Optical plastic films, polarizing plates and image display devices

Abstract

Provided is an optical plastic film having improved scratch resistance against both hard and soft objects. Provided is an optical plastic film having a first surface and a second surface located on the opposite side of the first surface, wherein in the flow direction and width direction of the plastic film, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle in the thickness direction satisfy a prescribed relationship.

Description

Technical Field

[0001] The present invention relates to an optical plastic film, a polarizing plate and an image display device. Background Art

[0002] Various optical plastic films are often used in optical components such as image display devices. For example, in an image display device having a polarizing plate on a display element, a plastic film (polarizing element protective film) is used to protect a polarizing element constituting the polarizing plate.

[0003] Plastic films for image display devices, such as polarizer protective films, preferably have excellent mechanical strength. Therefore, as the plastic film for image display devices, it is preferable to use a stretched plastic film.

[0004] In addition, stretched plastic films are excellent in abrasion resistance. Therefore, as in Patent Documents 1 and 2, stretched plastic films having an improved elastic modulus have been proposed.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2019-8293 (Claim 4)

[0008] Patent Document 2: Japanese Patent Application No. 2018-538572 (Claim 7) Summary of the invention

[0009] Problems to be solved by the invention

[0010] As described in Patent Documents 1 and 2, a stretched plastic film having a high elastic modulus can improve scratch resistance when scratched by a hard object such as a pencil or a stylus pen. However, a stretched plastic film having a high elastic modulus generally generates scratches earlier than a stretched plastic film having a low elastic modulus when repeatedly rubbed by a soft object such as a cloth.

[0011] On the other hand, although stretched plastic films with low elastic modulus have good scratch resistance when repeatedly rubbed with soft objects such as cloth, they will be scratched immediately when scratched with hard objects such as pencils and styluses.

[0012] As described above, improving the scratch resistance of a plastic film against both hard and soft objects is a trade-off.

[0013] An object of the present invention is to provide an optical plastic film capable of improving scratch resistance against both hard and soft objects, a polarizing plate, and an image display device.

[0014] Means for solving problems

[0015] The present invention provides the following optical plastic film, polarizing plate and image display device.

[0016] [1] An optical plastic film having a first surface and a second surface located on the opposite side of the first surface, and satisfying the following conditions.

[0017] <Condition 1>

[0018] Regarding the indentation hardness of the cross section in the flow direction of the plastic film, the softer hardness of the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side is defined as MD1, and the indentation hardness of the cross section in the middle of the thickness direction is defined as MD2. In addition, regarding the indentation hardness of the cross section in the width direction of the plastic film, the softer hardness of the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side is defined as TD1, and the indentation hardness of the cross section in the middle of the thickness direction is defined as TD2. Under this premise, MD2 / MD1 and TD2 / TD1 are both greater than 1.01 and less than 1.30.

[0019] [2] The optical plastic film according to the above [1], further satisfying the following condition 2.

[0020] <Condition 2>

[0021] When the larger of the product of MD1 and MD2 and the product of TD1 and TD2 is defined as X1 and the smaller one is defined as X2, X1 / X2 is 1.30 or less.

[0022] [3] The optical plastic film according to [1] or [2], further satisfying the following condition 3.

[0023] <Condition 3>

[0024] A sample of 50 mm in the flow direction and 50 mm in the width direction is cut from the plastic film. The directions of the slow axis of the sample are measured at four locations 10 mm from the four corners to the center and at the center of the sample, a total of five locations. When the angles formed by the direction of the slow axis of each measurement location with any one of the flow direction and the width direction of the sample are defined as D1, D2, D3, D4, and D5, respectively, the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0 degrees or more.

[0025] [4] The optical plastic film according to any one of [1] to [3] above, further satisfying the following condition 4.

[0026] <Condition 4>

[0027] A sample with a size of 50 mm in the flow direction and 50 mm in the width direction was cut from the plastic film. The in-plane phase difference of the sample was measured at four locations 10 mm from the four corners to the center and the center of the sample, a total of five locations. When the in-plane phase difference of the five locations is defined as Re1, Re2, Re3, Re4, and Re5, the average of Re1 to Re5 is less than 500 nm.

[0028] [5] The optical plastic film according to any one of [1] to [4] above, further satisfying the following condition 5.

[0029] <Condition 5>

[0030] A sample with a size of 50 mm in the flow direction and 50 mm in the width direction was cut from the plastic film. The phase difference in the thickness direction of the sample was measured at four locations 10 mm from the four corners to the center and at the center of the sample, a total of five locations. When the phase difference in the thickness direction of the five locations was defined as Rth1, Rth2, Rth3, Rth4, and Rth5, the average of Rth1 to Rth5 was 2000 nm or more.

[0031] [6] A polarizing plate comprising: a polarizing element; a transparent protective plate A disposed on one side of the polarizing element; and a transparent protective plate B disposed on the other side of the polarizing element, wherein at least one of the transparent protective plate A and the transparent protective plate B is an optical plastic film as described in any one of [1] to [5] above.

[0032] [7] An image display device comprising: a display element; and a plastic film disposed on a light emitting surface side of the display element, wherein the plastic film is the optical plastic film according to any one of [1] to [5] above.

[0033] [8] The image display device according to [7] above, comprising a polarizing element between the display element and the plastic film.

[0034] Effects of the Invention

[0035] The optical plastic film, polarizing plate and image display device of the present invention can improve the scratch resistance against both hard and soft objects. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a three-dimensional diagram of a sample used to measure the cross-sectional hardness of a plastic film.

[0037] Figure 2 It is a cross-sectional view for explaining the measurement positions of the cross-sectional hardness on the first surface side and the cross-sectional hardness on the second surface side under conditions 1 and 2.

[0038] Figure 3 It is a top view for explaining the measurement positions of five locations under conditions 3 to 6.

[0039] Figure 4 This is a cross-sectional view showing one embodiment of the image display device of the present invention.

[0040] Figure 5 It is a cross-sectional view showing another embodiment of the image display device of the present invention.

[0041] Figure 6 It is a figure which shows schematically the state of a continuous folding test. DETAILED DESCRIPTION

[0042] Hereinafter, embodiments of the present invention will be described.

[0043] [Plastic film for optical use]

[0044] The optical plastic film of the present invention has a first surface and a second surface located on the opposite side to the first surface, and satisfies the following conditions.

[0045] <Condition 1>

[0046] Regarding the indentation hardness of the cross section in the flow direction of the plastic film, the softer hardness of the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side is defined as MD1, and the indentation hardness of the cross section in the middle of the thickness direction is defined as MD2. In addition, regarding the indentation hardness of the cross section in the width direction of the plastic film, the softer hardness of the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side is defined as TD1, and the indentation hardness of the cross section in the middle of the thickness direction is defined as TD2. Under this premise, MD2 / MD1 and TD2 / TD1 are both greater than 1.01 and less than 1.30.

[0047] <Measurement of Conditions 1 and 2>

[0048] Conditions 1 and 2 define the indentation hardness of a cross section in the flow direction of the plastic film and the indentation hardness of a cross section in the width direction of the plastic film.

[0049] Regarding Condition 1 and Condition 2, in order to measure the cross-sectional hardness of a plastic film, it is first necessary to prepare a sample for measurement. The sample is prepared, for example, by the following steps (A1) to (A2).

[0050] (A1) Two cut samples were prepared by cutting an optical plastic film into a size of 2 mm in the flow direction and 10 mm in the width direction. Then, two cut samples S were prepared. Figure 1The embedded sample shown is embedded with resin R. The embedding resin is preferably epoxy resin.

