JP7670089B2 - Cover member for foldable flexible display, base film for cover member for foldable flexible display, and display device including same - Google Patents

Description

The present invention relates to a cover member for a foldable flexible display , a base film for the cover member for a foldable flexible display , and a display device including the same. More specifically, the present invention relates to a cover member that improves the handleability of the cover member and does not leave fold marks on the base film provided on the cover member, a base film for the cover member, and a display device including the same.

Recently, there has been much development of foldable or rollable flexible displays. These displays are composed of a display unit that includes a cover glass unit (hereinafter, also referred to as a "cover member" in this invention) for protecting the display and a polarizing plate.

When using a glass substrate as the transparent substrate for the cover glass unit, flexibility is required, and therefore the glass substrate itself needs to be thin. In response to this demand, a method for manufacturing thin glass has been disclosed (see, for example, Patent Document 1).

On the other hand, an invention has been disclosed that controls waviness on the surface of a polymer layer when laminating the polymer layer (protective film) onto a glass substrate for the purpose of protecting the glass substrate (see, for example, Patent Document 2).

In the flexible display having the folding specifications, the bending radius of the cover member tends to become smaller when the display is folded due to the thinning of each component. In such a case, according to the study by the inventors, it was found that the protective film (also called "substrate film" in the present invention) provided on the cover member needs to have an improved protective function (push strength) to the extent that the glass substrate does not crack when the cover member is folded, and also needs to have a property that does not leave fold marks on the protective film when the cover member is repeatedly bent.

However, in the cover member constructed as a laminate of a glass substrate and a protective film produced by the above-disclosed technical means, it was difficult to achieve both the improvement of the protective function when made thinner and the property of not leaving creases on the protective film when repeatedly bent.

International Publication No. 2017/066924 Special Publication No. 2002-534305

The present invention has been made in consideration of the above-mentioned problems and circumstances, and the problem to be solved is to provide a cover member for a foldable flexible display that not only improves the handleability of the cover member, but also provides a base film for the cover member that does not leave fold marks on the base film provided on the cover member when repeatedly bent, and a display device equipped with them.

In the course of investigating the causes of the above problems in order to solve the above problems, the inventors discovered that when a cover member has a base film and a transparent base material having a specific thickness, and a specific relationship is satisfied between the stress of the base film and the strain thereon, the cover member is not only easier to handle, but also has the property of not leaving fold marks on the base film provided in the cover member.

In other words, the above problems of the present invention are solved by the following means.

1. A cover member for a foldable flexible display having a substrate film and a transparent substrate ,
The thickness of the base film is in the range of 1 μm or more and less than 15 μm,
The thickness of the transparent substrate is in the range of 5 μm or more and less than 50 μm,
the substrate film contains a cellulose ester-based resin, a cycloolefin-based resin, a fumaric acid diester-based resin, a polypropylene-based resin, a (meth)acrylic resin, a polyester-based resin, a polyarylate-based resin, a polyimide-based resin, a styrene-based resin, or a composite resin thereof;
A cover member for a foldable flexible display, characterized in that, when the relationship between tensile stress and tensile elongation at break of the base film is measured under conditions of 23°C, 55% RH, and a pulling speed of 50 mm/min, the slope of a straight line connecting the origin and the breaking point in a stress-strain curve , with the vertical axis representing stress (MPa) and the horizontal axis representing tensile elongation at break (%), is within a range of 1.1 MPa/% to 25.0 MPa/% .

2. When the surface of the base film that is bonded to the transparent base material is surface A and the back surface of the base film opposite surface A is surface B,
2. The cover member for a foldable flexible display according to claim 1, wherein the film density (ρ _A ) of the A side is smaller than the film density (ρ _B ) of the B side.

3. When the surface of the base film that is attached to the transparent base material is surface A and the back surface of the base film opposite surface A is surface B,
3. The cover member for _a foldable flexible display described in claim 1 or 2, characterized in that the ratio (ρ _A /ρ _{B ) of the film density (ρ B )} of the B side to the film density (ρ A ) of the A side is within a range of 0.80 to 0.95.

4. The cover member for a foldable flexible display according to item 1 or 2, wherein the base film contains rubber particles in a range of 40 to 85% by mass.

5. The cover member for a foldable flexible display according to item 1 or 2, wherein the elastic modulus of the transparent substrate is within a range of 55 to 80 GPa, and the ratio of the elastic modulus of the transparent substrate to the elastic modulus of the substrate film (elastic modulus of transparent substrate/elastic modulus of substrate film) is 30 or more.
6. The cover member for a foldable flexible display according to item 1 or 2, wherein the base film contains at least a polyimide resin.
7. The cover member for a foldable flexible display according to item 1 or 2, wherein the base film does not contain rubber particles.

8. The thickness of the base film is in the range of 1 μm or more and less than 15 μm,
the substrate film contains a cellulose ester-based resin, a cycloolefin-based resin, a fumaric acid diester-based resin, a polypropylene-based resin, a (meth)acrylic resin, a polyester-based resin, a polyarylate-based resin, a polyimide-based resin, a styrene-based resin, or a composite resin thereof;
When the relationship between tensile stress and tensile elongation at break of the substrate film is measured under conditions of 23°C, 55% RH, and a pulling speed of 50 mm/min, the slope of a straight line connecting the origin and the breaking point in a stress-strain curve, with the vertical axis representing stress (MPa) and the horizontal axis representing tensile elongation at break (%) , is within a range of 1.1 MPa/% to 25.0 MPa/% .

9. The substrate film is attached to a transparent substrate,
When the surface of the base film that is attached to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B,
9. The substrate film for a cover member of a foldable flexible display according to claim 8 , wherein the film density (ρ _A ) of the A side is smaller than the film density (ρ _B ) of the B side.

10. The substrate film is attached to a transparent substrate,
When the surface of the base film that is attached to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B,
10. The substrate film for a cover member of a foldable flexible display according to item 8 or 9, characterized in that the ratio (ρ _A /ρ _B ) of the film density (ρ _B ) of the side B to the film density (ρ A ) of _the side A is within a range of 0.80 to 0.95.

11. The substrate film for a cover member of a foldable flexible display according to item 8 or 9 , characterized in that the substrate film contains rubber particles in a range of 40 to 85% by mass.

12. A display device comprising the cover member of a foldable flexible display according to item 1 or 2 , or the base film for the cover member of a foldable flexible display according to item 8 or 9 .

The above-mentioned means of the present invention can provide a cover member that improves the handleability of the cover member and does not leave creases on the base film provided on the cover member when repeatedly bent, a base film for the cover member, and a display device provided with them.

Although the mechanism by which the effects of the present invention are expressed or acted upon has not been clarified, it is speculated as follows.

The cover member of the present invention is characterized in that it is made of a laminate of a substrate film having a specific thickness range and a transparent substrate, and that the slope of the line connecting the origin and the breaking point on the stress-strain curve of the substrate film is 1.1 or more and 25.0 or less.

These characteristics not only improve the handleability of the cover member, but also allow the cover member to be obtained such that the base film provided on the cover member does not leave creases when repeatedly bent.

In this invention, "handleability of the cover member" refers to the fact that the base film of the cover member exerts its protective function (push strength) during the opening and closing operation by folding the cover member, and does not cause cracks or the like in the transparent base material. In addition, "fold marks" refer to the fact that, with repeated opening and closing operations by folding, the base film whitens at the folding site, which is observed as a fold line, and in addition, the base film peels off from the transparent base material, causing deterioration in visibility such as image distortion.

When the slope of the line connecting the origin and the breaking point in the stress-strain curve of the base film exceeds 25.0, i.e., when the base film is stiff and reaches the breaking point quickly, a sudden increase in stress occurs especially on the outside of the base film when the base film is stretched, i.e., when the device is closed, and thus a difference in the deterioration of physical properties occurs between the outside and inside during repeated opening and closing operations, causing whitening of the folded parts and poor adhesion, which are visually recognized as the "fold marks." Furthermore, it is presumed that the transparent substrate is prone to cracking because stress is directly concentrated on the transparent substrate when it is pressed in.

On the other hand, if the slope of the straight line is less than 1.1 and the base film is soft, the difference in physical property deterioration between the outside and inside is unlikely to occur during repeated opening and closing operations. However, as the base film and the transparent base material expand and contract repeatedly during the opening and closing operations, the base film will float up due to poor adhesion at the folded portion, reducing visibility. Furthermore, it is presumed that the base film is too soft when pressed together with the transparent base material, causing the transparent base material to physically deform too much, making it more likely to crack.

Cross-sectional view of a laminate of a base film and a transparent base 1 is a cross-sectional view of another embodiment of a laminate of a substrate film and a transparent substrate. Schematic diagram of the laminate folded Schematic diagram of another embodiment of a laminate folded. A graph showing the stress-strain curve of the base film and the slope of the line connecting the origin and the breaking point FIG. 1 is a schematic diagram showing a method for producing a base film according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing an example of a method for producing a glass substrate used in a transparent substrate; Schematic diagram showing lamination of a substrate film with a support and a transparent substrate FIG. 1 is a schematic diagram showing lamination of a substrate film with a support and a transparent substrate according to another embodiment. An example of application of a cover member to an organic EL display, which is an example of the display device of the present invention

The cover member for a foldable flexible display of the present invention is a cover member for a foldable flexible display having a base film and a transparent base material , the base film has a thickness in the range of 1 μm or more and less than 15 μm, the transparent base material has a thickness in the range of 5 μm or more and less than 50 μm, the base film contains a cellulose ester resin, a cycloolefin resin, a fumaric acid diester resin, a polypropylene resin, a (meth)acrylic resin, a polyester resin, a polyarylate resin, a polyimide resin, a styrene resin, or a composite resin thereof, and when the relationship between the tensile stress and the tensile breaking elongation of the base film is measured under conditions of 23° C. and 55% RH and a tensile speed of 50 mm/min, the slope of the straight line connecting the origin and the breaking point in the stress-strain curve with the vertical axis being stress (MPa) and the horizontal axis being tensile breaking elongation (%) is in the range of 1.1 MPa/% to 25.0 MPa/% . This feature is a technical feature common to or corresponding to the following embodiments.

As an embodiment of the present invention, from the viewpoint of manifestation of the effects of the present invention, when the surface of the base film that is bonded to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B, it is preferable that the film density (ρ _A ) of surface A is smaller than the film density (ρ _B ) of surface B, and it is preferable that the value of the ratio (ρ _A /ρ _B ) of the film density (ρ _B ) of surface B to the film density ₍ ρ A ) of surface A is within a range of 0.80 to 0.95. This is because, when the cover member is folded towards the transparent base material, a greater tensile stress is applied to surface B of the base film than to surface A, but if the film density of surface A is low, this tensile stress can be alleviated.

It is also preferable that the base film contains rubber particles in the range of 40 to 85% by mass. The improved impact resistance of the film due to the addition of a large amount of rubber particles leads to improved resistance to creases when the film is repeatedly bent.

It is preferable that the elastic modulus of the transparent substrate is within a range of 55 to 80 GPa, and that the elastic modulus ratio of the transparent substrate to the base film (elastic modulus of transparent substrate/elastic modulus of base film) is 30 or more. This is because, in order to provide a cover member that has improved pressing strength and does not leave creases using a transparent substrate with a low elastic modulus due to its thinning, it is necessary to use a base film with a lower elastic modulus, that is, by setting the elastic modulus ratio (elastic modulus of transparent substrate/elastic modulus of base film) to 30 or more, the protective function of the transparent substrate can be further improved.

The substrate film for the cover member of the foldable flexible display of the present invention has a thickness (also referred to as "film thickness") in the range of 1 μm or more and less than 15 μm, and the substrate film contains a cellulose ester resin, a cycloolefin resin, a fumaric acid diester resin, a polypropylene resin, a (meth)acrylic resin, a polyester resin, a polyarylate resin, a polyimide resin, a styrene resin, or a composite resin thereof. When the relationship between the tensile stress and the tensile elongation at break is measured for the substrate film under conditions of 23° C. and 55% RH and a tensile speed of 50 mm/min, the slope of the line connecting the origin and the break point in the stress-strain curve with the vertical axis being stress (MPa) and the horizontal axis being tensile elongation at break (%) is 1.1 MPa/% or more and 25.0 MPa/% or less. By adjusting the slope of the line within the above range, the indentation strength and fold mark resistance of the substrate film are improved, and the handleability when the substrate film is provided in the cover member is improved.

When the surface of the base film that is bonded to the transparent substrate is designated as surface A and the back surface of the base film opposite surface A is designated as surface B, it is preferable that the film density (ρ _A ) of surface A is smaller than the film density (ρ _B ) of surface B, and from the viewpoint of the above-mentioned effects, it is more preferable that the ratio (ρ _A /ρ _B ) of the film density (ρ _{B )} of surface B to the film density (ρ A ) of surface A is within a range of 0.80 to 0.95.

It is also preferable that the base film contains rubber particles in the range of 40 to 85% by mass.

The display device of the present invention is characterized by comprising the cover member of the present invention or the base film for the cover member of the present invention. This display device not only improves the handleability of the cover member, but also makes it possible to obtain a display device having a foldable display or a rollable display in which the base film of the cover member does not leave creases when repeatedly bent.

The present invention, its components, and the forms and modes for implementing the present invention are described in detail below. Note that in this application, "~" is used to mean that the numerical values before and after it are included as the lower and upper limits.

