, the tensile strength (MPa), and the number of folds of the optical films obtained in Examples and Comparative Examples are shown in Table 1.

TABLE-US-00001 TABLE 1 Retardation In-plane in thickness Optical Optical Tensile retardation direction transmittance transmittance strength Number of Resin Tg (° C.) (nm) (nm) at 350 nm at 500 nm (MPa) folds (folds) Example 1 6FDA- 181 5.9 55.6 1.1 90.5 90 More than DAB 200K Example 2 6FDA- 180 6.6 54.6 1.0 90.4 92 More than DAB 200K Example 3 6FDA- 180 18.9 59.9 1.1 90.4 90 More than DAB 200K Comparative COC 163 1.6 5.7 88.5 89.3 60 0.4K Example 1 Comparative 6FDA- 145 6.1 59.5 1.8 90.3 84 More than Example 2 DAB 200K

[0150] As shown in Table 1, it was confirmed that the optical films obtained in Examples 1 to 3 had a remarkably low optical transmittance at 350 nm and a remarkably high Tg as compared with Comparative Example 1. In addition, it was confirmed that the optical films obtained in Examples 1 and 2 had a low retardation in a thickness direction and a remarkably high Tg as compared with Comparative Example 2. Furthermore, it was confirmed that the optical film obtained in Example 3 had a significantly high Tg as compared with Comparative Example 2. Therefore, it was found that the optical films obtained in Examples 1 to 3 had a low retardation, high heat resistance, and high UV-blocking property.

[0151] Furthermore, it was confirmed that the optical films obtained in Example 1 to 3 were higher in optical transmittance at 500 nm, tensile strength, and the number of folds than the optical film of Comparative Example 1, and higher in optical transmittance at 500 nm and tensile strength than the optical film of Comparative Example 2. Accordingly, it was found that the optical films obtained in Examples 1 to 3 were also superior in transparency, tensile strength, and bending resistance.