[0051] The embedded sample can be obtained, for example, as follows: after the cut sample is placed in a silicon embedding plate (silicon capsule), the embedding resin is poured, and after the embedding resin is cured (in the case of the epoxy resin manufactured by Struers as exemplified below, it is preferably placed at room temperature for 12 hours for curing), the cut sample and the embedding resin that embeds the cut sample are taken out of the silicon embedding plate (silicon capsule), thereby obtaining the embedded sample. The silicon embedding plate (silicon capsule) can be, for example, a silicon embedding plate (silicon capsule) manufactured by DOSAKA EM. The embedding epoxy resin can be, for example, a substance obtained by mixing the trade name "EpoFix" manufactured by Struers with the trade name "EpoFix Curing Agent" manufactured by the same company in a ratio of 10:1.2. It should be noted that the two cut samples were cut from a close area (within an area of ​​50 mm×50 mm). In addition, the dimensions of the two cut samples are 2 mm±0.2 mm in the flow direction×10 mm±1 mm in the width direction.

[0052] (A2) One embedded sample is cut vertically along the flow direction with a diamond knife to produce a sample for measuring the indentation hardness of the cross section in the flow direction in which the cross section in the flow direction is exposed. Another embedded sample is cut vertically along the width direction with a diamond knife to produce a sample for measuring the indentation hardness of the cross section in the width direction in which the cross section in the width direction is exposed. The embedded sample is preferably cut to pass through the center of the cut sample.

[0053] As an apparatus for cutting the embedded sample, for example, there is the trade name "Ultramicrotome EM UC7" manufactured by Leica Microsystems.

[0054] Using the sample for measuring the indentation hardness of the cross section in the flow direction prepared as described above, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle of the thickness direction are measured to calculate MD1 and MD2.

[0055] Similarly, using the sample for measuring the indentation hardness of the cross section in the width direction prepared as described above, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle of the thickness direction are measured to calculate TD1 and TD2.

[0056] It should be noted that in this specification, the indentation hardness of the cross section on the first surface side in the flow direction and the width direction, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle in the thickness direction refer to the average value of 5 measured values.

[0057] The indentation hardness of the cross section was measured by vertically pressing a Berckovich indenter (material: diamond triangular pyramid) into the cross section of the sample. The indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side were as follows: Figure 2 As shown, the measurement was performed at a position 2.0 μm inside the first and second surfaces ( Figure 2 The positions of (i) and (iii) correspond to the measurement sites). It should be noted that, Figure 2 Corresponds to Figure 1 In addition, Figure 2 The "d" refers to the thickness direction of the cut sample of the plastic film. In addition, Figure 2 The "(ii)" refers to the middle position in the thickness direction of the cut sample of the plastic film.

[0058] The indentation hardness is preferably measured under the following conditions.

[0059] <Measurement Conditions>

[0060] ·Indenter used: Berckovich indenter (Model: TI-0039, manufactured by HYSITRON)

[0061] Pressing conditions: displacement control method

[0062] Maximum indentation depth: 200nm

[0063] Load application time: 20 seconds (speed: 10 nm / second)

[0064] Holding time: 5 seconds at maximum pressing depth

[0065] Load unloading time: 20 seconds (speed: 10nm / second)

[0066] The indentation hardness can be calculated as follows.

[0067] First, the indentation depth h (nm) corresponding to the indentation load F (N) is continuously measured to create a load-displacement curve. The load-displacement curve created is analyzed as the maximum indentation load F max (N) divided by the projected area A where the pressure head meets the plastic film p (mm 2 ) can be used to calculate the indentation hardness H IT (Following formula (1)).

[0068] H IT =F max / A p …(1)

[0069] Here, Ap It is the projected contact area corrected for the curvature of the indenter tip using the device's standard method.

[0070] Before measuring the indentation hardness, a standard adjustment is preferably carried out.

[0071] Standard adjustment can be performed, for example, by performing an indentation test using a standard sample with known indentation hardness and complex elastic modulus, and confirming that the indentation hardness and complex elastic modulus obtained from the test results are within reference values.

[0072] It is preferred that standard adjustment is performed each time the sample is changed. However, if the samples are the same, it is preferred to continuously perform multiple indentation hardness measurements from the perspective of operating efficiency. That is, it is preferred to perform the measurement as described in (1) below. It should be noted that in (1) below, the measurement of the indentation hardness of the cross section in the flow direction and the measurement of the indentation hardness of the cross section in the width direction can be reversed in order.

[0073] (1) After the above standard adjustment was implemented, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the thickness direction were measured five times for the sample for measuring the indentation hardness of the cross section in the flow direction, and MD1 and MD2 were calculated. The above standard adjustment was implemented again, and the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the thickness direction were measured five times for the sample for measuring the indentation hardness of the cross section in the width direction, and TD1 and TD2 were calculated.

[0074] In addition, when measuring the indentation hardness for a long time, it is preferred to perform standard adjustment at least before the elapse of 12 hours. For example, even if standard adjustment is not performed every time the sample is changed, it is preferred to perform standard adjustment at least before the elapse of 12 hours.

[0075] In this specification, the various measurement atmospheres of Conditions 1 and 2, and Conditions 3 to 6 described later, are 23°C ± 5°C and 40% to 65% RH unless otherwise specified. Before measurement, the sample is exposed to the above atmosphere for 30 minutes or more.

[0076] <Flow direction and width direction>

[0077] Optical plastic films are available in, for example, sheet-like and roll-like forms.

[0078] In the case of a roll form, the flow direction of the roll and the width direction of the roll can be easily confirmed.

[0079] On the other hand, in the case of a sheet-like form, when the flow direction and the width direction can be easily confirmed as in a uniaxially stretched film, the flow direction and the width direction may be determined based on this confirmation (in the case of a uniaxially stretched film, the direction of the slow axis is usually the width direction).

[0080] When it is difficult to confirm the flow direction and width direction of the sheet, the flow direction and width direction may be identified as described in the following (1) and (2).

[0081] (1) When the sheet is rectangular or square, the flow direction or width direction can be determined by the four sides constituting the rectangle or square. The sheet-like film is produced by punching a roll-like film. In addition, in order to improve the yield of the sheet, it is necessary to punch the sheet along the flow direction and width direction of the roll. Therefore, when the sheet is rectangular or square, it can be said that it is technical common sense that the direction of the four sides constituting the rectangle or square is consistent with the flow direction or width direction.

[0082] (2) When the sheet is in a shape other than a rectangle or square (an ellipse, triangle, polygon other than a triangle, etc.), draw a rectangle or square with the largest area that does not protrude from the shape, and determine the flow direction or width direction based on the drawn rectangle or square in the same manner as in (1) above.

[0083] It should be noted that in the identification of the above (1) and (2), it is not possible to distinguish which of the two directions is the flow direction and which is the width direction. However, conditions 1 to 6 of this specification are parameters that are valid even if the flow direction and the width direction are reversed as long as the two directions can be determined as the flow direction and the width direction. Therefore, the flow direction and the width direction can be identified using the methods of the above (1) and (2).

[0084] <Condition 1>

[0085] Condition 1 stipulates that both MD2 / MD1 and TD2 / TD1 exceed 1.01 and are 1.30 or less.

[0086] When both MD2 / MD1 and TD2 / TD1 exceed 1.01, it means that the indentation hardness of the cross section of the plastic film is greater on the inside than on the surface. When the surface hardness of two different objects is the same, the object with a harder inside can improve the scratch resistance when the surface of the object is rubbed with a hard object. Therefore, by making both MD2 / MD1 and TD2 / TD1 exceed 1.01, the scratch resistance to hard objects can be improved regardless of the direction of friction.

[0087] On the other hand, MD2 / MD1 and TD2 / TD1 being both 1.30 or less means that the indentation hardness of the cross section of the plastic film is not too large inside compared to the surface side. When the surface of the plastic film is rubbed with a soft object, the stress during friction can be easily released when the inside of the plastic film is soft. Therefore, by making MD2 / MD1 and TD2 / TD1 both 1.30 or less, the scratch resistance to soft objects can be improved regardless of the direction of friction.

[0088] Under condition 1, both MD2 / MD1 and TD2 / TD1 are preferably greater than 1.01 and 1.20 or less, more preferably 1.02 or more and 1.15 or less, and even more preferably 1.02 or more and 1.10 or less.