<Overview of the cover member of the present invention>
The cover member for a foldable flexible display of the present invention is a cover member for a foldable flexible display having a base film and a transparent base material , wherein the thickness of the base film is within a range of 1 μm or more and less than 15 μm, the thickness of the transparent base material is within a range of 5 μm or more and less than 50 μm, the base film contains a cellulose ester resin, a cycloolefin resin, a fumaric acid diester resin, a polypropylene resin, a (meth)acrylic resin, a polyester resin, a polyarylate resin, a polyimide resin, a styrene resin, or a composite resin thereof, and when the relationship between the tensile stress and the tensile breaking elongation of the base film is measured under conditions of 23° C. and 55% RH and a tensile speed of 50 mm/min, the slope of the straight line connecting the origin and the breaking point in a stress-strain curve with the vertical axis representing stress (MPa) and the horizontal axis representing tensile breaking elongation (%) is within a range of 1.1 MPa/% to 25.0 MPa/% .

Figure 1 shows a cross-sectional view of a laminate of a base film and a transparent base material in the cover member of the present invention, and a schematic diagram of the cover member (laminate) when folded.

Figure 1A shows the minimum configuration of the cover member 10 of the present invention, in which a base film 1 is attached to a transparent base material 3 via an adhesive layer 4 to form a laminate. When the base film 1 is attached to the transparent base material 3, the surface of the base film 1 that is attached to the transparent base material 3 is called side A, and the back side of the base film opposite side A is called side B.

Figure 1B shows another embodiment of the cover member 10 of the present invention. It shows an embodiment in which a base film 1 is attached to a transparent base material 3 via an adhesive layer 4, and a base film 2 is further attached to the transparent base material 3 via an adhesive layer 4 to form a laminate. The base film 2 may be a film having the same specifications as the base film 1, or may be a film having a different material and specifications. For example, it may be a functional film such as a hard coat film to improve surface hardness.

Figure 1C is a schematic diagram of the cover member 10 shown in Figure 1A folded toward the transparent substrate 3. Here, the bending radius when folded means the inner radius R of the folded portion when the cover member 10 is folded in Figure 1C or Figure 1D.

As mentioned above, when the cover member is folded onto the transparent substrate 3 side, more tensile stress is applied to the B side of the substrate film than to the A side, but if the film density on the A side is low, this tensile stress can be alleviated.

Figure 1D is a schematic diagram of the cover member 10 shown in Figure 1B folded onto the transparent substrate 3 side. Even in this case, the B-side of the substrate film 1 is subjected to more tensile stress than the A-side, but if the film density on the A-side is low, this tensile stress can be alleviated.

The stress-strain curve of the present invention shows the relationship between the tensile stress of the base film and the tensile elongation at break, measured in accordance with JIS K7127:1999.

The substrate film is cut into a size of 100 mm (MD direction: longitudinal direction) x 10 mm (TD direction: width direction) to obtain a sample film. This sample film is conditioned for 24 hours in an environment of 23°C and 55% RH, and the conditioned sample film is pulled in the MD direction with a chuck distance of 50 mm using Orientec's Tensilon RTC-1225A in accordance with JIS K7127:1999 to obtain a stress-strain curve until breakage. The stress-strain curve is expressed as the vertical axis representing stress (MPa) and the horizontal axis representing tensile elongation at break (%). The stress-strain curve is measured at 23°C, 55% RH, and a pulling speed of 50 mm/min.

The method for determining the slope of the straight line according to the present invention from the stress-strain curve of the base film obtained by the above measurements is explained using figures.

Figure 2 is a graph showing the stress-strain curve of the base film and the slope of the line connecting the origin and the break point.

When the base film is subjected to a tensile test, it reaches a breaking point X after elongation. When a straight line Y is drawn connecting the breaking point X and the origin (0 point), the slope α of this line is defined in the present invention as "the slope of the straight line connecting the origin and the breaking point in the stress-strain curve of the base film." As described above, in order to obtain the effects of the present invention, the slope α must be 1.1 or more and 25.0 or less. By being highly elastic but not too hard, it is possible to obtain a cover member that not only improves the handling of the cover member by improving the pressing strength of the base film, but also leaves no crease marks on the base film itself provided on the cover member when repeatedly bent.

The components of the present invention are described in detail below. However, the present invention is not limited to these components in the following description.

[1] Substrate Film [1.1] Overview of Substrate Film In the following description, the "substrate film" will be described as the "substrate film 1" in FIG. 1 unless otherwise specified.

The base film according to the present invention is characterized in that its thickness is in the range of 1 μm or more and less than 15 μm, and the slope of the line connecting the origin and the breaking point in the stress-strain curve of the base film is 1.1 or more and 25.0 or less. In addition to improving the handling properties such as the pressing strength when used in a cover member, when used in the cover member, it has the effect of preventing fold marks from being left on the base film itself when repeatedly bent.

As mentioned above, if the slope of the line exceeds 25.0, when the base film is stretched, i.e., when the device is closed, a sudden increase in stress occurs, particularly on the outside of the base film, and when the device is repeatedly opened and closed, a difference in the deterioration of physical properties occurs between the outside and inside, causing whitening of the folded parts and poor adhesion, which are visible as fold marks. Furthermore, when the transparent substrate is pressed in, stress is directly concentrated on the transparent substrate, making the transparent substrate prone to cracking.

On the other hand, if the slope of the straight line is less than 1.1 and the base film is soft, the base film will expand and contract repeatedly as a whole as it moves together with the transparent base material during the opening and closing process, causing the base film to lift up due to poor adhesion at the folded portion, reducing visibility and being visible as fold marks. Furthermore, because the base film is too soft when the base film is pressed in together with the transparent base material, the amount of physical deformation of the transparent base material becomes too large, making the transparent base material prone to cracking.

If the film thickness is less than 1 μm, the base film will be weak and the pressing strength of the cover member will decrease. If the film thickness is 15 μm or more, the base film will be too strong and the fold marks will be more likely to occur with repeated opening and closing of the display, so it is necessary to keep the film thickness within the above range.

Furthermore, when the surface of the base film that is bonded to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B, it is preferable that the film density (ρ _A ) of surface A is smaller than the film density (ρ _B ) of surface B, and it is preferable that the ratio (ρ _A /ρ _B ) of the film density (ρ _B ) of surface B to the film density (ρ A ) of surface _A is within a range of 0.80 to 0.95. This is because, when the cover member is folded towards the transparent base material, a greater tensile stress is applied to surface B of the base film than to surface A, but if the film density of surface A is low, this tensile stress can be alleviated.

<Measurement of density of sides A and B of base film>
The density of the surface (side A and side B) of the substrate film is measured using X-ray reflectivity (XRR). When X-rays are incident on the film surface at a very shallow angle, they are totally reflected, and when the angle of incident X-rays is equal to or greater than the critical angle of total reflection, the X-rays penetrate into the film and the reflectivity decreases. The reflectivity profile measured by the XRR method can be analyzed using dedicated reflectivity analysis software. In the present invention, when the angle at which the reflectivity starts to decrease is θa, the density at which the fitting error between the measurement result and the calculation result is the smallest in the range of 2θ from 2θa to 2θa+0.1° is the surface density. At this time, fitting is performed with the surface roughness in the range of 0 nm to 1 nm.

The substrate film is cut into a size of 30 mm x 30 mm, fixed to a sample stage, and measured under the following measurement conditions.
(Measurement conditions)
・Apparatus: Thin film X-ray diffraction apparatus (ATX-G manufactured by Rigaku Corporation)
・Sample size: 30mm x 30mm
・Incidence X-ray wavelength: 1.5405Å
Measurement range (θ): 0 to 6°
Analysis software: Reflectance analysis software GXRR (Rigaku Corporation)

In order to set the ratio (ρ _A /ρ B ) of the film density (ρ _B ) of the side _B to the film density (ρ _A ) of the side A within the range of 0.80 to 0.95, the ratio can be adjusted in the production process of the base film by utilizing the following knowledge, for example.

(a) Change the amount of rubber particles or microparticles relative to the resin. For example, increasing the amount of rubber particles or microparticles promotes the diffusion of the solvent, making it less likely that there will be a difference in density between the front and back surfaces.

(b) Change the solids concentration in the dope. For example, if the solids content is high, there is relatively little solvent, making it difficult for density differences to occur.

(c) Change the drying method. For example, by adding spot drying (applying a heater to the surface) in addition to slow drying, the density difference can be increased.

By combining the means derived from the findings of (a) to (c), it is possible to adjust the density ratio of the A side to the B side (ρ _A /ρ _B ) to within the range of 0.80 to 0.95.

[1.2] Materials for the Base Film The resin used for the base film according to the present invention is not particularly limited, and examples thereof include cellulose ester resins, cycloolefin resins, fumaric acid diester resins, polypropylene resins, (meth)acrylic resins, polyester resins, polyarylate resins, polyimide resins, and styrene resins or composite resins thereof. Preferred resins include (meth)acrylic resins, styrene-(meth)acrylate copolymers, cycloolefin resins, and polyimide resins, which contain a linear polymer material having a carbonyl group in the side chain, or a polymer material having a cyclic structure in the main chain, from the viewpoint of controlling physical properties such as bending resistance and improving optical properties.

<(Meth)acrylic resin>
The (meth)acrylic resin used in the base film preferably contains at least a structural unit (U1) derived from methyl methacrylate and a structural unit (U2) derived from phenylmaleimide. The (meth)acrylic resin containing the structural unit (U2) derived from phenylmaleimide has the advantage of reducing the photoelastic coefficient of the base film and making it less susceptible to unevenness even when it absorbs moisture and expands.

The (meth)acrylic resin may further contain other structural units in addition to those mentioned above. Examples of such other structural units include (meth)acrylic acid alkyl esters such as adamantyl acrylate; (meth)acrylic acid cycloalkyl esters such as 2-ethylhexyl acrylate, and the like. Among these, it is preferable to further contain a structural unit (U3) derived from an acrylic acid alkyl ester, from the viewpoint of reducing the deterioration of brittleness due to the inclusion of the structural unit (U2) derived from phenylmaleimide.

That is, it is more preferable that the (meth)acrylic resin contains a structural unit (U1) derived from methyl methacrylate, a structural unit (U2) derived from phenylmaleimide, and a structural unit (U3) derived from an alkyl acrylate.

The content of the structural unit (U1) derived from methyl methacrylate is preferably in the range of 50 to 95% by mass, and more preferably in the range of 70 to 90% by mass, based on the total structural units constituting the (meth)acrylic resin.

The structural unit (U2) derived from phenylmaleimide has a relatively rigid structure, and therefore can improve the mechanical strength of the base film. In addition, the structural unit (U2) derived from phenylmaleimide has a relatively bulky structure, and therefore can have microvoids that can move rubber particles in the resin matrix, and therefore can make it easier for the rubber particles to be concentrated in the surface layer of the base film.

The content of the structural unit (U2) derived from phenylmaleimide is preferably in the range of 1 to 25% by mass relative to all structural units constituting the (meth)acrylic resin. When the content of the structural unit (U2) derived from phenylmaleimide is 1% by mass or more, the base film has excellent storage stability in a high humidity environment. When the content is 25% by mass or less, the brittleness of the base film is unlikely to be excessively impaired. From the above viewpoint, the content of the structural unit (U2) derived from phenylmaleimide is more preferably in the range of 7 to 15% by mass.

The structural unit (U3) derived from an alkyl acrylate ester can impart appropriate flexibility to the resin, improving the brittleness caused by, for example, the inclusion of a structural unit (U2) derived from phenylmaleimide.

The alkyl acrylate ester is preferably an alkyl acrylate ester having an alkyl moiety with 1 to 7 carbon atoms, preferably 1 to 5 carbon atoms. Examples of alkyl acrylate esters include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-hydroxyethyl acrylate, hexyl acrylate, and 2-ethylhexyl acrylate.

The content of the structural unit (U3) derived from an alkyl acrylate is preferably in the range of 1 to 25% by mass relative to the total structural units constituting the (meth)acrylic resin. When the content of the structural unit (U3) derived from an alkyl acrylate is 1% by mass or more, the (meth)acrylic resin can be given an appropriate degree of flexibility, so that the base film does not become too brittle and is less likely to break. When the content of the structural unit (U3) derived from an alkyl acrylate is 25% by mass or less, the Tg of the base film does not become too low, and the base film has excellent storage stability in a high humidity environment. From the above viewpoint, the content of the structural unit (U3) derived from an alkyl acrylate is more preferably in the range of 5 to 15% by mass.

The ratio of the structural unit (U2) derived from phenylmaleimide to the total amount of the structural unit (U2) derived from phenylmaleimide and the structural unit (U3) derived from an alkyl acrylate is preferably in the range of 20 to 70% by mass. If the ratio is 20% by mass or more, the elastic modulus of the base film is easily increased, and if it is 70% by mass or less, the base film does not become too brittle.

The glass transition temperature (Tg) of the (meth)acrylic resin is preferably 100°C or higher, and more preferably in the range of 120 to 150°C. If the Tg of the (meth)acrylic resin is within the above range, the heat resistance of the base film is easily improved. In order to adjust the Tg of the (meth)acrylic resin, it is preferable to adjust the content of, for example, the structural unit (U2) derived from phenylmaleimide or the structural unit (U3) derived from an alkyl acrylate.

The weight average molecular weight (Mw) of the (meth)acrylic resin is not particularly limited and can be adjusted according to the purpose. The weight average molecular weight of the (meth)acrylic resin is preferably 100,000 or more, more preferably 1,000,000 or more, from the viewpoint of, for example, promoting entanglement between resin molecules to increase the toughness of the substrate film and make it less likely to break, and from the viewpoint of appropriately increasing the humidity expansion coefficient and making it easier to adjust the curl amount to a level preferable for adhesion. If the weight average molecular weight of the (meth)acrylic resin is 1,000,000 or more, the toughness of the obtained substrate film can be increased. As a result, in the manufacturing process, when the substrate film is transported as a laminated film described later, the substrate film can be prevented from breaking due to the transport tension, and transport stability can be improved. From the same viewpoint, the weight average molecular weight of the (meth)acrylic resin is more preferably in the range of 1,500,000 to 3,000,000. The method for measuring the weight average molecular weight is as follows.