[0089] The absolute values ​​of MD1, MD2, TD1 and TD2 are not particularly limited as long as they are in a range that provides appropriate mechanical strength, and are usually 150 MPa to 350 MPa, preferably 170 MPa to 300 MPa, more preferably 200 MPa to 270 MPa, and further preferably 220 MPa to 250 MPa.

[0090] One embodiment of the optical plastic film of the present invention preferably further satisfies the following condition 2.

[0091] <Condition 2>

[0092] When the larger of the product of MD1 and MD2 and the product of TD1 and TD2 is defined as X1 and the smaller one is defined as X2, X1 / X2 is 1.30 or less.

[0093] A small X1 / X2 means that the anisotropy of hardness in the flow direction and the width direction is small. Therefore, by setting X1 / X2 to 1.30 or less, it is possible to suppress scratches in a specific direction when an object (such as a pen tip) hits the plastic film (hereinafter, this performance is sometimes referred to as "scratch resistance"). In addition, by setting X1 / X2 to 1.30 or less, it is possible to easily suppress the remaining bending marks or fractures after the bending test.

[0094] X1 / X2 is more preferably 1.25 or less, and further preferably 1.20 or less. The lower limit of X1 / X2 is about 1.03, preferably 1.05 or more, and more preferably 1.10 or more.

[0095] One embodiment of the optical plastic film of the present invention preferably further satisfies the following condition 3.

[0096] <Condition 3>

[0097] A sample of 50 mm in the flow direction and 50 mm in the width direction is cut from the plastic film. The directions of the slow axis of the sample are measured at four locations 10 mm from the four corners to the center and at the center of the sample, a total of five locations. When the angles formed by the direction of the slow axis of each measurement location with any one of the flow direction and the width direction of the sample are defined as D1, D2, D3, D4, and D5, respectively, the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0 degrees or more.

[0098] Condition 3 stipulates that the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is 5.0 degrees or more. By setting the difference to 5.0 degrees or more, it is possible to suppress the occurrence of black vision in at least the area of ​​the sample when observing with polarized sunglasses.

[0099] Conventional optical plastic films are designed so that the direction of the slow axis does not deviate, but the plastic film satisfying condition 3 intentionally deviates the direction of the slow axis, which is different from the conventional optical film structure. In addition, it can be said that the plastic film satisfying condition 3 is also characterized by focusing on the deviation of the slow axis in a relatively small area such as 50 mm long and 50 mm wide.

[0100] Furthermore, from the viewpoint of being able to improve the folding resistance of the plastic film, it is preferable to satisfy condition 3.

[0101] On the other hand, for a general oriented film with a consistent slow axis, the film breaks after the bending test, or a strong bending mark remains. Specifically, a general uniaxially stretched film breaks when a bending test is performed along the slow axis, and a strong bending mark remains when a bending test is performed in a direction orthogonal to the slow axis. In addition, a general biaxially stretched film leaves a strong bending mark when a bending test is performed in a direction orthogonal to the slow axis.

[0102] The plastic film satisfying condition 3 is preferred in that it is possible to suppress the bending marks from remaining or the breakage from occurring after the bending test regardless of the bending direction.

[0103] The difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is preferably 6.0 degrees or more, more preferably 8.0 degrees or more, and further preferably 10.0 degrees or more.

[0104] It should be noted that if the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is too large, the orientation of the plastic film tends to decrease and the mechanical strength tends to decrease. Therefore, the difference is preferably 20.0 degrees or less, more preferably 17.0 degrees or less, and further preferably 15.0 degrees or less.

[0105] D1 to D5 of the optical plastic film according to one embodiment of the present invention are preferably 5 to 30 degrees or 60 to 85 degrees, more preferably 7 to 25 degrees or 65 to 83 degrees, further preferably 10 to 23 degrees or 67 to 80 degrees.

[0106] By setting D1 to D5 to 5 degrees or more or 85 degrees or less, black vision can be easily suppressed when observing with polarized sunglasses. In addition, by setting D1 to D5 to 30 degrees or less or 60 degrees or more, the reduction in mechanical strength caused by the reduction in orientation of the plastic film can be easily suppressed.

[0107] One embodiment of the optical plastic film of the present invention preferably further satisfies the following condition 4.

[0108] <Condition 4>

[0109] A sample with a size of 50 mm in the flow direction and 50 mm in the width direction was cut from the plastic film. The in-plane phase difference of the four locations 10 mm from the four corners of the sample to the center and the center of the sample was measured. When the in-plane phase difference of the five locations is defined as Re1, Re2, Re3, Re4, and Re5, the average of Re1 to Re5 is less than 600 nm.

[0110] Condition 4 stipulates that the average of Re1 to Re5 is 600 nm or less. By setting the average of Re1 to Re5 to 600 nm or less, it is possible to easily suppress the spots of the rainbow pattern (rainbow spots) at least in the region of the sample when observed with the naked eye.

[0111] The average of Re1 to Re5 is more preferably 300 nm or less, further preferably 250 nm or less, and further preferably 200 nm or less. The lower limit of the average of Re1 to Re5 is not particularly limited, but is usually about 50 nm, and preferably 100 nm or more.

[0112] Re1 to Re5 are each preferably 600 nm or less, more preferably 300 nm or less, further preferably 250 nm or less, and further preferably 200 nm or less.

[0113] The difference between the maximum value of Re1 to Re5 and the minimum value of Re1 to Re5 is preferably 200 nm or less, more preferably 150 nm or less, and further preferably 100 nm or less.

[0114] The optical plastic film according to one embodiment of the present invention preferably satisfies the following condition 5.

[0115] <Condition 5>

[0116] A sample with a size of 50 mm in the flow direction and 50 mm in the width direction was cut from the plastic film. The phase difference in the thickness direction of the sample was measured at four locations 10 mm from the four corners to the center and at the center of the sample, a total of five locations. When the phase difference in the thickness direction of the five locations was defined as Rth1, Rth2, Rth3, Rth4, and Rth5, the average of Rth1 to Rth5 was 2000 nm or more.

[0117] By satisfying condition 5, the degree of stretching of the optical plastic film can be made close to uniform biaxiality, and the mechanical strength of the optical plastic film can be improved. In addition, by satisfying condition 5, black vision when observing from an oblique direction through polarized sunglasses can be easily suppressed.

[0118] The average of Rth1 to Rth5 is more preferably 3000 nm or more, more preferably 4000 nm or more. The upper limit of the average of Rth1 to Rth5 is about 10000 nm, preferably 8000 nm or less, more preferably 7000 nm or less.

[0119] Furthermore, Rth1 to Rth5 are each preferably 2000 nm to 10000 nm, more preferably 3000 nm to 8000 nm, and further preferably 4000 nm to 7000 nm.

[0120] The difference between the maximum value of Rth1 to Rth5 and the minimum value of Rth1 to Rth5 is preferably 200 nm or less, more preferably 150 nm or less, and further preferably 100 nm or less.

[0121] The optical plastic film according to one embodiment of the present invention preferably further satisfies the following condition 6.

[0122] <Condition 6>

[0123] The average of Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4 and Re5 / Rth5 is 0.10 or less.

[0124] A small ratio (Re / Rth) of the in-plane phase difference (Re) to the phase difference in the thickness direction (Rth) means that the stretching degree of the optical plastic film is close to equal biaxiality. Therefore, by making the ratio 0.10 or less, the mechanical strength of the optical plastic film can be improved. The ratio is more preferably 0.07 or less, and further preferably 0.05 or less. The lower limit of the ratio is about 0.01.

[0125] Re1 / Rth1, Re2 / Rth2, Re3 / Rth3, Re4 / Rth4 and Re5 / Rth5 are each preferably 0.10 or less, more preferably 0.07 or less, and further preferably 0.05 or less. The lower limits of these ratios are about 0.01.

[0126] <Measurement of Conditions 3 to 6>

[0127] A sample of 50 mm in length and 50 mm in width was cut from any position of the plastic film for use under conditions 3 to 6. The five measurement points under conditions 3 to 6 are one in the center and four points 10 mm from the four corners of the sample toward the center, a total of five points ( Figure 3 5 parts of the black circle).

[0128] The in-plane phase difference (Re) of condition 4 and the phase difference in the thickness direction (Rth) of condition 5 are expressed by the following equations (1) and (2) from the refractive index nx in the direction of the slow axis, which is the direction in which the refractive index of each measurement position is the largest, the refractive index ny in the direction of the fast axis, which is the direction orthogonal to the slow axis, the refractive index nz in the thickness direction of the plastic film, and the thickness T [nm] of the plastic film. It should be noted that in this specification, the in-plane phase difference (Re) and the phase difference in the thickness direction (Rth) refer to the values ​​at a wavelength of 550nm.

[0129] In-plane retardation (Re) = (nx-ny) × T [nm] (1)

[0130] Retardation in the thickness direction (Rth) = ((nx+ny) / 2-nz) × T [nm] (2)

[0131] The direction of the slow axis, the in-plane phase difference (Re), and the phase difference in the thickness direction (Rth) can be measured, for example, using the trade name "RETS-100" manufactured by Otsuka Electronics Co., Ltd., or the trade names "KOBRA-WR" and "PAM-UHR100" manufactured by Oji Scientific Instruments Co., Ltd.

[0132] When measuring the in-plane retardation (Re) or the like using the trade name "RETS-100" manufactured by Otsuka Electronics Co., Ltd., it is preferable to prepare for the measurement according to the following steps (A1) to (A4).

[0133] (A1) First, in order to stabilize the light source of RETS-100, turn on the light source and leave it for more than 60 minutes. Then, select the rotating analyzer method and the θ mode (the mode for angular phase difference measurement and Rth calculation). Since the θ mode is selected, the stage is a tilted rotating stage.

[0134] (A2) Next, the following measurement conditions were input into RETS-100.

[0135] (Measurement conditions)

[0136] Retardation measurement range: Rotating analyzer method

[0137] ·Measurement of spot diameter:

[0138] Tilt angle range: 0°

[0139] ·Measurement wavelength range: 400nm~800nm

[0140] Average refractive index of the plastic film (for example, N=1.617 in the case of PET film)

[0141] Thickness: Thickness measured separately using SEM or optical microscope

[0142] (A3) Next, no sample is placed in the device to obtain background data. The device is a closed system and this operation is performed each time the light source is turned on.

[0143] (A4) Then, the sample is placed on a stage in the device and measurement is performed.

[0144] Under Condition 3, as long as all of D1 to D5 use the same direction as the reference for the angle formed with the slow axis direction (flow direction or width direction), either the flow direction or the width direction may be used as the reference.

[0145] When multiple samples of 50 mm in length x 50 mm in width can be cut from a sheet of plastic film, the proportion of samples satisfying the specified conditions among the multiple samples is preferably 50% or more, more preferably 70% or more, further preferably 90% or more, and further preferably 100% or more.

[0146] Furthermore, when a plurality of samples of 50 mm in length and 50 mm in width can be cut from a rolled plastic film, it is preferred that the samples cut from a specific position in the width direction of the roll satisfy the specified conditions in most of the flow direction of the roll. By satisfying this configuration, when the plastic film at a specific position in the width direction of the roll is picked up, a plastic film that exhibits a specified effect can be produced.

[0147] <Plastic film>

[0148] The laminated structure of the plastic film may be a single-layer structure or a multi-layer structure, of which a single-layer structure is preferred.

[0149] As described later, in order to improve mechanical strength and suppress rainbow spots, the plastic film is preferably a stretched plastic film with a small in-plane phase difference. In addition, in order to reduce the in-plane phase difference of the stretched plastic film, it is important to make the longitudinal and transverse stretches close to each other. Regarding the fine stretching control, it is difficult to control it in a multilayer structure due to the different physical properties of each layer, but it is easy to control it in a single-layer structure, which is preferred from this point of view.

[0150] As the resin component constituting the plastic film, polyester, triacetyl cellulose (TAC), cellulose diacetate, cellulose acetate butyrate, polyamide, polyimide, polyether sulfone, polysulfone, polypropylene, polymethylpentene, polyvinyl chloride, polyvinyl acetal, polyether ketone, polymethyl methacrylate, polycarbonate, polyurethane and amorphous olefin (Cyclo-Olefin-Polymer (cycloolefin polymer): COP) etc. can be cited. Among these, polyester is preferred from the aspect of easy improvement of mechanical strength. That is, the optical plastic film is preferably a polyester film.

[0151] Examples of the polyester constituting the polyester film include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), etc. Among these, PET is preferred because it has low intrinsic birefringence and is easy to reduce in-plane phase difference.

[0152] The plastic film may also contain additives such as ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, antigelling agents and surfactants.

[0153] The thickness of the plastic film is preferably 15 μm to 60 μm, more preferably 20 μm to 55 μm, and further preferably 30 μm to 50 μm. By making the thickness 15 μm or more, the mechanical strength can be easily improved. In addition, by making the thickness 60 μm or less, the in-plane phase difference can be easily reduced.

[0154] The haze of the optical plastic film in JIS K7136:2000 is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably 1.0% or less.

[0155] The total light transmittance of the optical plastic film in accordance with JIS K7361-1:1997 is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more.

[0156] In order to improve the mechanical strength, the plastic film is preferably a stretched plastic film, more preferably a stretched polyester film. In addition, the stretched polyester film is more preferably a single-layer structure of a polyester resin layer.

[0157] The stretched plastic film can be obtained by stretching a resin layer containing the components constituting the plastic film. The stretching method can include biaxial stretching such as stepwise biaxial stretching and synchronous biaxial stretching, and uniaxial stretching such as longitudinal uniaxial stretching. Among these, biaxial stretching that is easy to reduce the in-plane phase difference and easy to improve the mechanical strength is preferred. That is, the stretched plastic film is preferably a biaxially stretched plastic film. In addition, in the biaxially stretched plastic film, a biaxially stretched polyester film is preferred, and a biaxially stretched polyethylene terephthalate film is more preferred. It should be noted that, from the aspect of easily satisfying condition 2, the biaxially stretched plastic film preferably makes the stretching ratios in the flow direction and the width direction close.

[0158] -Gradually bi-directional stretching-

[0159] In stepwise biaxial stretching, the cast film is stretched in the flow direction and then stretched in the width direction of the film.

[0160] The stretching in the flow direction is usually implemented by the difference in the peripheral speed of the stretching roller, and can be carried out in one stage or in multiple stages using multiple stretching rollers. From the perspective of suppressing excessive deviations of optical properties such as in-plane phase difference, it is preferred to bring multiple nip rollers close to the stretching roller. The stretching ratio in the flow direction is usually 2 to 15 times, and from the perspective of suppressing excessive deviations of optical properties such as in-plane phase difference, it is preferably 2 to 7 times, more preferably 3 to 5 times, and further preferably 3 to 4 times.

[0161] From the viewpoint of suppressing excessive variation in optical properties such as in-plane retardation, the stretching temperature is preferably from the glass transition temperature of the resin to the glass transition temperature + 100°C. In the case of PET, it is preferably 70°C to 120°C, more preferably 80°C to 110°C, and even more preferably 95°C to 110°C.

[0162] Regarding the stretching temperature, by rapidly increasing the film temperature and shortening the stretching interval at low temperature, the average value of the in-plane phase difference tends to be reduced. On the other hand, by slowly increasing the film temperature and extending the stretching interval at low temperature, the orientation is improved, the average value of the in-plane phase difference is increased, and the deviation of the slow axis tends to be reduced.

[0163] It should be noted that when heating during stretching, it is preferred to use a heater that generates turbulence. By heating with wind containing turbulence, a temperature difference is generated in a small area within the film surface, and the orientation axis is slightly offset due to the temperature difference, which can easily satisfy condition 3.