<Gel permeation chromatography>
Solvent: methylene chloride Column: Shodex K806, K805, K803G (three columns manufactured by Showa Denko K.K. were connected and used)
Column temperature: 25°C
Sample concentration: 0.1% by mass
Detector: RI Model 504 (GL Sciences)
Pump: L6000 (Hitachi, Ltd.)
Flow rate: 1.0mL/min
Calibration curve: A calibration curve was used using 13 samples of standard polystyrene STK standard polystyrene (manufactured by Tosoh Corporation) in the range of Mw = 500 to 2,800,000. It is preferable to use the 13 samples at approximately equal intervals.

<Styrene/(meth)acrylate copolymer>
Styrene-(meth)acrylate copolymers (hereinafter also referred to as styrene-acrylic resins) have excellent transparency when used in substrate films. In addition, the moisture absorption expansion coefficient can be adjusted by the copolymerization ratio of the styrene portion, so curling of the laminate can be controlled by changing these ratios.

Styrene-acrylic resin is formed by addition polymerization of at least a styrene monomer and a (meth)acrylic acid ester monomer. The styrene monomer includes styrene represented by _the structural formula _CH2 =CH- _C6H5 , as well as styrene derivatives having known side chains or functional groups in the styrene structure.

In addition to acrylic acid esters and methacrylic acid esters represented by CH(R ₁ )=CHCOOR ₂ (R ₁ represents a hydrogen atom or a methyl group, and R ₂ represents an alkyl group having 1 to 24 carbon atoms), the (meth)acrylic acid ester monomers include acrylic acid ester derivatives and methacrylic acid ester derivatives having known side chains or functional groups in the structure of these esters.

Examples of styrene monomers include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, α-methylstyrene, p-phenylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-tert-butylstyrene, p-n-hexylstyrene, p-n-octylstyrene, p-n-nonylstyrene, p-n-decylstyrene, and p-n-dodecylstyrene.

Examples of (meth)acrylic acid ester monomers include acrylic acid ester monomers such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate (2EHA), stearyl acrylate, lauryl acrylate, and phenyl acrylate; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isopropyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, lauryl methacrylate, phenyl methacrylate, diethylaminoethyl methacrylate, and dimethylaminoethyl methacrylate.

In this specification, the term "(meth)acrylic acid ester monomer" is a general term for "acrylic acid ester monomer" and "methacrylic acid ester monomer" and means one or both of them. For example, "methyl (meth)acrylate" means one or both of "methyl acrylate" and "methyl methacrylate".

The (meth)acrylic acid ester monomer may be one or more kinds. For example, it is possible to form a copolymer using a styrene monomer and two or more kinds of acrylic acid ester monomers, to form a copolymer using a styrene monomer and two or more kinds of methacrylic acid ester monomers, or to form a copolymer using a styrene monomer in combination with an acrylic acid ester monomer and a methacrylic acid ester monomer.

From the viewpoint of easy control of plasticity, the weight average molecular weight (Mw) of the above styrene-acrylic resin is preferably in the range of 5,000 to 150,000, and more preferably in the range of 30,000 to 120,000.

The styrene-acrylic resin used in the present invention may be a commercially available product, and an example of such a resin is MS resin "TX320XL" manufactured by Denka Co., Ltd.

<Rubber particles>
In the base film according to the present invention, particularly when a (meth)acrylic resin or a styrene-(meth)acrylate copolymer is used, it is preferable that the base film contains rubber particles in a range of 40 to 85 mass % from the viewpoint of imparting toughness (suppleness) and improving fold mark resistance.

The rubber particles are particles containing a rubber-like polymer. The rubber-like polymer is a soft crosslinked polymer with a glass transition temperature of 20°C or less. Examples of such crosslinked polymers include butadiene-based crosslinked polymers, (meth)acrylic crosslinked polymers, and organosiloxane-based crosslinked polymers. Among these, (meth)acrylic crosslinked polymers are preferred, and acrylic crosslinked polymers (acrylic rubber-like polymers) are more preferred, from the viewpoint of having a small refractive index difference with (meth)acrylic resins and being less likely to impair the transparency of the substrate film.

In other words, the rubber particles are preferably particles containing an acrylic rubber-like polymer (a).

Regarding the acrylic rubber polymer (a):
The acrylic rubber-like polymer (a) is a crosslinked polymer containing a structural unit derived from an acrylic acid ester as a main component. The term "containing as a main component" means that the content of the structural unit derived from an acrylic acid ester is within the range described below. The acrylic rubber-like polymer (a) is preferably a crosslinked polymer containing a structural unit derived from an acrylic acid ester, a structural unit derived from another monomer copolymerizable therewith, and a structural unit derived from a polyfunctional monomer having two or more radical polymerizable groups (non-conjugated reactive double bonds) in one molecule.

The acrylic acid ester is preferably an alkyl acrylate ester having an alkyl group of 1 to 12 carbon atoms, such as methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, sec-butyl acrylate, isobutyl acrylate, benzyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, or n-octyl acrylate. The acrylic acid ester may be one type or two or more types.

The content of structural units derived from acrylic acid esters is preferably in the range of 40 to 80 mass % of all structural units constituting the acrylic rubber-like polymer (a1), and more preferably in the range of 50 to 80 mass %. When the content of acrylic acid esters is within the above range, it is easy to impart sufficient toughness to the protective film.

The copolymerizable other monomer is a monomer copolymerizable with the acrylic acid ester other than a polyfunctional monomer. In other words, the copolymerizable monomer does not have two or more radically polymerizable groups. Examples of the copolymerizable monomer include methacrylic acid esters such as methyl methacrylate; styrenes such as styrene and methylstyrene; (meth)acrylonitriles; (meth)acrylamides; and (meth)acrylic acid. Of these, it is preferable that the copolymerizable other monomer includes a styrene. The copolymerizable other monomer may be one type or two or more types.

The content of structural units derived from other copolymerizable monomers is preferably in the range of 5 to 55% by mass, and more preferably in the range of 10 to 45% by mass, based on the total structural units constituting the acrylic rubber-like polymer (a).

Examples of polyfunctional monomers include allyl (meth)acrylate, triallyl cyanurate, triallyl isocyanurate, diallyl phthalate, diallyl malate, divinyl adipate, divinyl benzene, ethylene glycol di(meth)acrylate, diethylene glycol (meth)acrylate, triethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tetra(meth)acrylate, dipropylene glycol di(meth)acrylate, and polyethylene glycol di(meth)acrylate.

The content of the structural units derived from the polyfunctional monomer is preferably in the range of 0.05 to 10% by mass, and more preferably in the range of 0.1 to 5% by mass, based on the total structural units constituting the acrylic rubber-like polymer (a). If the content of the polyfunctional monomer is 0.05% by mass or more, the degree of crosslinking of the resulting acrylic rubber-like polymer (a) is easily increased, so that the hardness and rigidity of the resulting substrate film are not excessively impaired, and if it is 10% by mass or less, the toughness of the substrate film is not easily impaired.

The monomer composition constituting the acrylic rubber-like polymer (a) can be measured, for example, by the peak area ratio detected by pyrolysis GC-MS.

The glass transition temperature (Tg) of the rubbery polymer is preferably 0°C or lower, and more preferably -10°C or lower. If the glass transition temperature (Tg) of the rubbery polymer is 0°C or lower, the film can be given appropriate toughness. The glass transition temperature (Tg) of the rubbery polymer is measured in the same manner as described above.

The glass transition temperature (Tg) of the rubbery polymer can be adjusted by the composition of the rubbery polymer. For example, in order to lower the glass transition temperature (Tg) of the acrylic rubbery polymer (a), it is preferable to increase the mass ratio of the acrylic acid ester having an alkyl group with 4 or more carbon atoms to the other copolymerizable monomer in the acrylic rubbery polymer (a) (for example, 3 or more, preferably in the range of 4 to 10).

The particles containing the acrylic rubber-like polymer (a) may be particles made of the acrylic rubber-like polymer (a), or particles having a hard layer made of a hard crosslinked polymer (c) having a glass transition temperature of 20°C or higher and a soft layer made of the acrylic rubber-like polymer (a) arranged around it (these are also called "elastomers"); or particles made of an acrylic graft copolymer obtained by polymerizing a mixture of monomers such as methacrylic acid esters in at least one stage in the presence of the acrylic rubber-like polymer (a). The particles made of the acrylic graft copolymer may be core-shell type particles having a core containing the acrylic rubber-like polymer (a) and a shell covering the core.

Regarding core-shell type rubber particles containing acrylic rubber-like polymer:
(Core part)
The core portion contains an acrylic rubber-like polymer (a) and may further contain a hard crosslinked polymer (c) as necessary. That is, the core portion may have a soft layer made of an acrylic rubber-like polymer and a hard layer made of a hard crosslinked polymer (c) disposed inside the soft layer.

The crosslinked polymer (c) may be a crosslinked polymer mainly composed of a methacrylic acid ester. In other words, the crosslinked polymer (c) is preferably a crosslinked polymer containing a structural unit derived from an alkyl methacrylic acid ester, a structural unit derived from another monomer copolymerizable therewith, and a structural unit derived from a polyfunctional monomer.

The methacrylic acid alkyl ester may be the above-mentioned methacrylic acid alkyl ester; the other copolymerizable monomer may be the above-mentioned styrenes or acrylic acid esters; the polyfunctional monomer may be the same as the above-mentioned polyfunctional monomers.

The content of structural units derived from methacrylic acid alkyl esters may be in the range of 40 to 100% by mass relative to all structural units constituting the crosslinked polymer (c). The content of structural units derived from other copolymerizable monomers may be in the range of 60 to 0% by mass relative to all structural units constituting the other crosslinked polymer (c). The content of structural units derived from polyfunctional monomers may be in the range of 0.01 to 10% by mass relative to all structural units constituting the other crosslinked polymer.

(Shell part)
The shell portion contains a methacrylic polymer (b) (another polymer) containing, as a main component, a structural unit derived from a methacrylic acid ester, which is graft-bonded to the acrylic rubber-like polymer (a). "Containing as a main component" means that the content of the structural unit derived from a methacrylic acid ester falls within the range described below.

The methacrylic acid ester constituting the methacrylic polymer (b) is preferably a methacrylic acid alkyl ester having an alkyl group of 1 to 12 carbon atoms, such as methyl methacrylate. The methacrylic acid ester may be one type or two or more types.

The content of the methacrylic acid ester is preferably 50% by mass or more relative to all structural units constituting the methacrylic polymer (b). When the content of the methacrylic acid ester is 50% by mass or more, compatibility with a methacrylic resin containing structural units derived from methyl methacrylate as a main component is easily obtained. From the above viewpoint, the content of the methacrylic acid ester is more preferably 70% by mass or more relative to all structural units constituting the methacrylic polymer (b).

The methacrylic polymer (b) may further contain structural units derived from other monomers copolymerizable with the methacrylic acid ester. Examples of other copolymerizable monomers include acrylic acid esters such as methyl acrylate, ethyl acrylate, and n-butyl acrylate; and (meth)acrylic monomers having an alicyclic, heterocyclic, or aromatic ring (ring-containing (meth)acrylic monomers) such as benzyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and phenoxyethyl (meth)acrylate.

The content of structural units derived from copolymerizable monomers is preferably 50% by mass or less, and more preferably 30% by mass or less, of all structural units constituting the methacrylic polymer (b).

In this embodiment, when the base film is not stretched, the shape of the rubber particles may be close to a perfect sphere. In other words, when observing the cross section or surface of the base film, the aspect ratio of the rubber particles may be about 1 to 2.

The average particle size of the rubber particles is preferably in the range of 100 to 400 nm. If the average particle size of the rubber particles is 100 nm or more, it is easy to impart sufficient toughness and stress relaxation properties to the base film, and if it is 400 nm or less, the transparency of the base film is less likely to be impaired. From the same viewpoint, it is more preferable that the average particle size of the rubber particles is in the range of 150 to 300 nm.

The average particle size of rubber particles can be calculated using the following method:

The average particle size of rubber particles can be measured as the average of the circle-equivalent diameters of 100 particles obtained by SEM or TEM photography of the surface or slice of the base film. The circle-equivalent diameter can be determined by converting the projected area of the particle obtained by photography into the diameter of a circle having the same area. In this case, the rubber particles observed by SEM observation and/or TEM observation at a magnification of 5000 times are used to calculate the average particle size.

The amount of rubber particles contained is not particularly limited, but is preferably in the range of 40 to 85% by mass, and more preferably in the range of 45 to 75% by mass, relative to the base film.

<Cycloolefin Resin>
The cycloolefin resin used in the substrate film is preferably a polymer of a cycloolefin monomer, or a copolymer of a cycloolefin monomer and another copolymerizable monomer.

The cycloolefin monomer is preferably a cycloolefin monomer having a norbornene skeleton, and more preferably a cycloolefin monomer having a structure represented by the following general formula (A-1) or (A-2).

In general formula (A-1), R ¹ to R ⁴ each independently represent a hydrogen atom, a hydrocarbon group having 1 to 30 carbon atoms, or a polar group. p represents an integer of 0 to 2. However, R ¹ to R ⁴ do not all represent hydrogen atoms at the same time, R ¹ and R ² do not both represent hydrogen atoms, and R ³ and R ⁴ do not both represent hydrogen atoms.