[0164] The film stretched in the flow direction can be provided with functions such as lubricity, adhesion, antistatic properties, etc. by in-line coating. In addition, surface treatment such as corona treatment, flame treatment, plasma treatment, etc. can be performed as needed before in-line coating.

[0165] The coating film formed by in-line coating is extremely thin, about 10 nm to 2000 nm (the coating film is stretched to be thinner by stretching). In this specification, such a thin layer is not counted as the number of layers constituting the plastic film.

[0166] Regarding the stretching in the width direction, usually, a tenter method is used, and the film is stretched in the width direction while being conveyed while being held at both ends by a clamp. The stretching ratio in the width direction is usually 2 to 15 times, and from the aspect of suppressing excessive deviation of optical properties such as in-plane phase difference, it is preferably 2 to 5 times, more preferably 3 to 5 times, and further preferably 3 to 4.5 times. In addition, it is more preferable to increase the width stretching ratio compared to the longitudinal stretching ratio.

[0167] The stretching temperature is preferably the glass transition temperature of the resin to the glass transition temperature + 120°C, and the temperature preferably increases from the upstream to the downstream. Specifically, when the transverse stretching zone is divided into two parts, the difference between the upstream temperature and the downstream temperature is preferably 20°C or more, more preferably 30°C or more, further preferably 35°C or more, and further preferably 40°C or more. In the case of PET, the first stage stretching temperature is preferably 80°C to 120°C, more preferably 90°C to 110°C, and further preferably 95°C to 105°C.

[0168] For the plastic film that has been gradually biaxially stretched as described above, in order to impart flatness and dimensional stability, it is preferred to perform a heat treatment in a tenter at a temperature above the stretching temperature and below the melting point. Specifically, in the case of PET, it is preferred to perform heat fixing in the range of 150°C to 255°C, more preferably 200°C to 250°C. At this time, by performing heat fixing at a temperature as high as possible below the melting point, the crystallinity inside the film can be maintained, and on the other hand, the crystallinity on the surface of the film is slightly reduced, and condition 1 can be easily satisfied. In addition, from the perspective of suppressing excessive deviations in optical properties such as in-plane phase difference, it is preferred to perform additional stretching by heat treatment of 1% to 10% in the first half of the heat treatment.

[0169] After the plastic film is heat treated, it is slowly cooled to room temperature and then rolled up. In addition, as needed, relaxation treatment can be used in combination during heat treatment or slow cooling. From the perspective of suppressing excessive deviations of optical properties such as in-plane phase difference, the relaxation rate during heat treatment is preferably 0.5% to 5%, more preferably 0.5% to 3%, further preferably 0.8% to 2.5%, and further preferably 1% to 2%. In addition, from the perspective of suppressing excessive deviations of optical properties such as in-plane phase difference, the relaxation rate during slow cooling is preferably 0.5% to 3%, more preferably 0.5% to 2%, further preferably 0.5% to 1.5%, and further preferably 0.5% to 1.0%. From the perspective of planarity, the temperature during slow cooling is preferably 80°C to 150°C, more preferably 90°C to 130°C, further preferably 100°C to 130°C, and further preferably 100°C to 120°C.

[0170] -Synchronous bidirectional stretching-

[0171] In synchronous biaxial stretching, the cast film is introduced into a synchronous twin-screw tenter, and the film is conveyed while being held at both ends by a clamp, and is stretched synchronously and / or stepwise in the flow direction and the width direction. Synchronous biaxial stretching machines include a pantograph type, a screw type, a drive motor type, and a linear motor type, but the stretching ratio can be arbitrarily changed, and a drive motor type or a linear motor type that can perform relaxation treatment at any location is preferred.

[0172] The ratio of simultaneous biaxial stretching is generally 6 to 50 times in terms of area ratio, and is preferably 8 to 30 times, more preferably 9 to 25 times, further preferably 9 to 20 times, and further preferably 10 to 15 times in terms of suppressing excessive deviation of optical properties such as in-plane phase difference.

[0173] In the case of simultaneous biaxial stretching, in order to suppress the orientation difference within the plane, it is preferred that the stretching ratios in the flow direction and the width direction be the same and the stretching speeds be substantially the same.

[0174] From the perspective of suppressing excessive deviation in optical properties such as in-plane retardation, the stretching temperature in simultaneous biaxial stretching is preferably from the glass transition temperature of the resin to the glass transition temperature + 120° C. In the case of PET, it is preferably 80° C. to 160° C., more preferably 90° C. to 150° C., and even more preferably 100° C. to 140° C.

[0175] The film subjected to simultaneous biaxial stretching is preferably subjected to heat treatment at a temperature above the stretching temperature and below the melting point in a heat fixation chamber in a tenter in order to impart flatness and dimensional stability. The conditions for this heat treatment are the same as those for the heat treatment after the sequential biaxial stretching.

[0176] <Bending resistance>

[0177] The plastic film preferably does not produce cracks or breaks after the folding test shown in the embodiment is performed 100,000 times (more preferably after 300,000 times). In addition, for the plastic film, it is preferred that after the folding test shown in the embodiment is performed 100,000 times (more preferably after 300,000 times), when the measurement sample is placed on a horizontal platform, the angle at which the end of the sample floats from the platform is less than 20 degrees, more preferably less than 15 degrees. The angle of floating from the end of the sample of less than 15 degrees means that it is difficult to produce creases due to folding. In addition, it is preferred that the plastic film shows the above results (no cracks, breaks and creases caused by folding. The floating angle of the end of the sample after the test is less than 20 degrees) in any direction in the flow direction and width direction.

[0178] <Thickness>

[0179] From the perspective of mechanical strength, the optical plastic film is preferably 10 μm or more, more preferably 20 μm or more, and further preferably 25 μm or more. In addition, from the perspective of reducing the in-plane phase difference, the optical plastic film is preferably 100 μm or less, more preferably 75 μm or less, and further preferably 50 μm or less. It should be noted that from the perspective of improving the bending resistance, the thickness is also preferably 50 μm or less.

[0180] <Purpose>

[0181] As described above, the plastic film of the present invention can improve the scratch resistance when scratched by hard objects such as pencils and styluses, and the scratch resistance when repeatedly rubbed by soft objects such as cloth. Therefore, the optical plastic film of the present invention can be suitably used as a plastic film for image display devices, and in particular, can be suitably used as a plastic film for image display devices equipped with touch panels.

[0182] In addition, the plastic film of one embodiment of the present invention that satisfies condition 2 or 3 can suppress the remaining bending marks or the occurrence of breakage after the bending test regardless of the bending direction, and can therefore be suitably used as a plastic film for curved image display devices or foldable image display devices.

[0183] Examples of the plastic film for the image display device include plastic films used as base materials for various functional films such as polarizer protective films, surface protective films, antireflection films, and conductive films constituting touch panels.

[0184] [Optical layered body]

[0185] The optical plastic film of the present invention may be further formed with functional layers such as a protective layer, an antireflection layer, a hard coat layer, an antiglare layer, a phase difference layer, an adhesive layer, a transparent conductive layer, an antistatic layer and an antifouling layer to form an optical layered body.

[0186] The functional layer of the optical layered body preferably includes an antireflection layer. The antireflection layer is preferably disposed on the outermost surface of the plastic film on the side having the functional layer.

[0187] By including an antireflection layer as a functional layer of the optical layered body, rainbow spots can be easily suppressed.

[0188] In addition, the functional layer more preferably includes a hard coat layer and an antireflection layer. When the functional layer includes a hard coat layer and an antireflection layer, it is preferred that the hard coat layer and the antireflection layer are sequentially disposed on the optical plastic film.

[0189] The hard coat layer and the anti-reflection layer may be commonly used hard coat layers and anti-reflection layers.

[0190] [Polarizing plate]

[0191] The polarizing plate of the present invention comprises: a polarizing element; a transparent protective plate A disposed on one side of the polarizing element; and a transparent protective plate B disposed on the other side of the polarizing element, wherein at least one of the transparent protective plate A and the transparent protective plate B is the optical plastic film of the present invention.