In general formula (A-1), the hydrocarbon group having 1 to 30 carbon atoms represented by R ¹ to R ⁴ is, for example, preferably a hydrocarbon group having 1 to 10 carbon atoms, and more preferably a hydrocarbon group having 1 to 5 carbon atoms. The hydrocarbon group having 1 to 30 carbon atoms may further have a linking group containing, for example, a halogen atom, an oxygen atom, a nitrogen atom, a sulfur atom, or a silicon atom. Examples of such linking groups include divalent polar groups such as a carbonyl group, an imino group, an ether bond, a silyl ether bond, and a thioether bond. Examples of the hydrocarbon group having 1 to 30 carbon atoms include a methyl group, an ethyl group, a propyl group, and a butyl group.

Examples of the polar group represented by R ¹ to R ⁴ in general formula (A-1) include a carboxy group, a hydroxy group, an alkoxy group, an alkoxycarbonyl group, an aryloxycarbonyl group, an amino group, an amide group, and a cyano group. Among them, a carboxy group, a hydroxy group, an alkoxycarbonyl group, and an aryloxycarbonyl group are preferred, and from the viewpoint of ensuring solubility during solution casting, an alkoxycarbonyl group and an aryloxycarbonyl group are preferred.

In general formula (A-1), p is preferably 1 or 2 from the viewpoint of increasing the heat resistance of the optical film. When p is 1 or 2, the resulting polymer becomes bulky and the glass transition temperature is likely to be improved. In addition, there is an advantage that it becomes somewhat responsive to humidity, making it easier to control the curl balance of the laminate.

In formula (A-2), ^R5 represents a hydrogen atom, a hydrocarbon group having 1 to 5 carbon atoms, or an alkylsilyl group having an alkyl group having 1 to 5 carbon atoms. ^R6 represents a carboxy group, a hydroxy group, an alkoxycarbonyl group, an aryloxycarbonyl group, an amino group, an amido group, a cyano group, or a halogen atom (a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom). p represents an integer of 0 to 2.

In formula (A-2), R ⁵ preferably represents a hydrocarbon group having 1 to 5 carbon atoms, and more preferably represents a hydrocarbon group having 1 to 3 carbon atoms.

In formula (A-2), R ⁶ preferably represents a carboxy group, a hydroxy group, an alkoxycarbonyl group or an aryloxycarbonyl group, and from the viewpoint of ensuring solubility during solution casting, an alkoxycarbonyl group or an aryloxycarbonyl group is more preferable.

In general formula (A-2), p preferably represents 1 or 2 from the viewpoint of improving the heat resistance of the optical film. When p represents 1 or 2, the resulting polymer becomes bulky, and the glass transition temperature is likely to be improved.

Cycloolefin monomers having a structure represented by general formula (A-2) are preferred from the viewpoint of improving solubility in organic solvents. In general, by breaking the symmetry of an organic compound, the crystallinity is reduced, and the solubility in organic solvents is improved. ^R5 and ^R6 in general formula (A-2) are substituted only on the ring-constituting carbon atoms on one side of the axis of symmetry of the molecule, so that the symmetry of the molecule is low. In other words, cycloolefin monomers having a structure represented by general formula (A-2) have high solubility, and are therefore suitable for producing optical films by a solution casting method.

The content of the cycloolefin monomer having the structure represented by general formula (A-2) in the polymer of cycloolefin monomers can be, for example, 70 mol% or more, preferably 80 mol% or more, and more preferably 100 mol% relative to the total of all cycloolefin monomers constituting the cycloolefin resin. If a certain amount or more of the cycloolefin monomer having the structure represented by general formula (A-2) is contained, the orientation of the resin is increased, and the phase difference (retardation) value is likely to increase.

Specific examples of cycloolefin monomers having a structure represented by general formula (A-1) are shown below as exemplary compounds 1 to 14, and specific examples of cycloolefin monomers having a structure represented by general formula (A-2) are shown below as exemplary compounds 15 to 34.

Examples of copolymerizable monomers that can be copolymerized with cycloolefin monomers include copolymerizable monomers that can be ring-opening copolymerized with cycloolefin monomers, and copolymerizable monomers that can be addition-copolymerized with cycloolefin monomers.

Examples of copolymerizable monomers capable of ring-opening copolymerization include cycloolefins such as cyclobutene, cyclopentene, cycloheptene, cyclooctene, and dicyclopentadiene.

Examples of copolymerizable monomers capable of addition copolymerization include unsaturated double bond-containing compounds, vinyl cyclic hydrocarbon monomers, and (meth)acrylates. Examples of unsaturated double bond-containing compounds include olefin compounds having 2 to 12 carbon atoms (preferably 2 to 8), examples of which include ethylene, propylene, and butene. Examples of vinyl cyclic hydrocarbon monomers include vinylcyclopentene monomers such as 4-vinylcyclopentene and 2-methyl-4-isopropenylcyclopentene. Examples of (meth)acrylates include alkyl (meth)acrylates having 1 to 20 carbon atoms, such as methyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate.

The content of the cycloolefin monomer in a copolymer of a cycloolefin monomer and a copolymerizable monomer can be, for example, in the range of 20 to 80 mol %, preferably 30 to 70 mol %, based on the total of all monomers constituting the copolymer.

As mentioned above, cycloolefin resins are polymers obtained by polymerizing or copolymerizing cycloolefin monomers having a norbornene skeleton, preferably cycloolefin monomers having a structure represented by general formula (A-1) or (A-2), examples of which include the following:

1) Ring-opening polymer of cycloolefin monomer; 2) Ring-opening copolymer of cycloolefin monomer and a copolymerizable monomer capable of ring-opening copolymerization therewith; 3) Hydrogenated product of ring-opening (co)polymer of 1) or 2) above; 4) (co)polymer obtained by cyclizing ring-opening (co)polymer of 1) or 2) above by Friedel-Crafts reaction and then hydrogenating it; 5) Saturated copolymer of cycloolefin monomer and unsaturated double bond-containing compound; 6) Addition copolymer of cycloolefin monomer and vinyl cyclic hydrocarbon monomer and hydrogenated product thereof; 7) Alternating copolymer of cycloolefin monomer and (meth)acrylate.

The polymers 1) to 7) above can be obtained by known methods, for example, the methods described in JP-A-2008-107534 and JP-A-2005-227606. For example, the catalyst and solvent used in the ring-opening copolymerization 2) above can be those described in paragraphs 0019 to 0024 of JP-A-2008-107534. The catalyst used in the hydrogenation 3) and 6) above can be those described in paragraphs 0025 to 0028 of JP-A-2008-107534. The acidic compound used in the Friedel-Crafts reaction 4) above can be those described in paragraph 0029 of JP-A-2008-107534. The catalyst used in the addition polymerization 5) to 7) above can be those described in paragraphs 0058 to 0063 of JP-A-2005-227606. The alternating copolymerization reaction of 7) above can be carried out, for example, by the method described in paragraphs 0071 and 0072 of JP 2005-227606 A.

Among them, the polymers of 1) to 3) and 5) above are preferred, and the polymers of 3) and 5) above are more preferred. That is, the cycloolefin resin preferably contains at least one of the structural units represented by the following general formula (B-1) and the structural units represented by the following general formula (B-2) in that the glass transition temperature and light transmittance of the resulting cycloolefin resin can be increased, and more preferably contains only the structural unit represented by the general formula (B-2) or contains both the structural unit represented by the general formula (B-1) and the structural unit represented by the general formula (B-2). The structural unit represented by the general formula (B-1) is a structural unit derived from the cycloolefin monomer represented by the above-mentioned general formula (A-1), and the structural unit represented by the general formula (B-2) is a structural unit derived from the cycloolefin monomer represented by the above-mentioned general formula (A-2).

In formula (B-1), X represents --CH.dbd.CH-- or _{--CH.sub.2CH.sub.2--} , and ^R.sub.1 to _R.sub.4 and ^p have the same meanings as ^R.sub.1 to ^R.sub.4 and p in formula (A-1), respectively.

In formula (B-2), X represents --CH.dbd.CH-- or _{--CH.sub.2CH.sub.2--} , and ^{R.sub.5 to} _R.sub.6 and p have the same meanings as ^R.sub.5 to ^R.sub.6 and p in formula (A-2), respectively.

The cycloolefin resin used in the present invention may be a commercially available product. Examples of commercially available cycloolefin resins include Arton G (e.g., G7810, etc.), Arton F, Arton R (e.g., R4500, R4900, and R5000, etc.), and Arton RX (e.g., RX4500, etc.), all manufactured by JSR Corporation.

The intrinsic viscosity [η]inh of the cycloolefin resin, measured at 30° C., is preferably in the range of 0.2 to 5 cm ³ /g, more preferably in the range of 0.3 to 3 cm ³ /g, and even more preferably in the range of 0.4 to 1.5 cm ³ /g.

The number average molecular weight (Mn) of the cycloolefin resin is preferably in the range of 8,000 to 100,000, more preferably in the range of 10,000 to 80,000, and even more preferably in the range of 12,000 to 50,000. The weight average molecular weight (Mw) of the cycloolefin resin is preferably in the range of 20,000 to 300,000, more preferably in the range of 30,000 to 250,000, and even more preferably in the range of 40,000 to 200,000. The number average molecular weight and weight average molecular weight of the cycloolefin resin can be measured in polystyrene equivalent by the aforementioned gel permeation chromatography (GPC).

When the intrinsic viscosity [η]inh, number average molecular weight, and weight average molecular weight are within the above ranges, the cycloolefin resin has good heat resistance, water resistance, chemical resistance, mechanical properties, and moldability as a substrate film.

The glass transition temperature (Tg) of cycloolefin resins is usually 110°C or higher, preferably in the range of 110 to 350°C, more preferably in the range of 120 to 250°C, and even more preferably in the range of 120 to 220°C. If the Tg is 110°C or higher, deformation under high temperature conditions is easily suppressed. On the other hand, if the Tg is 350°C or lower, molding processing is easy and deterioration of the resin due to heat during molding processing is also easily suppressed.

The content of cycloolefin resin is preferably 70% by mass or more, and more preferably 80% by mass or more, relative to the base film.

<Particles>
When the substrate film according to the present invention uses a cycloolefin resin, it is also preferable that the substrate film contains fine particles.

As the fine particles used in the present invention, examples of inorganic compounds include silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, calcium carbonate, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, and calcium phosphate. Fine particles of organic compounds can also be preferably used. Examples of organic compounds include polytetrafluoroethylene, cellulose acetate, polystyrene, polymethyl methacrylate, polypropyl methacrylate, polymethyl acrylate, polyethylene carbonate, acrylic styrene resins, silicone resins, polycarbonate resins, benzoguanamine resins, melamine resins, polyolefin powders, polyester resins, polyamide resins, polyimide resins, polyethylene fluoride resins, and pulverized fractions of organic polymer compounds such as starch, and polymer compounds synthesized by suspension polymerization.

The fine particles preferably contain silicon because they reduce turbidity, and silicon dioxide is particularly preferred. For example, Aerosil R972, R972V, R974, R812, 200, 200V, 300, R202, OX50, and TT600 (all manufactured by Nippon Aerosil Co., Ltd.) are commercially available and can be used.

<Polyimide resin>
The polyimide resin may be a polymerization reaction product of a tetracarboxylic dianhydride and a diamine.

The tetracarboxylic dianhydride may be any of aromatic tetracarboxylic dianhydrides, aliphatic tetracarboxylic dianhydrides, and alicyclic tetracarboxylic dianhydrides, but is preferably an aromatic tetracarboxylic dianhydride. The diamine may be any of aromatic diamines, aliphatic diamines, and alicyclic diamines, but is preferably an aromatic diamine.

The weight-average molecular weight Mw of the polyimide resin is not particularly limited, but from the viewpoint of increasing the toughness of the base film and making it less likely to break due to conveying tension, it is preferably in the range of 100,000 to 300,000, and more preferably in the range of 130,000 to 250,000. The method for measuring the weight-average molecular weight Mw of the polyimide resin is the same as described above.

The content of polyimide resin is preferably 60% by mass or more, and more preferably 70% by mass or more, relative to the base film.

[1.3] Physical properties <Retardation Ro and Rt>
The substrate film according to the present invention can function as an optical film such as a retardation film by being laminated on the surface of a polarizer.

When the substrate film is used as a retardation film for IPS mode, for example, the in-plane retardation Ro measured at a measurement wavelength of 590 nm and in an environment of 23°C and 55% RH is preferably in the range of 0 to 10 nm, more preferably in the range of 0 to 5 nm. The retardation Rt in the thickness direction of the substrate film is preferably in the range of -40 to 40 nm, more preferably in the range of -25 to 25 nm.

Ro and Rt are defined by the following formulas:

Formula (a): Ro = ( _nx - _ny ) x d
Formula (b): Rt=((n _x + n _y )/2-n _z )×d
(Wherein,
_nx represents the refractive index in the in-plane slow axis direction of the base film (the direction in which the refractive index is maximum),
n _y represents the refractive index in a direction perpendicular to the in-plane slow axis of the substrate film,
n _z represents the refractive index in the thickness direction of the substrate film,
d represents the film thickness (nm) of the base film.

The in-plane slow axis of the substrate film can be confirmed using an automatic birefringence meter Axoscan (Axo Scan Mueller Matrix Polarimeter, manufactured by Axometrics).

Ro and Rt can be measured using the following method:

1) The substrate film is conditioned for 24 hours in an environment of 23°C and 55% RH. The average refractive index of this film is measured using an Abbe refractometer, and the film thickness d is measured using a commercially available micrometer.