[0192] The polarizing plate is used to impart antireflection properties in combination with a λ / 4 phase difference plate, for example. In this case, the λ / 4 phase difference plate is disposed on the display element of the image display device, and the polarizing plate is disposed on the observer side relative to the λ / 4 phase difference plate.

[0193] In addition, when the polarizing plate is used for a liquid crystal display device, it is used to give the function of a liquid crystal light valve. In this case, the liquid crystal display device is arranged in the order of a lower polarizing plate, a liquid crystal display element, and an upper polarizing plate, and the absorption axis of the polarizing element of the lower polarizing plate is arranged orthogonal to the absorption axis of the polarizing element of the upper polarizing plate. In this configuration, it is preferred to use the polarizing plate of the present invention as the upper polarizing plate.

[0194] <Transparent protective plate>

[0195] The polarizing plate of the present invention uses the optical plastic film of the present invention as at least one of the transparent protective plate A and the transparent protective plate B. In a preferred embodiment, both the transparent protective plate A and the transparent protective plate B are the optical plastic films of the present invention.

[0196] When one of the transparent protective plate A and the transparent protective plate B is the optical plastic film of the present invention, the other transparent protective plate is not particularly limited, but is preferably an optically isotropic transparent protective plate. Optically isotropic means that the in-plane phase difference is 20 nm or less, preferably 10 nm or less, and more preferably 5 nm or less. Examples of optically isotropic transparent substrates include acrylic films and triacetyl cellulose (TAC) films.

[0197] When one of the transparent protective plate A and the transparent protective plate B is the optical plastic film of the present invention, it is preferable to use the optical plastic film of the present invention as the transparent protective plate on the light emitting side.

[0198] <Polarizing Element>

[0199] As polarizing elements, for example, sheet-type polarizing elements such as polyvinyl alcohol film, polyvinyl formal film, polyvinyl acetal film, ethylene-vinyl acetate copolymer saponified film, etc., which are dyed with iodine or the like and stretched; wire grid polarizing elements composed of a large number of metal wires arranged in parallel; coated polarizing elements coated with lyotropic liquid crystal or dichroic host-guest material; multilayer thin film polarizing elements, etc. It should be noted that these polarizing elements may be reflective polarizing elements having the function of reflecting non-transmitted polarized components.

[0200] The polarizing element is preferably arranged so that its absorption axis is approximately parallel or approximately perpendicular to the flow direction or width direction of the optical plastic film. Approximately parallel means within 0 degrees ± 5 degrees, preferably within 0 degrees ± 3 degrees, and more preferably within 0 degrees ± 1 degree. Approximately perpendicular means within 90 degrees ± 5 degrees, preferably within 90 degrees ± 3 degrees, and more preferably within 90 degrees ± 1 degree.

[0201] [Image display device]

[0202] The image display device of the present invention comprises: a display element; and a plastic film disposed on the light emitting surface side of the display element, wherein the plastic film is the optical plastic film of the present invention.

[0203] Figure 4 and Figure 5 1 is a cross-sectional view showing an embodiment of an image display device 100 according to the present invention.

[0204] Figure 4 and Figure 5 The image display device 100 has a light emitting surface side ( Figure 4 and Figure 5 The upper side of the optical device has a plastic film 10. Figure 4 and Figure 5 Each of the image display devices 100 has a polarizing element 31 between the display element 20 and the optical plastic film 10. Figure 4 and Figure 5 In the embodiment, a transparent protective plate A (32) and a transparent protective plate B (33) are stacked on both sides of the polarizing element 31. Figure 5 In the image display device, an optical plastic film 10 is used as a transparent protective plate A (32).

[0205] It should be noted that the image display device 100 is not limited to Figure 4 and Figure 5 For example, Figure 4 and Figure 5 In the figure, the components constituting the image display device 100 are arranged at a certain interval, but the components may be integrated via an adhesive layer, etc. In addition, the image display device may also have components (other plastic films, functional layers, etc.) not shown.

[0206] <Display Components>

[0207] Examples of the display element include a liquid crystal display element, an EL display element (an organic EL display element, an inorganic EL display element), and a plasma display element. Furthermore, examples of the display element include an LED display element such as a Micro LED display element.

[0208] When the display element of the display device is a liquid crystal display element, a backlight is required on the surface of the liquid crystal display element on the opposite side to the resin sheet.

[0209] In addition, the image display device may be an image display device having a touch panel function.

[0210] Examples of the touch panel include a resistive film type, an electrostatic capacitance type, an electromagnetic induction type, an infrared type, and an ultrasonic type.

[0211] The touch panel function may be added to the display element as in a built-in touch panel liquid crystal display element, or a touch panel may be mounted on the display element.

[0212] In addition, as described above, the optical plastic film of the present invention can suppress the bending marks remaining or the occurrence of fractures after the bending test. Therefore, the image display device of the present invention can exert a more significant effect when it is a curved image display device or a foldable image display device, which is preferred from this point of view.

[0213] It should be noted that, when the image display device is a curved image display device or a foldable image display device, the display element is preferably an organic EL display element.

[0214] <Plastic film>

[0215] The image display device of the present invention has the optical plastic film of the present invention on the light emitting surface side of the display element. The plastic film may be a single sheet or two or more sheets.

[0216] Examples of the plastic film disposed on the light emitting surface side of the display element include plastic films used as substrates for various functional films such as polarizer protective films, surface protective films, antireflection films, and conductive films constituting touch panels.

[0217] <Other plastic films>

[0218] The image display device of the present invention may include other plastic films within a range not impairing the effects of the present invention.

[0219] As the other plastic film, a plastic film having optical isotropy is preferable.

[0220] Example

[0221] Next, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[0222] 1. Measurement and evaluation

[0223] The atmosphere for the following measurements and evaluations was a temperature of 23°C ± 5°C and a humidity of 40% RH to 65% RH. Before the measurements and evaluations, the samples were exposed to the above atmosphere for 30 minutes or more.

[0224] 1-1. In-plane phase difference (Re), thickness direction phase difference (Rth), and direction of slow axis

[0225] A sample with a size of 50 mm in the flow direction and 50 mm in the width direction is cut out from the optical plastic film of the embodiment and comparative example produced or prepared in "2" described later. The in-plane phase difference, the phase difference in the thickness direction and the direction of the slow axis are measured at four locations 10 mm from the four corners of the cut sample to the center and a total of five locations in the center of the sample. The averages of Re1 to Re5 calculated from the measurement results are shown in Table 1. The measuring device used is the trade name "RETS-100 (measurement spot: diameter 5 mm)" manufactured by Otsuka Electronics Co., Ltd. It should be noted that regarding the direction of the slow axis, the flow direction (MD direction) of the plastic film is used as the reference 0 degree, and the measurement is performed in the range of 0 to 90 degrees.

[0226] 1-2. Indentation hardness of cross section

[0227] Two cut samples were prepared by cutting the sample with a size of 50 mm in the flow direction and 50 mm in the width direction from the sample cut in 1-1 to a size of 2 mm in the flow direction and 10 mm in the width direction. Then, two cut samples were prepared. Figure 1 The embedded sample is shown as embedded in resin. It should be noted that the embedded sample is prepared according to the preferred methods exemplified in (A1) and (A2) of the main text of the specification.

[0228] An embedded sample is cut vertically along the flow direction with a diamond knife to make a sample A for measuring the indentation hardness of the cross section in the flow direction in which the cross section in the flow direction is exposed. Another embedded sample is cut vertically along the width direction with a diamond knife to make a sample B for measuring the indentation hardness of the cross section in the width direction in which the cross section in the width direction is exposed. It should be noted that the device for cutting the embedded sample uses the trade name "Ultramicrotome EM UC7" manufactured by Leica Microsystems. In addition, it is cut into the center of the cut sample through the embedded sample. It should be noted that when cutting, it is first roughly cut (rough trimming), and finally trimmed precisely under the conditions of "speed: 1.00mm / s" and "feed: 70nm" so that the cross section through the center of the sample is roughly flat. In addition, after precise trimming, it is confirmed with a microscope that there are no foreign matter and concave-convex substances that hinder the measurement in the cross section.

[0229] Next, using sample A, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle of the thickness direction were measured, and MD1 and MD2 were calculated. Similarly, using sample B, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle of the thickness direction were measured, and TD1 and TD2 were calculated. The results are shown in Table 1. As described in the main text of the specification, MD1, MD2, TD1, and TD2 refer to the average value of 5 measured values.

[0230] like Figure 2 As shown, the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side were measured at a position 2.0 μm inside from the first surface and the second surface.

[0231] Indentation hardness was measured by vertically pressing a Berckovich indenter (material: diamond triangular pyramid) into a cross section using a HYSITRON product number "TI950 TriboIndenter" as a measuring device and using the application software (TriboScan Version 9.6.0.2) attached to the device under the following conditions.

[0232] In addition, in the above-mentioned measurement, the following standard adjustment is implemented: before measuring the indentation hardness of each sample, a standard sample (fused quartz (5-0098) manufactured by HYSITRON) with known indentation hardness and complex elastic modulus is used to implement an indentation test, and it is confirmed that the indentation hardness and complex elastic modulus obtained from the test results are within the reference value. That is, after implementing the above-mentioned standard adjustment, for sample A, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle of the thickness direction are measured 5 times. And, after completing the measurement of sample A, the above-mentioned standard adjustment is implemented again, and for sample B, the indentation hardness of the cross section on the first surface side, the indentation hardness of the cross section on the second surface side, and the indentation hardness of the cross section in the middle of the thickness direction are measured 5 times.

[0233] In Table 1, the case where condition 1 (MD2 / MD1 and TD2 / TD1 are both greater than 1.01 and less than 1.30) is satisfied is recorded as "Y", and the case where condition 1 is not satisfied is recorded as "N".

[0234] <Measurement Conditions>

[0235] ·Indenter used: Berckovich indenter (Model: TI-0039, manufactured by HYSITRON)

[0236] Pressing conditions: displacement control method

[0237] Maximum indentation depth: 200nm

[0238] Load application time: 20 seconds (speed: 10 nm / second)

[0239] Holding time: 5 seconds at maximum pressing depth

[0240] Load unloading time: 20 seconds (speed: 10nm / second)

[0241] 1-3. Scratch resistance 1 (scratch resistance to hard objects)

[0242] A pencil with a hardness of F specified in JIS S6006 was pressed against the surface of the optical plastic film of the embodiment and the comparative example, and a pencil hardness test (4.9N load) specified in JIS K5600-5-4:1999 was performed. The test was performed in both the flow direction and the width direction. The result was recorded as "A" when there was no scratch on the surface of the plastic film in all directions, and recorded as "C" when there was a scratch on the surface of the plastic film in at least one direction.

[0243] 1-4. Scratch resistance 2 (scratch resistance to soft objects)

[0244] A 300-gauge cotton flannel cloth was pressed against the surface of the optical plastic film of the Example and the Comparative Example at a load of 500 g / cm 2 After rubbing back and forth 1000 times, visually check for scratches under fluorescent light. Tests were conducted in both the flow direction and the width direction. The test device used was a Gakushin abrasion tester (product number "AB-301" manufactured by TESTER SANGYO). The result was recorded as "A" when there were no scratches on the surface of the plastic film in all directions, and "C" when there were scratches on the surface of the plastic film in at least one direction.

[0245] 1-5.Bending resistance

[0246] <Width direction>

[0247] Cut out a long strip sample with a short side (width direction) of 30 mm x a long side (flow direction) of 100 mm from the optical plastic film of the embodiment and the comparative example. Fix both ends of the short side (30 mm) of the sample to a durability tester (product name "DLDMLH-FS", manufactured by YUASA SYSTEM Co., Ltd.) (fix the area 10 mm from the front end) and perform a continuous folding test of 100,000 folds of 180 degrees. The folding speed is set to 120 times per minute. A more detailed method of the folding test is shown below.

[0248] After the folding test, the long strip sample was placed on a horizontal platform, and the angle at which the end of the sample floated from the platform was measured. The results are shown in Table 1. It should be noted that when the sample broke in the middle, it was recorded as "broken".

[0249] <Flow direction>

[0250] Strip samples with a short side (flow direction) of 30 mm x a long side (width direction) of 100 mm were cut out from the optical plastic films of Examples and Comparative Examples, and the same evaluation as above was performed.

[0251] <Details of folding test>

[0252] like Figure 6 As shown in (A), in the continuous folding test, first, the side 10C and the side 10D facing the side 10C of the plastic film 10 are fixed by the parallelly arranged fixing parts 60. The fixing parts 60 can slide in the horizontal direction.

[0253] Then, if Figure 6 As shown in (B), the fixing parts 60 are moved so as to approach each other, thereby folding and deforming the plastic film 10, and then Figure 6As shown in (C), the fixing part 60 is moved to a position where the interval between two opposing sides of the plastic film 10 fixed by the fixing part 60 is 2 mm, and then the fixing part 60 is moved in the reverse direction to eliminate the deformation of the plastic film 10.

[0254] Through Figure 6 By moving the fixing part 60 as shown in (A) to (C), the plastic film 10 can be folded 180 degrees. In addition, the continuous folding test is performed in such a way that the curved part 10E of the plastic film 10 does not protrude from the lower end of the fixing part 60, and the interval when the fixing part 60 is closest is controlled to 2 mm, thereby making it possible to make the interval between the two sides of the optical film 10 facing each other 2 mm.

[0255] 1-6. Rainbow spots

[0256] The sample (sample prepared in 1-1) cut out of the optical plastic of the embodiment and the comparative example was placed on the observation side polarizing plate of the image display device with the following structure so that the TD direction of the sample was parallel to the horizontal direction of the screen. Then, the image display device was lit in a dark room environment, and observed from various angles with the naked eye, and the presence or absence of rainbow spots was evaluated according to the following criteria.

[0257] A: No rainbow spots can be observed.

[0258] B: Rainbow spots are observed in a very small area.

[0259] C: Rainbow spots were observed in most areas.

[0260] <Configuration of Image Display Device>

[0261] (1) Backlight: White LED or cold cathode tube

[0262] (2) Light source side polarizing plate: TAC films are provided as protective films on both sides of a polarizing element made of PVA and iodine, and the polarizing element is arranged so that the absorption axis direction of the polarizing element is perpendicular to the horizontal direction of the screen.

[0263] (3) Image display unit: Liquid crystal unit

[0264] (4) Observation-side polarizing plate: A polarizing plate using a TAC film as a polarizing element protective film of a polarizing element composed of PVA and iodine was arranged so that the absorption axis of the polarizing element was perpendicular to the parallel direction of the screen.

[0265] (5) Size: 10 inches diagonally

[0266] 1-7. Black Vision

[0267] The sample cut out of the optical plastic of the embodiment and the comparative example (the sample prepared in 1-1) was arranged on the observation side polarizing plate of the image display device of the structure shown in 1-6, so that the TD direction of the sample was parallel to the horizontal direction of the screen. Then, in a state where the image arranged with the sample was displayed in the vertical direction, the image display device prepared in the embodiment and the comparative example was observed from the front through polarized sunglasses that absorb S polarized light, and black vision was evaluated according to the following criteria.

[0268] A: All areas have no black vision.

[0269] B: Black vision occurs in a very small area.

[0270] C: Black vision occurs in most areas.

[0271] 2. Production and preparation of stretched polyester film

[0272] [Example 1]

[0273] 1 kg of PET (melting point 258°C, absorption center wavelength: 320 nm) and 0.1 kg of ultraviolet absorber (2,2'-(1,4-phenylene)bis(4H-3,1-benzoxazine-4-one) were melt-mixed at 280°C using a kneader to produce pellets containing the ultraviolet absorber. The pellets and PET with a melting point of 258°C were put into a single screw extruder, melt-mixed at 280°C, extruded from a T-die, and cast onto a casting drum whose surface temperature was controlled to 25°C to obtain a cast film. The amount of the ultraviolet absorber in the cast film was 1 part by mass relative to 100 parts by mass of PET.