2) After humidity conditioning, the retardation Ro and Rt of the film at a measurement wavelength of 590 nm are measured in an environment of 23°C and 55% RH using an automatic birefringence meter Axoscan (Axo Scan Mueller Matrix Polarimeter, manufactured by Axometrics).

The phase differences Ro and Rt of the substrate film can be adjusted, for example, by the type of resin, the stretching conditions, and the drying conditions. For example, Rt can be lowered by increasing the drying temperature.

[1.4] Method for Producing Substrate Film The form of the substrate film according to the present invention is not particularly limited, and may be, for example, a strip-like form. That is, the substrate film according to the present invention is preferably wound into a roll in a direction perpendicular to its width direction to form a roll body.

[Production method]
The method for producing a base film according to the present invention includes the steps of 1) obtaining a solution for a base film, 2) applying the obtained base film solution to the surface of a support, and 3) removing the solvent from the applied solution for a base film to form a base film.

1) Step of Obtaining a Solution for a Substrate Film A solution for a substrate film (also called a "dope") containing the above-mentioned resin and a solvent is prepared.

The solvent used in the base film solution is not particularly limited as long as it can disperse or dissolve the resin well. For example, organic solvents used in the present invention include alcohols (methanol, ethanol, diols, triols, tetrafluoropropanol, etc.), glycols, cellosolves, ketones (acetone, methyl ethyl ketone, etc.), carboxylic acids (formic acid, acetic acid, etc.), carbonates (ethylene carbonate, propylene carbonate, etc.), esters (ethyl acetate, propyl acetate, etc.), ethers (isopropyl ether, THF, etc.), amides (dimethyl sulfoxide, etc.), hydrocarbons (heptane, etc.), nitriles (acetonitrile, etc.), aromatics (cyclohexylbenzene, toluene, xylene, chlorobenzene, etc.), alkyl halides (dichloromethane (also called "methylene chloride"), etc.), amines (1,4-diazabicyclo[2.2.2]octane, diazabicycloundecene, etc.), lactones, etc.

Among them, it is preferable that the solvent for the base film has a boiling point of 100°C or less under atmospheric pressure, is a chlorine-based solvent, and more specifically, is dichloromethane (also called "methylene chloride"), from the viewpoint of ease of handling when preparing a dope for the base film and forming a film. This is preferable from the viewpoint of high solubility and fast drying speed, which allows the film quality of the coating film to be adjusted when preparing a dope for the base film and forming a film. It is also possible to add a hydrophilic solvent. Examples of hydrophilic solvents include ketones and alcohols, and alcohols are preferable. More preferable are isopropanol, ethanol, methanol, etc., and most preferable is methanol. The amount added is preferably in the range of 1 to 20 mass%, more preferably in the range of 3 to 10 mass%.

From the viewpoint of making it easier to adjust the viscosity to the range described below, the resin concentration of the solution for the base film is preferably in the range of, for example, 1.0 to 20% by mass. Furthermore, from the viewpoint of reducing the amount of shrinkage of the coating film when it dries, the resin concentration of the solution for the base film is preferably moderately high, more preferably more than 5% by mass and not more than 20% by mass, and even more preferably more than 5% by mass and not more than 15% by mass. Furthermore, by adjusting the solution concentration, the time until the film is formed can be shortened, and the drying time can also be a means of controlling the surface condition of the base film. To increase the concentration, a mixed solvent may be used as appropriate.

The viscosity of the solution for the base film is not particularly limited as long as it is sufficient to form a base film of the desired thickness, but is preferably in the range of 5 to 5000 mPa·s, for example. If the viscosity of the solution for the base film is 5 mPa·s or more, a base film of an appropriate thickness is easily formed, and if it is 5000 mPa·s or less, the occurrence of unevenness in the film thickness due to an increase in the viscosity of the solution can be suppressed. From the same viewpoint, the viscosity of the solution for the base film is more preferably in the range of 100 to 1000 mPa·s. The viscosity of the solution for the base film can be measured at 25°C using an E-type viscometer.

2) Step of applying the solution for the base film Next, the obtained solution for the base film is applied to the surface of the support. Specifically, the obtained solution for the base film is coated on the surface of the support. The laminate of the support and the base film is also called a "laminate film."

<Support>
The support is a film that supports the substrate film during its formation, and typically includes a resin film. The thickness of the support is preferably 50 μm or less. Although the support is thin, it needs to have a certain degree of strength (flexibility and rigidity) as a support, so the thickness of the support is preferably in the range of 15 to 45 μm, and more preferably in the range of 20 to 40 μm.

Examples of resins that can be used include cellulose ester resins, cycloolefin resins, polypropylene resins, acrylic resins, polyester resins, polyarylate resins, and styrene resins or composite resins thereof. Among these, it is preferable to use polyester resins, which have excellent storage stability in high humidity environments.

Examples of resin films include polyester-based resins (e.g., polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), etc.). Among these, polyester-based resin films containing polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are preferred from the viewpoint of ease of handling.

The resin film may be heat-treated (heat-relaxed) or stretched.

The purpose of the heat treatment is to reduce residual stress in the resin film (such as residual stress due to stretching), and although there are no particular limitations, it can be performed in the range of (Tg + 60) to (Tg + 180) °C, where Tg is the glass transition temperature of the resin that constitutes the resin film.

The stretching process is performed to increase the residual stress of the resin film, and is preferably performed, for example, in two axial directions of the resin film. The stretching process can be performed under any conditions, for example, at a stretch ratio of about 120 to 900%. Whether the resin film is stretched can be confirmed, for example, by whether there is an in-plane slow axis (an axis extending in the direction in which the refractive index is maximum). The stretching process may be performed before or after laminating the base film, but it is preferable to stretch the resin film before laminating it.

The polyester resin film (also simply called polyester film) can be a commercially available product, for example, polyethylene terephthalate film TN100 (manufactured by Toyobo Co., Ltd.) or MELINEX ST504 (manufactured by Teijin DuPont Films Co., Ltd.) can be suitably used.

The support may further have a release layer provided on the surface of the resin film. The release layer can make it easier to peel the support from the base film when producing the cover member.

The release layer may contain a known release agent and is not particularly limited. Examples of release agents contained in the release layer include silicone-based release agents and non-silicone-based release agents.

Examples of silicone-based release agents include known silicone-based resins. Examples of non-silicone-based release agents include long-chain alkyl pendant polymers obtained by reacting long-chain alkyl isocyanates with polyvinyl alcohol or ethylene-vinyl alcohol copolymers, olefin-based resins (e.g., copolymerized polyethylene, cyclic polyolefins, polymethylpentene), polyarylate resins (e.g., polycondensates of aromatic dicarboxylic acid components and dihydric phenol components), fluororesins (e.g., polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), PFA (copolymer of tetrafluoroethylene and perfluoroalkoxyethylene), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), ETFE (copolymer of tetrafluoroethylene and ethylene)), etc.

The thickness of the release layer is not particularly limited as long as it is thick enough to exhibit the desired releasability, but it is preferably in the range of 0.1 to 1.0 μm, for example.

The support may contain a plasticizer as an additive. The plasticizer is not particularly limited, but is preferably selected from polyhydric alcohol ester plasticizers, phthalate ester plasticizers, citric acid plasticizers, fatty acid ester plasticizers, phosphate ester plasticizers, polycarboxylic acid ester plasticizers, polyester plasticizers, etc.

The support may also contain an ultraviolet absorber. Examples of ultraviolet absorbers that may be used include benzotriazole-based, 2-hydroxybenzophenone-based, and salicylic acid phenyl ester-based ones. Examples include triazoles such as 2-(5-methyl-2-hydroxyphenyl)benzotriazole, 2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, and 2-(3,5-di-t-butyl-2-hydroxyphenyl)benzotriazole, and benzophenones such as 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-octoxybenzophenone, and 2,2'-dihydroxy-4-methoxybenzophenone.

Furthermore, the support used in the present invention preferably contains fine particles to improve transportability.

Examples of fine particles that can be used include inorganic compounds such as silicon dioxide, titanium dioxide, aluminum oxide, zirconium oxide, calcium carbonate, calcium carbonate, talc, clay, calcined kaolin, calcined calcium silicate, hydrated calcium silicate, aluminum silicate, magnesium silicate, and calcium phosphate. Fine particles of organic compounds can also be used. Examples of organic compounds that can be used include polytetrafluoroethylene, cellulose acetate, polystyrene, polymethyl methacrylate, polypropyl methacrylate, polymethyl acrylate, polyethylene carbonate, acrylic styrene resins, silicone resins, polycarbonate resins, benzoguanamine resins, melamine resins, polyolefin powders, polyester resins, polyamide resins, polyimide resins, polyethylene fluoride resins, and pulverized fractions of organic polymer compounds such as starch, and polymer compounds synthesized by suspension polymerization.

The fine particles preferably contain silicon because they reduce turbidity, and silicon dioxide is particularly preferred. For example, Aerosil R972, R972V, R974, R812, 200, 200V, 300, R202, OX50, and TT600 (all manufactured by Nippon Aerosil Co., Ltd.) are commercially available and can be used.

The support used in the present invention can be manufactured by the usual methods such as inflation, T-die, calendar, cutting, casting, emulsion, and hot press. However, from the viewpoints of suppressing coloration, foreign matter defects, and optical defects such as die lines, the solution casting and melt casting methods are preferred as film-forming methods. Furthermore, with the solution casting method, the temperature during the processing is low, which makes it possible to impart high functionality by using various additives.

When forming a film by the solution casting method, the method for manufacturing the support preferably includes a step of preparing a dope by dissolving and dispersing a thermoplastic resin and additives such as the above-mentioned fine particles in a solvent (dissolving step; dope preparation step), a step of casting the dope onto an endless metal support that moves infinitely (casting step), a step of drying the cast dope as a web (solvent evaporation step), a step of peeling it off from the metal support (peeling step), a step of drying, stretching, and maintaining the width (stretching, maintaining the width, and drying step), and a step of winding up the finished film into a roll (winding step).

It is preferable to form the substrate film of the present invention by the following method using the support produced as described above.

The method for applying the solution for the substrate film is not particularly limited, and may be a known method such as back-roll coating, gravure coating, spin coating, wire bar coating, or roll coating. Among these, the back-coating method is preferred from the viewpoint of forming a coating film that is thin and has a uniform thickness.

3) Step of forming a base film Next, the solvent is removed from the base film solution applied to the support to form a base film.

Specifically, the solution for the base film applied to the support is dried. Drying can be performed, for example, by blowing air or heating. In particular, drying by blowing air is preferable from the viewpoint of easily suppressing curling of the base film, and furthermore, it is preferable to have a difference in wind speed between the initial stage and the latter stage of drying in terms of controlling the film thickness deviation described below. Specifically, the higher the initial wind speed, the larger the film thickness deviation, and the lower the initial wind speed, the smaller the film thickness deviation tends to be.

Therefore, by adjusting the drying conditions (e.g., drying temperature, drying air volume, drying time, etc.), the density of the base film can be controlled and the film thickness of the base film can be adjusted to satisfy the following formula 1.

Formula 1: 5<|Average thickness of three randomly selected points in the width direction (B)−average thickness (A)|/average thickness (A)×100<20(%)
Here, the average film thickness (A) is the average of film thickness values at 10 points randomly selected from the film.

When the value represented by formula 1 exceeds 5%, the surface has a moderate unevenness, which has the effect of improving adhesion to the upper layer, and when it is less than 20%, the surface unevenness is not too large, so it does not affect the coatability and smoothness of the upper layer.

The film thickness can be measured using a commercially available film thickness measuring device, for example, the F20-UV (manufactured by Filmetrics) as a film thickness measuring system.

The thickness deviation of the base film is preferably adjusted to within a range of 0.5±0.2 μm. It is preferable to adjust the film quality to be coarser from the viewpoint of improving adhesion to the upper layer, and specifically, it is preferable to increase the drying speed, preferably 0.0015 to 0.05 kg/hr· ^m2 , and more preferably 0.002 to 0.05 kg/hr· ^m2 .

The drying rate is expressed as the mass of solvent that evaporates per unit time and unit area. The drying rate can usually be adjusted by the drying temperature. The drying temperature depends on the type of solvent used, but can be, for example, in the range of 50 to 200°C ((Tb-50) to (Tb+50)°C, where Tb is the boiling point of the solvent used). Temperature control may be performed in multiple stages. After a certain degree of drying has progressed, the drying rate and film quality can be controlled by drying at a higher temperature.

As described above, the base film according to this embodiment can be in a strip shape. Therefore, it is preferable that the method for producing a laminated film according to this embodiment further includes a step 4) of winding the strip-shaped laminated film into a roll to form a roll body.

4) Step of Winding the Substrate Film to Obtain a Roll The obtained strip-shaped substrate film is wound into a roll in a direction perpendicular to its width direction to obtain a roll.

The length of the strip-shaped base film is not particularly limited, but can be, for example, about 100 to 10,000 m. The width of the strip-shaped laminate film is preferably 1 m or more, and more preferably in the range of 1.1 to 4 m. From the viewpoint of improving the uniformity of the film, it is more preferably in the range of 1.3 to 2.5 m.

[Manufacturing equipment]
The method for producing the substrate film used in the present invention can be carried out, for example, by using a production apparatus shown in FIG.

Figure 3 is a schematic diagram of a manufacturing apparatus B200 for carrying out the method for manufacturing a base film according to this embodiment. The manufacturing apparatus B200 has a supply section B210, a coating section B220, a drying section B230, a cooling section B240, and a winding section B250. Ba to Bd indicate transport rolls that transport the support B110.

The supply unit B210 has a payout device (not shown) that pays out the roll body B201 of the belt-shaped support B110 wound around a core.