[0274] The obtained cast film was heated with a roller set at 95°C, and then stretched 3.3 times in the flow direction while heating from both sides of the film with a radiation heater in such a way that the film temperature at a point 150mm in a stretching interval of 400mm (starting from stretching roller A and ending at stretching roller B. Stretching rollers A and B each have two nip rollers) reached 103°C, and then temporarily cooled. It should be noted that when heating with the radiation heater, a wind of 92°C and 4m / s was blown to the film from the opposite side of the film of the radiation heater, thereby generating turbulence on the inside and outside of the film, disturbing the temperature uniformity of the film.

[0275] Next, both surfaces of the uniaxially stretched film were subjected to corona discharge treatment in air, the wetting tension of the base film was set to 55 mN / m, and a "lubricating layer coating liquid comprising a polyester resin having a glass transition temperature of 18°C, a polyester resin having a glass transition temperature of 82°C, and silica particles having an average particle size of 100 nm" was applied online to the corona discharge treated surfaces of both surfaces of the film to form an lubricating layer.

[0276] Next, the uniaxially stretched film was introduced into a tenter, preheated with hot air at 95°C, and then stretched 4.5 times in the film width direction at a temperature of 105°C in the first stage and 140°C in the second stage. Here, when the transverse stretching section is divided into two parts, the stretching is performed in two stages in such a way that the stretching amount of the film at the midpoint of the transverse stretching section (film width at the measuring point - film width before stretching) is 80% of the stretching amount at the end of the transverse stretching section. The transversely stretched film was directly heat-treated from 180°C in stages in the tenter using hot air at a heat treatment temperature of 245°C, then, a 1% relaxation treatment was performed in the width direction under the same temperature conditions, and then, after being rapidly cooled to 100°C, a 1% relaxation treatment was performed in the width direction, and then it was rolled up to obtain the optical plastic film (biaxially stretched polyester film, thickness 40μm) of Example 1.

[0277] [Example 2]

[0278] An optical plastic film (biaxially stretched polyester film, thickness 40 μm) of Example 2 was obtained in the same manner as in Example 1 except that the point where the film temperature reached 103° C. was changed to a point 200 mm in the stretching section 400 mm.

[0279] [Example 3]

[0280] An optical plastic film (biaxially oriented polyester film, thickness 50 μm) of Example 3 was obtained in the same manner as Example 1 except that the thickness of the cast film of Example 1 was increased, the stretching ratio in the flow direction was changed from 3.3 times to 3.5 times, and the stretching ratio in the width direction was changed from 4.5 times to 5.0 times.

[0281] [Example 4]

[0282] An optical plastic film (biaxially oriented polyester film, thickness 42 μm) of Example 4 was obtained in the same manner as Example 1 except that the thickness of the cast film of Example 1 was increased, the stretching ratio in the flow direction was changed from 3.3 times to 3.5 times, and the stretching ratio in the width direction was changed from 4.5 times to 5.0 times.

[0283] [Comparative Example 1]

[0284] As the optical plastic film of Comparative Example 1, a commercially available biaxially oriented polyester film (manufactured by Toyobo Co., Ltd., trade name: Cosmoshine A4100, thickness: 50 μm) was prepared.

[0285] [Comparative Example 2]

[0286] As the optical plastic film of Comparative Example 2, a biaxially stretched polyester film having a three-layer structure (three layers of crystalline polyester / non-crystalline polyester / crystalline polyester) of Example 13 of JP-A-2018-59078 was produced.

[0287] [Table 1]

[0288] Table 1

[0289]

[0290] From the results in Table 1, it can be confirmed that the optical plastic films of Examples 1 to 4 satisfying Condition 1 can improve the scratch resistance for both hard and soft objects. In addition, the optical plastic films of Examples 1 to 4 can suppress black vision by satisfying Condition 3, and further suppress the remaining bending marks and the occurrence of cracks after the bending test regardless of the bending direction. In addition, it can be confirmed that the optical plastic films of Examples 1 and 2 can suppress rainbow spots by satisfying Condition 4.

[0291] Explanation of symbols

[0292] 10: Plastic film for optics

[0293] 20: Display components

[0294] 30: Polarizer

[0295] 31: Polarization element

[0296] 32: Transparent protective plate A

[0297] 33: Transparent protective plate B

[0298] 50: Shell

[0299] 100: Image display device

[0300] S: Cut sample of plastic film

[0301] R: embedding resin

[0302] d: Thickness direction of the cut sample of the plastic film

Claims

1. An optical plastic film, which is a stretched plastic film, having a first surface and a second surface located on the opposite side of the first surface, and which satisfies the following condition 1: <Condition 1> Regarding the indentation hardness of the cross section in the flow direction of the stretched plastic film, the softer hardness of the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side is defined as MD1, and the indentation hardness of the cross section in the middle of the thickness direction is defined as MD2; in addition, regarding the indentation hardness of the cross section in the width direction of the stretched plastic film, the softer hardness of the indentation hardness of the cross section on the first surface side and the indentation hardness of the cross section on the second surface side is defined as TD1, and the indentation hardness of the cross section in the middle of the thickness direction is defined as TD2; under this premise, MD2 / MD1 and TD2 / TD1 are both greater than 1.01 and less than 1.

30.

2. The optical plastic film according to claim 1, further satisfying the following condition 2: <Condition 2> When the larger of the product of MD1 and MD2 and the product of TD1 and TD2 is defined as X1 and the smaller one is defined as X2, X1 / X2 is 1.30 or less.

3. The optical plastic film according to claim 1 or 2, further satisfying the following condition 3: <Condition 3> A sample with a size of 50 mm in the flow direction and 50 mm in the width direction is cut from the stretched plastic film; the directions of the slow axes of four locations 10 mm forward from the four corners of the sample to the center and a total of five locations in the center of the sample are measured; when the angles formed by any one of the flow direction and the width direction of the sample and the direction of the slow axis of each measured location are defined as D1, D2, D3, D4, and D5, respectively, the difference between the maximum value of D1 to D5 and the minimum value of D1 to D5 is greater than 5.0 degrees.

4. The optical plastic film according to claim 1 or 2, further satisfying the following condition 4: <Condition 4> A sample with a size of 50 mm in the flow direction and 50 mm in the width direction is cut from the stretched plastic film; the in-plane phase differences of four locations 10 mm forward from the four corners of the sample to the center and the center of the sample, totaling five locations, are measured; when the in-plane phase differences of the five locations are defined as Re1, Re2, Re3, Re4, and Re5, respectively, the average of Re1 to Re5 is less than 500 nm.

5. The optical plastic film according to claim 1 or 2, further satisfying the following condition 5: <Condition 5> A sample with a size of 50 mm in the flow direction and 50 mm in the width direction is cut from the stretched plastic film; the phase difference in the thickness direction of four locations 10 mm forward from the four corners of the sample to the center and the center of the sample, a total of five locations, is measured; when the phase difference in the thickness direction of the five locations is defined as Rth1, Rth2, Rth3, Rth4, and Rth5, respectively, the average of Rth1 to Rth5 is greater than 2000 nm.

6. A polarizing plate comprising: a polarizing element; a transparent protective plate A disposed on one side of the polarizing element; and a transparent protective plate B disposed on the other side of the polarizing element, wherein: At least one of the transparent protective plate A and the transparent protective plate B is the optical plastic film according to any one of claims 1 to 5.

7. An image display device comprising: a display element; and a plastic film disposed on the light emitting surface side of the display element, wherein: The plastic film is the optical plastic film according to any one of claims 1 to 5. 8 . The image display device according to claim 7 , further comprising a polarizing element between the display element and the plastic film.

Images