The coating section B220 is a coating device that includes a backup roll B221 that holds the support B110, a coating head B222 that applies a solution for the base film to the support B110 held by the backup roll B221, and a reduced pressure chamber B223 provided upstream of the coating head B222.

The flow rate of the solution for the base film discharged from the coating head B222 can be adjusted by a pump (not shown). The flow rate of the solution for the base film discharged from the coating head B222 is set to an amount that can stably form a coating layer of a predetermined thickness when continuously coated under the previously adjusted conditions of the coating head B222.

The reduced pressure chamber B223 is a mechanism for stabilizing the bead (a pool of coating liquid) formed between the substrate film solution from the coating head B222 and the support B110 during coating, and the degree of reduced pressure is adjustable. The reduced pressure chamber B223 is connected to a reduced pressure blower (not shown) so that the interior can be reduced in pressure. The reduced pressure chamber B223 is in a state where there is no air leakage, and the gap with the backup roll is adjusted to be narrow, so that a stable bead of coating liquid can be formed.

The drying section B230 is a drying device that dries the coating applied to the surface of the support B110, and has a drying chamber B231, an inlet B232 for drying gas, and an outlet B233. The temperature and volume of the drying air are determined appropriately depending on the type of coating and the type of support B110. By setting conditions such as the temperature and volume of the drying air and the drying time in the drying section B230, the amount of residual solvent in the coating after drying can be adjusted. The amount of residual solvent in the coating after drying can be measured by comparing the unit mass of the coating after drying with the mass of the coating after it has been thoroughly dried.

(Residual Solvent Amount)
Since the base film is obtained by applying a solution for the base film, the solvent derived from the solution may remain in the base film. The amount of the residual solvent can be controlled by the concentration of the solvent and the coating solution used, the wind speed for drying the base film, the drying temperature and time, the conditions of the drying room (open air or internal air circulation), the heating temperature of the back roll during application, etc.

As mentioned above, rapid drying makes the film sparser, allowing the surface condition to be controlled.

From the viewpoint of curl balance of the base film, it is preferable that the residual solvent amount in the base film satisfies the following formula 2, where the residual solvent amount in the base film is _S1 .

Formula 2 10<S ₁ <1000 (ppm)
Specifically, the residual solvent content of the substrate film is more preferably less than 800 ppm, and more preferably from 500 to less than 700 ppm, taking into consideration the curl balance of the substrate film. In addition, by selecting a solvent and coating process that leaves the solvent on the substrate, the adhesion between the substrate and the substrate film is improved. The residual solvent content of the substrate is preferably in the range of 10 to 100 ppm.

The amount of residual solvent in the support and base film can be measured by headspace gas chromatography. In headspace gas chromatography, a sample is sealed in a container, heated, and when the container is filled with volatile components, the gas in the container is quickly injected into a gas chromatograph, and mass analysis is performed to identify the compounds and quantify the volatile components. The headspace method makes it possible to observe all peaks of the volatile components using a gas chromatograph, and by using an analytical method that utilizes electromagnetic interactions, it is also possible to quantify volatile substances, monomers, etc. with high accuracy.

The cooling section B240 cools the temperature of the support B110 having the coating film (substrate film) obtained by drying in the drying section B230, and adjusts it to an appropriate temperature. The cooling section B240 has a cooling chamber B241, a cooling air inlet B242, and a cooling air outlet B243. The temperature and volume of the cooling air can be determined appropriately depending on the type of coating film and the type of support B110. Furthermore, if an appropriate cooling temperature can be achieved without providing the cooling section B240, the cooling section B240 may not be necessary.

The winding section B250 is a winding device (not shown) that winds up the support B110 on which the base film is formed to obtain a roll body B251.

[2] Transparent substrate Here, "transparent" means that the total light transmittance measured after humidity control in an environment of 23°C and 55% RH is 80% or more, preferably 85% or more, more preferably 90% or more, and particularly preferably 95% or more. The total light transmittance can be measured according to JIS 7573 (Plastics - Determination of total light transmittance and total light reflectance).

The transparent substrate according to the present invention is a flexible transparent substrate having a thickness of 5 μm or more and less than 50 μm. If the thickness is less than 5 μm, the strength cannot be maintained, resulting in a high rate of breakage during processing, a rapid deterioration in yield, and poor durability, such as breakage due to repeated opening and closing of the display. Furthermore, if the thickness is 50 μm or more, the overall thickness of the cover member becomes too thick, resulting in poor flexibility (such as the occurrence of cracks) and weight, so it is necessary that the thickness is within the above range. A more preferable thickness is 10 μm or more and less than 30 μm.

It is also preferable that the elastic modulus of the transparent substrate according to the present invention is within the range of 55 to 80 GPa, and that the elastic modulus ratio between the elastic modulus of the transparent substrate and the substrate film (elastic modulus of transparent substrate/elastic modulus of substrate film) is 30 or more. This is because, in order to provide a cover member that has improved pressing strength and does not leave creases using a transparent substrate with a low elastic modulus due to its thinning, it is necessary to use a substrate film with a lower elastic modulus, that is, by setting the elastic modulus ratio to the transparent substrate (elastic modulus of transparent substrate/elastic modulus of substrate film) to 30 or more, the protective function of the transparent substrate can be further improved.

<Measurement of Elastic Modulus>
The conditions for measuring the elastic modulus (also called tensile elastic modulus) of the base film and the transparent base material are set as follows, and the elastic modulus is determined by linear regression between strains of 0.05 and 0.25%.

The tensile modulus in the MD direction is measured in accordance with JIS K7127 (1999) by the following method.
1) A substrate film or a transparent substrate is cut into a size of 100 mm (MD) x 10 mm (TD) to prepare a test piece.
2) Using a Tensilon RTC-1225A manufactured by Orientec Co., Ltd., the test piece is pulled in the longitudinal direction (MD) of the test piece at a pulling speed of 50 mm/min with a chuck distance of 50 mm, and the tensile modulus in the MD is measured. The measurement is performed at 23°C and 55% RH.

[2.1] Glass Substrate Because of its excellent durability, flatness, and the like, the transparent substrate according to the present invention is preferably a thin glass substrate. Examples of such a substrate include soda-lime glass and silicate glass. Of these, silicate glass is preferred, and more specifically, silica glass or borosilicate glass is more preferred.

The glass constituting the glass substrate is preferably alkali-free glass that does not substantially contain alkaline components, specifically glass with an alkaline component content of 1000 ppm or less. The alkaline component content in the glass substrate is preferably 500 ppm or less, and more preferably 300 ppm or less. Glass substrates containing alkaline components are prone to the phenomenon of soda blowing due to the replacement of cations on the film surface. This is because the density of the film surface layer is likely to decrease, and the glass substrate is likely to break.

The glass substrate can be formed by a commonly known method, such as the float method, the down-draw method, or the overflow down-draw method. Among these, the overflow down-draw method is preferred because the surface of the glass substrate does not come into contact with the forming member during forming, and the surface of the resulting glass substrate is less likely to be scratched.

The glass substrate used in the present invention can be obtained by grinding a thick glass, such as borosilicate glass, to the desired thickness, but it is difficult to obtain a glass substrate of less than 200 μm by grinding and polishing a thick glass sheet.

Therefore, to obtain the glass substrate according to the present invention having a thickness of 5 μm or more and less than 50 μm, it is preferable to use a special float method.

The thinner the glass, the more difficult it is to handle and process the ultra-thin glass plate, the weaker the glass, and the higher the probability of breakage. In order to make it easier to handle and process thin glass substrates, a method has been proposed in which the thin glass substrate is processed while temporarily attached to a thicker support substrate (hereinafter also referred to as a "carrier substrate"), and the support substrate is peeled off as a post-processing step to obtain a thin glass substrate.

For example, according to the aforementioned International Publication No. 2017/066924, soda-lime glass having a thickness of less than 100 μm can be manufactured by the following process.

(Step 1) A step of forming a first surface of a glass substrate 22 so that it is in contact with a glass carrier substrate having a bonding surface, and attaching a contact film (also called a "contact film") having adhesive properties to a second surface opposite the first surface.

That is, as shown in FIG. 4 (step 1), the manufacturing method of the above glass substrate includes a step of pouring a material for forming the glass substrate to a desired thickness into a carrier substrate 21 having sufficient strength and a thickness that is easy to process, forming a first surface of the glass substrate 22 so that it is in contact with the carrier substrate 21, and then attaching a contact film 23 to the second surface on the opposite side of the glass substrate 22.

(Step 2) Next, the glass base material 22 is peeled off from the carrier substrate 21 by the contact film 23 having high adhesive strength (FIG. 4 (Step 2)).
(Step 3) is characterized by including a step of removing the contact film 23 from the second surface of the glass substrate 22 peeled off from the carrier substrate by a weakening treatment (electromagnetic radiation irradiation 24) that weakens the adhesive strength of the contact film (FIG. 4 (Step 3)).

That is, the contact film 23 is used to hold the glass substrate securely, and thus has a protective function for the glass substrate 22, thereby protecting, for example, the exposed surface of the glass substrate 22 from mechanical damage that may occur thereto, and allowing safe and easy handling.

The contact film can comprise a hard or soft material depending on the specific requirements, with polyethylene terephthalate (PET), polyethylene (PE) or polyolefin (PO) being preferred materials.

The contact film is typically adhered to the glass substrate by an adhesive layer formed of an adhesive on one side of the substrate. However, it is not excluded that the substrate of the contact film itself has adhesive properties.

The adhesive strength between the contact film and the second surface of the glass substrate is selected so that sufficient force can be transferred to the glass substrate during peeling by the peeling device to eliminate the bonding force between the carrier substrate and the glass substrate during compression.

The contact film may be provided as a foil or tape, which may be wound, for example, from a roll, or may be provided as a sheet. Preferably, the contact film has a thickness of 50 μm or more, more preferably 80 μm or more, even more preferably 125 μm or more, and especially preferably 150 μm or more.

The glass substrate is preferably made from an alkali-containing glass composition, as described above. Preferred glass materials are, for example, lithium aluminosilicate glass, soda-lime glass, borosilicate glass, alkali metal aluminosilicate glass, and aluminosilicate glasses with low alkali content. Alkali-free compositions are also preferred. Such glasses are preferably made by, for example, the downdraw process, the overflow downdraw process, or the float process described above.

The carrier substrate preferably has a thickness of at least 100 μm, preferably at least 300 μm, more preferably at least 500 μm, and a maximum dimension of at least 3 inches (1 inch is 2.54 cm), preferably at least 6 inches, more preferably at least 8 inches, and especially preferably at least 12 inches. In particular, the carrier substrate may have a glass substrate first generation size or larger, for example second to eighth generation, or even larger, for example substrate sizes of 1×1 m to 3×3 m. The carrier substrate may have various shapes, such as rectangular, quadratic or elliptical, or circular.

The glass substrate, together with the contact film, is peeled off from the carrier substrate by the adhesive force of the contact film, and then the contact film is peeled off to obtain a single glass substrate 22 that becomes the transparent substrate 3 shown in Figure 1.

In a preferred embodiment, before removing the contact film from the glass substrate, the contact film is preferably subjected to an adhesion weakening treatment to reduce the adhesion between the contact film and the ultra-thin substrate. The weakening treatment is preferably selected so as to reduce the adhesion to 0.5 N/25 mm or less.
The weakening treatment can be selected to be in a particular wavelength range, such as infrared or ultraviolet light or visible light. The selected electromagnetic radiation can be narrow band or cover a broader band or can be laser radiation, depending on the adhesive material used.

Preferably, it is selected outside the visible spectrum so that its adhesive strength does not deteriorate under exposure to visible light. There are a variety of adhesive materials commercially available that can be at least partially deactivated by exposure to such electromagnetic radiation, and the preferred choice will depend on the particular requirements.

Alternatively, if the adhesion of the contact film can be reduced by increasing or decreasing the temperature, it may be advantageous to perform a heat treatment as a weakening process.

The electromagnetic radiation is preferably applied from the outside of the contact film, i.e., from the side not bonded to the glass substrate.
Suitable adhesive materials are widely available, for example, under the trade name "NDS4150-20", and the corresponding weakening treatment includes exposure to ultraviolet light at a wavelength of 365 nm.

Specific examples of manufacturing glass substrates will be described in the Examples section. Thin glass manufactured by SCHOTT, Nippon Electric Glass, etc. can be used as the glass substrate described above.

[2.2] Thermoplastic resin substrate The transparent substrate according to the present invention can be a thermoplastic resin film, and the thermoplastic resin is not particularly limited, and examples thereof include cellulose ester resin, cycloolefin resin, fumaric acid diester resin, polypropylene resin, (meth)acrylic resin, polyester resin, polyarylate resin, polyimide resin, and styrene resin or a composite resin thereof. Among them, it is preferable to use a thermoplastic resin film using a polyimide resin as the transparent substrate 3 shown in FIG. 1 in terms of optical properties and physical properties.

<Polyimide resin>
Polyimide resins can be obtained, for example, by synthesizing a polyamic acid (polyimide precursor) from an acid anhydride and a diamine compound, and then imidizing the polyamic acid with heat or a catalyst.

The acid anhydride used in the synthesis of polyimide is not particularly limited, but examples thereof include aromatic tetracarboxylic dianhydrides such as biphenyltetracarboxylic dianhydride (BPDA), terphenyltetracarboxylic dianhydride, benzophenonetetracarboxylic dianhydride, pyromellitic anhydride (PMDA), oxydiphthalic dianhydride, diphenylsulfonetetracarboxylic dianhydride, hexafluoroisopropylidenediphthalic dianhydride, and cyclobutanetetracarboxylic dianhydride.

The diamine compound used in the synthesis of polyimide is not particularly limited, but examples thereof include aromatic diamines such as p-phenylenediamine (PDA), m-phenylenediamine, 2,4-diaminotoluene, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylether (ODA), 3,4'-diaminodiphenylether, 3,3'-dimethyl-4,4'-diaminobiphenyl, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 3,7-diamino-dimethyldibenzothiophene-5,5'-dioxide, 4,4'-diaminobenzophenone, 4,4'-bis(4-aminophenyl)sulfide, 4,4'-diaminobenzanilide, and 1,4-bis(4-aminophenoxy)benzene.

In addition, commercially available polyimide varnishes whose main component is a polyimide precursor, such as U-Varnish S (manufactured by Ube Industries, Ltd.) and Ecrios (manufactured by Mitsui Chemicals, Inc.), can be used.

When using ultraviolet-transmitting polyimides depending on the purpose, for example, commercially available polyimide precursors (polyamic acids) as the main component may be used, such as TORMED-S, X (manufactured by IST), Spiceria HR003, GR003 (both manufactured by Somar), FLUPI (manufactured by NTT), Type HM (manufactured by Toyobo), Type-C (manufactured by Mitsui Chemicals), PI-100 (manufactured by Maruzen Chemical), HDN-20D (manufactured by New Japan Chemical), CBDA-6FDAC (manufactured by Central Glass), and Q-VR-X-1655 (manufactured by PI Giken).

[3] Manufacturing method of cover member [3.1] Configuration example of cover member On at least one surface of the transparent substrate, the base film of the present invention (base film 1) is arranged in the configuration shown in Fig. 1A. As described later, when the display device is a display unit having a polarizing plate, it is a preferred embodiment that the base film 1 side of the cover member of the present invention is bonded to the polarizing plate via a pressure-sensitive adhesive layer.

The arrangement of the base film relative to the transparent substrate is preferably such that the A-side of the base film, which is the back side (B-side) of one side ( _A- side) of the base film, is adjusted so that the ratio (ρ _A /ρ _B ) of the film density (ρ _A ) of the A-side to the B-side film density (ρ B ) is within a range of 0.80 to 0.95.

It is preferable that the substrate film 2 described below is disposed on the surface of the transparent substrate opposite the substrate film 1 (Figure 1B).

The substrate film 1 or substrate film 2 is attached to the transparent substrate 3 by a pressure-sensitive adhesive layer or an adhesive layer.

Figure 5 is a schematic diagram showing the bonding of a substrate film with a support to a transparent substrate.

Figure 5A shows a laminate of the base film 1 formed on the aforementioned support 5, and the surface of the base film 1 in contact with the support 5 corresponds to surface B.

Next, as shown in FIG. 5B, the A-side of the base film 1 is attached to the transparent substrate 3 via the adhesive layer 4, and then the support 5 is peeled off.

[3.2] Pressure-sensitive adhesive layer The pressure-sensitive adhesive layer is preferably a pressure-sensitive adhesive composition containing a base polymer, a prepolymer and/or a crosslinkable monomer, a crosslinking agent and a solvent, which is dried and partially crosslinked. That is, the pressure-sensitive adhesive composition may be at least partially crosslinked.

Examples of pressure-sensitive adhesive compositions include acrylic pressure-sensitive adhesive compositions using a (meth)acrylic polymer as the base polymer, silicone pressure-sensitive adhesive compositions using a silicone polymer as the base polymer, and rubber pressure-sensitive adhesive compositions using rubber as the base polymer. Among these, acrylic pressure-sensitive adhesive compositions are preferred from the viewpoints of transparency, weather resistance, heat resistance, and processability.

The (meth)acrylic polymer contained in the acrylic pressure-sensitive adhesive composition may be a copolymer of an alkyl (meth)acrylate ester and a functional group-containing monomer that can be crosslinked with a crosslinking agent.

The (meth)acrylic acid alkyl ester is preferably an acrylic acid alkyl ester having an alkyl group with 2 to 14 carbon atoms.

Examples of functional group-containing monomers that can be crosslinked with the crosslinking agent include amide group-containing monomers, carboxyl group-containing monomers (e.g., acrylic acid), and hydroxyl group-containing monomers (e.g., hydroxyethyl acrylate).

Examples of crosslinking agents contained in the acrylic adhesive composition include epoxy crosslinking agents, isocyanate crosslinking agents, and peroxide crosslinking agents. The content of the crosslinking agent in the adhesive composition is usually, for example, 0.01 to 10 parts by mass per 100 parts by mass of the base polymer (solid content).

The adhesive composition may further contain various additives such as tackifiers, plasticizers, glass fibers, glass beads, metal powders, other fillers, pigments, colorants, fillers, antioxidants, UV absorbers, and silane coupling agents, as necessary.

The thickness of the adhesive layer is usually about 3 to 100 μm, and preferably in the range of 5 to 50 μm.

The surface of the adhesive layer is protected by a release film that has been treated for release. Examples of release films include plastic films such as acrylic films, polycarbonate films, polyester films, and fluororesin films.

[3.3] Adhesive Layer Instead of the above-mentioned adhesive layer, an adhesive layer can be used, and the adhesive layer 4 shown in FIG. 1 can be an adhesive layer.

The adhesive layer is disposed between the substrate film and the transparent substrate. It is also preferable to dispose it between a polarizing plate, which will be described later, to form a display device. The types of adhesive contained in the adhesive layers may be the same or different.

The adhesive layer may be a layer obtained from a water-soluble polymer, or may be a layer of a cured product of an active energy ray-curable adhesive. In the case of a water-soluble polymer, the adhesive layer may be formed, for example, via an adhesive made of a vinyl alcohol-based polymer, or an adhesive made of at least a water-soluble crosslinking agent of a vinyl alcohol-based polymer such as boric acid, borax, glutaraldehyde, melamine, or oxalic acid. Such an adhesive layer is formed as a coating and drying layer of an aqueous solution, and when preparing the aqueous solution, other additives and catalysts such as acids can be added as necessary.

The active energy ray-curable adhesive may be a photoradical polymerizable composition or a photocationically polymerizable composition. Of these, a photocationically polymerizable composition is preferred.

The photocationically polymerizable composition contains an epoxy compound and a photocationic polymerization initiator.

An epoxy compound is a compound having one or more, preferably two or more, epoxy groups in the molecule. Examples of epoxy compounds include hydrogenated epoxy compounds (glycidyl ethers of polyols having alicyclic rings) obtained by reacting epichlorohydrin with alicyclic polyols; aliphatic epoxy compounds such as polyglycidyl ethers of aliphatic polyhydric alcohols or their alkylene oxide adducts; and alicyclic epoxy compounds having one or more epoxy groups bonded to an alicyclic ring in the molecule. Only one type of epoxy compound may be used, or two or more types may be used in combination.

The photocationic polymerization initiator can be, for example, an aromatic diazonium salt; an onium salt such as an aromatic iodonium salt or an aromatic sulfonium salt; or an iron-arene complex.

The photocationic polymerization initiator may further contain additives such as cationic polymerization accelerators such as oxetane and polyol, photosensitizers, ion trapping agents, antioxidants, chain transfer agents, tackifiers, thermoplastic resins, fillers, flow control agents, plasticizers, defoamers, antistatic agents, leveling agents, and solvents, as necessary.

The thickness of the adhesive layer is not particularly limited, but is preferably in the range of 0.01 to 10 μm, and more preferably in the range of 0.01 to 5 μm.

[3.4] Base film 2
As the opposing film of the base film 1, the base film 1 may be used, but another resin film may also be used, examples of which include a cycloolefin resin, a polypropylene resin, an acrylic resin, a polyester resin, a polyarylate resin, a cellulose ester resin, a styrene resin, or a composite resin thereof, etc. Among these, it is preferable to use a resin film containing a polyester resin as a resin having excellent storage stability under a high humidity environment.

Examples of the above resin films include polyester-based resins (e.g., polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), etc.). Among these, polyester-based resin films containing polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are preferred from the viewpoint of ease of handling.

The resin film may be heat-treated (heat-relaxed) or stretched.

The purpose of the heat treatment is to reduce residual stress in the resin film (such as residual stress due to stretching), and although there are no particular limitations, it can be performed in the range of (Tg + 60) to (Tg + 180) °C, where Tg is the glass transition temperature of the resin that constitutes the resin film.

The stretching process is performed to increase the residual stress of the resin film, and is preferably performed, for example, in two axial directions of the resin film. The stretching process can be performed under any conditions, for example, at a stretch ratio of about 120 to 900%. Whether the resin film is stretched can be confirmed, for example, by whether there is an in-plane slow axis (an axis extending in the direction in which the refractive index is maximum). The stretching process may be performed before or after laminating the base film, but it is preferable to stretch the resin film before laminating it.

The polyester resin film (also simply called polyester film) can be a commercially available product, for example, polyethylene terephthalate film TN100 (manufactured by Toyobo Co., Ltd.) or MELINEX ST504 (manufactured by Teijin DuPont Films Co., Ltd.) can be suitably used.

The thickness of the base film 2 can be adjusted as needed, but from the viewpoint of making the cover member thinner, it is preferable that the thickness is within the range of 10 to 100 μm, and more preferably within the range of 10 to 50 μm.

Furthermore, the base film 2 may be provided with a functional layer such as a hard coat layer.

(Hard Coat Layer)
The polyester film that is placed on the surface of the foldable display to protect the display preferably has a hard coat layer on its surface. The hard coat layer is preferably placed on the display surface side of the polyester film and used in the display. The resin that forms the hard coat layer can be an acrylic type, a siloxane type, an inorganic hybrid type, a urethane acrylate type, a polyester acrylate type, an epoxy type, or the like, without any particular limitation. In addition, two or more types of materials can be mixed and used, or particles such as inorganic filler or organic filler can be added.

The thickness of the hard coat layer is preferably in the range of 1 to 50 μm. If it is 1 μm or more, it will harden sufficiently and provide good pencil hardness. Furthermore, by keeping the thickness at 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, improving the handling properties of the film.

The hard coat layer can be applied by any method, including wire bar, gravure coater, die coater, knife coater, etc., without any particular limitations, and can be appropriately selected depending on the viscosity and film thickness.

The hard coat layer can be cured using energy rays such as ultraviolet rays and electron beams, or heat, and curing methods using ultraviolet rays or electron beams are preferred to reduce damage to the film.

The pencil hardness of the hard coat layer is preferably B or higher, more preferably H or higher, and particularly preferably 2H or higher. With a pencil hardness of B or higher, the layer is not easily scratched and does not reduce visibility. In general, a high pencil hardness of the hard coat layer is preferable, but a hard coat layer with a pencil hardness of 9H or lower is acceptable, and even 8H or lower is acceptable, and even 6H or lower can be used without any practical problems.

The hard coat layer used in the present invention can be used for the purpose of protecting the display by increasing the pencil hardness of the surface as described above, and preferably has a high transmittance. The transmittance of the hard coat film is preferably 87% or more, and more preferably 88% or more. If the transmittance is 87% or more, sufficient visibility can be obtained. The total light transmittance of the hard coat film is generally preferably as high as possible, but may be 99% or less, or may be 97% or less. In addition, the haze of the hard coat film is generally preferably low, and preferably 3% or less. The haze of the hard coat film is more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved.

The hard coat layer may further have other functions added thereto. For example, hard coat layers having functions added thereto, such as an antiglare layer having a certain pencil hardness as described above, an antiglare antireflection layer, an antireflection layer, a low reflection layer, and an antistatic layer, are also preferably used in the present invention.

[4] Display device The display device of the present invention is characterized by comprising the cover member or the base film for the cover member of the present invention. The display device of the present invention can be obtained, for example, by bonding the cover member of the present invention to the surface of the display device via a pressure-sensitive adhesive layer or an adhesive layer, preferably via a polarizing plate. The display device is a device having a display mechanism, and includes a light-emitting element or a light-emitting device as a light-emitting source. Examples of the display device include liquid crystal display devices, organic electroluminescence (EL) display devices, inorganic electroluminescence (EL) display devices, touch panel display devices, electron emission display devices (field emission display devices (FEDs, etc.), surface field emission display devices (SEDs)), electronic paper (display devices using electronic ink or electrophoretic elements), plasma display devices, projection display devices (grating light valve (GLV) display devices, display devices having digital micromirror devices (DMDs), etc.), and piezoelectric ceramic displays. The liquid crystal display devices include any of a transmissive liquid crystal display device, a semi-transmissive liquid crystal display device, a reflective liquid crystal display device, a direct-view liquid crystal display device, and a projection liquid crystal display device. These display devices may be display devices that display two-dimensional images, or stereoscopic display devices that display three-dimensional images. In particular, the display device of the present invention is preferably an organic EL display device or a touch panel display device, and more preferably an organic EL display device.

In addition, it is preferable that the folding display has a structure in which a single continuous display can be folded in half when carried, thereby reducing the size by half and improving portability. At the same time, it is desirable that the display is thin and lightweight. Therefore, the bending radius of the folding display is preferably 5 mm or less, and more preferably 3 mm or less. If the bending radius is 5 mm or less, the display can be made thin when folded. The smaller the bending radius, the better, but it may be 0.1 mm or more, or 0.5 mm or more. Even if it is 1 mm or more, the practicality is sufficiently good compared to conventional displays that do not have a folding structure. Here, the bending radius when folded means the inner radius R of the folded part when the cover member 10 is folded in the above-mentioned FIG. 1C or FIG. 1D.

Figure 6 shows an example of application of a cover member to an organic EL display, which is an example of a display device of the present invention.

The general configuration of an organic EL display is a display unit consisting of an organic EL layer 101 consisting of an electrode/electron transport layer/light emitting layer/hole transport layer/transparent electrode, and a polarizing plate 102 equipped with a retardation plate (λ/4 plate) to improve image quality. In a preferred embodiment of the display device 100 of the present invention, the base film 1 side of the cover member 10 of the present invention is bonded to the polarizing plate 102 via an adhesive layer 4.

In this embodiment, the display device of the present invention is a display device that not only improves the handleability of the cover member, but also leaves no fold marks on the base film provided on the cover member, and in particular, as a cover member for a foldable display, it has excellent visibility after repeated folding while maintaining mass productivity, and does not cause image distortion at the folded portion of the display. For example, a mobile terminal device equipped with the foldable display provides beautiful images, is highly functional, and is convenient in terms of portability, etc.

The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these. In the examples, the terms "parts" and "%" are used, but unless otherwise specified, they represent "parts by mass" or "% by mass."

<Preparation of base film>
<Preparation of Base Film 101>
(Support)
As a support, a polyethylene terephthalate film (PET film): (TN100 manufactured by Toyobo Co., Ltd., with a release layer containing a non-silicone based release agent, film thickness 38 μm) was used.

(Preparation of solution for substrate film 101)
The following components were mixed to obtain a solution for the base film 101.

Methylene chloride (boiling point 41° C.) 800 parts by mass Acrylic 1: MMA/PMI/MADA copolymer (60/20/20 mass ratio), Mw: 1.5 million, Tg: 137° C. (abbreviations indicate the following: MMA: methyl methacrylate, PMI: phenylmaleimide, and MADA: adamantyl acrylate) 20 parts by mass Rubber particles R1: 80 parts by mass Dispersant (polyoxyethylene lauryl ether sodium phosphate: molecular weight 332) Added in an amount of 0.006% by mass in the base film

(Preparation of Base Film 101)
The solution for the base film 101 was applied onto the release layer of the above support using a die by a backcoat method, and then the base film was dried in the drying step described below to form a base film with a thickness of 5 μm, thereby obtaining the base film 101.

(Initial drying)
1st step: 1 minute at 40°C 2nd step: 1 minute at 70°C 3rd step: 1 minute at 100°C 4th step: 2 minutes at 130°C (post-drying)
5th step: 110°C for 15 minutes

<Preparation of rubber particles R1>
The rubber particles used were prepared in the following manner.

The following materials were charged into an 8 L polymerization apparatus equipped with a stirrer:

Deionized water 180 parts by weight Polyoxyethylene lauryl ether phosphate 0.002 parts by weight Boric acid 0.473 parts by weight Sodium carbonate 0.473 parts by weight Sodium hydroxide 0.008 parts by weight

After thoroughly replacing the inside of the polymerization machine with nitrogen gas, the inside temperature was raised to 80°C, and 0.021 parts by mass of potassium persulfate was added as a 2% by mass aqueous solution. Next, a mixture of 21 parts by mass of monomer mixture (c') consisting of 84.6% by mass of methyl methacrylate, 5.9% by mass of butyl acrylate, 7.9% by mass of styrene, 0.5% by mass of allyl methacrylate, and 1.1% by mass of n-octyl mercaptan and 0.07 parts by mass of polyoxyethylene lauryl ether phosphoric acid was added continuously to the above solution over a period of 63 minutes. The polymerization reaction was continued for another 60 minutes to obtain the innermost hard polymer (c).

Then, 0.021 parts by mass of sodium hydroxide was added as a 2% by mass aqueous solution, and 0.062 parts by mass of potassium persulfate was added as a 2% by mass aqueous solution. Next, a mixture of 39 parts by mass of a monomer mixture (a') consisting of 80.0% by mass of butyl acrylate, 18.5% by mass of styrene, and 1.5% by mass of allyl methacrylate and 0.25 parts by mass of polyoxyethylene lauryl ether phosphoric acid was added continuously over 117 minutes. After the addition was completed, 0.012 parts by mass of potassium persulfate was added as a 2% by mass aqueous solution, and the polymerization reaction was continued for 120 minutes to obtain a soft layer (a layer consisting of acrylic rubber-like polymer (a)). The glass transition temperature (Tg) of the soft layer was -30°C. The glass transition temperature of the soft layer was calculated by averaging the glass transition temperatures of the homopolymers of each monomer constituting the acrylic rubber-like polymer (a) according to the composition ratio.

After that, 0.04 parts by mass of potassium persulfate was added as a 2% by mass aqueous solution, and 26.1 parts by mass of a monomer mixture (b') consisting of 97.5% by mass of methyl methacrylate and 2.5% by mass of butyl acrylate was continuously added over 78 minutes. The polymerization reaction was continued for another 30 minutes to obtain polymer (b).

The obtained polymer (b) was poured into a warm aqueous solution of 3% by mass sodium sulfate, salted out and coagulated. Next, after repeated dehydration and washing, it was dried to obtain acrylic graft copolymer particles (rubber particles R1) with a three-layer structure. The average particle diameter of the obtained rubber particles R1 was 200 nm.

The average particle size of the rubber particles was measured using the following method.

(Average particle size)
The dispersed particle size of the rubber particles in the obtained dispersion was measured using a zeta potential/particle size measuring system (ELSZ-2000ZS, manufactured by Otsuka Electronics Co., Ltd.).

<Preparation of Base Films 102 to 104>
Substrate films 102 to 104 were produced in the same manner as in the production of substrate film 101, except that the content of rubber particles R1 was changed to 65, 45, and 20 mass %, respectively.

<Preparation of Base Film 105>
(Support)
As a support, a polyethylene terephthalate film (PET film): (TN100 manufactured by Toyobo Co., Ltd., with a release layer containing a non-silicone based release agent, film thickness 38 μm) was used.

(Preparation of solution for substrate film 105)
The following components were mixed to obtain a solution for the base film 105.

Methylene chloride (boiling point 41° C.): 860 parts by weight Methanol (boiling point 65° C.): 40 parts by weight COP1 (G7810: ARTON G7810 manufactured by JSR Corporation, Mw: 140,000, cycloolefin resin having a carboxylic acid group) 100 parts by weight Antioxidant (Irganox 1076: manufactured by BASF: molecular weight 531)
Add an amount of 0.002% by mass to the base film.

(Preparation of Base Film 105)
The solution for the base film 105 was applied onto the release layer of the support using a die by a backcoat method, and then the base film was dried in the drying step described below to form a base film with a thickness of 5 μm, thereby obtaining the base film 105.

1st step: 1 min at 40°C 2nd step: 1 min at 70°C 3rd step: 1 min at 100°C 4th step: 2 min at 130°C

<Preparation of Base Film 106>
(Support)
As a support, a polyethylene terephthalate film (PET film): (TN100 manufactured by Toyobo Co., Ltd., with a release layer containing a non-silicone based release agent, film thickness 38 μm) was used.

(Preparation of Polyimide Powder)
Into a four-neck flask equipped with a dry nitrogen gas inlet tube, a condenser, a Dean-Stark condenser filled with toluene, and a stirrer, 5.146 g (20 mmol) of MeO-DABA represented by the following formula was placed, 20 mL of γ-butyrolactone (GBL) and 10 mL of toluene were added, and the mixture was stirred at room temperature under a nitrogen stream.

4.483 g (20 mmol) of cis,cis,cis-1,2,4,5-cyclohexanetetracarboxylic dianhydride (H-PMDA) powder was added to the mixture, and the mixture was heated and stirred at 80°C for 6 hours. The external temperature was then raised to 190°C, and the water generated during the imidization was azeotropically distilled off together with toluene. After 6 hours of continued heating, refluxing, and stirring, no more water was generated. The mixture was heated for 7 hours while distilling off the toluene, and after the toluene was distilled off, methanol was added to cause reprecipitation, yielding 6.4 g of polyimide powder (yield 72%).

(Preparation of solution for substrate film 106)
The following components were mixed to obtain a solution for the base film 106.

Methylene chloride (boiling point 41° C.) 900 parts by weight Polyimide 1 (above polyimide powder) 100 parts by weight

(Preparation of Base Film 106)
The solution for the base film 106 was applied onto the release layer of the support using a die by a backcoat method, and then the base film was dried in the drying step described below to obtain a base film 106 having a thickness of 5 μm.

(Initial drying)
1st step: 1 minute at 40°C 2nd step: 1 minute at 70°C 3rd step: 1 minute at 100°C 4th step: 2 minutes at 130°C (post-drying)
5th step: 110°C for 15 minutes

<Preparation of Base Films 107 and 108>
Substrate films 107 and 108 were prepared in the same manner as in the preparation of substrate film 101, except that the film thicknesses were changed to 10 μm and 14 μm, respectively.

<Preparation of Base Film 109>
A substrate film 109 was prepared in the same manner as in the preparation of the substrate film 105, except that the solid content concentration of COP1 (G7810) in the dope was adjusted to 20 mass %.

<Preparation of Base Film 110>
A substrate film 110 was prepared in the same manner as in the preparation of the substrate film 105, except that the dope described below in which silica fine particles were added was used.

(Preparation of Microparticle Additive Solution)
Silica microparticles (Aerosil R812: manufactured by Nippon Aerosil Co., Ltd., primary average particle size: 7 nm, apparent specific gravity 50 g/L) 4 parts by mass Dichloromethane 48 parts by mass Ethanol 48 parts by mass The above was stirred and mixed for 50 minutes with a dissolver, and then dispersed with a Manton-Gaulin. Furthermore, dispersion was performed with an attritor so that the particle size of the secondary particles became a predetermined size. This was filtered with Finemet NF manufactured by Nippon Seisen Co., Ltd. to prepare a microparticle-added liquid.

(Preparation of solution for substrate film 110)
A dope having the following composition was prepared. First, dichloromethane and ethanol were added to a pressurized dissolution tank. Cycloolefin resin (DOP): G7810 was added to a pressurized dissolution tank containing a mixed solution of dichloromethane and ethanol while stirring. Furthermore, 15 minutes after the start of solvent addition, the fine particle-added liquid prepared above was added, heated to 80°C, and completely dissolved while stirring. At this time, the temperature was raised from room temperature at 5°C/min, dissolved for 30 minutes, and then cooled at 3°C/min. The obtained solution was filtered using Azumi Filter Paper No. 244 manufactured by Azumi Filter Paper Co., Ltd. to prepare a dope.

(Dope Composition)
COP1 (G7810) 100 parts by weight Dichloromethane 200 parts by weight Ethanol 10 parts by weight Fine particle additive liquid 1 part by weight

<Preparation of Base Film 111>
Substrate film 111 was produced in the same manner as in the production of substrate film 102, except that during the production process, the PET film serving as a support was peeled off, side B was dried, and then a PET film was attached again to side B and the film was taken up.

<Preparation of Base Film 112>
A substrate film 112 was produced in the same manner as in the production of the substrate film 105, except that the rubber particles R1 were added to the dope so as to make the content thereof 65% by mass.

<Preparation of Base Film 113>
(Support)
As a support, Kapton film 200H/V (manufactured by DuPont-Toray Co., Ltd., film thickness 50 μm) was used.

(Preparation of Base Film 113)
In dehydrated dimethylacetamide (DMAc, manufactured by Tokyo Chemical Industry Co., Ltd., boiling point 165° C.), 2,2′-ditrifluoromethyl-4,4′-diaminobiphenyl (TFMB, manufactured by Toray Fine Chemicals Co., Ltd.) equivalent to 90 mol % relative to the total amount of diamines and 4,4′-diaminodiphenyl ether (DPE, manufactured by Tokyo Chemical Industry Co., Ltd.) equivalent to 10 mol % were dissolved under a nitrogen stream, and the liquid temperature was cooled to 5° C. in an ice-water bath. Thereto, 2-chloroterephthaloyl chloride (CTPC, manufactured by Nippon Light Metal Co., Ltd.) equivalent to 99 mol % relative to the total amount of diamines was added over 30 minutes while the system was kept in an ice-water bath under a nitrogen stream, and after the total amount was added, the mixture was stirred for about 2 hours to polymerize aromatic polyamide (polymer A). To the resulting solution, allyl glycidyl ether (neutralizing agent E, manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a neutralizing agent in an amount equivalent to 100 mol % relative to the amount of hydrogen chloride generated in the above reaction (i.e., the amount of amide groups in the aromatic polyamide), and the mixture was stirred for about 1 hour to obtain an aromatic polyamide solution consisting of polymer A (polyamide 1).

The obtained aromatic polyamide solution was cast in the form of a film on the support using an applicator at room temperature, and dried in a hot air oven at 115°C for 30 minutes and at 265°C for 5 minutes to obtain a substrate film 113 made of polymer A (polyamide 1) with a film thickness of 5 μm. Here, a safety oven SPH100 (manufactured by Espec Corporation) was used as the hot air oven, and it was used one hour after the temperature display reached the set temperature with the open/close damper at 50%.

<Preparation of Base Film 114>
(Support)
As a support, Kapton film 200H/V (manufactured by DuPont-Toray Co., Ltd., film thickness 50 μm) was used.

(Preparation of Base Film 114)
Into a 300 mL five-neck round-bottom flask equipped with a stainless steel half-moon stirring blade, a nitrogen inlet tube, a Dean-Stark equipped with a cooling tube, a thermometer, and a glass end cap, 9.76 g (0.028 mol) of 9,9-bis(4-aminophenyl)fluorene (manufactured by Taoka Chemical Co., Ltd.), 8.62 g (0.021 mol) of 2,2-bis[4-(4-aminophenoxy)phenyl]propane (manufactured by Wakayama Seika Kogyo Co., Ltd.), 6.72 g (0.021 mol) of 4,4'-diamino-2,2'-bis(trifluoromethyl)biphenyl (manufactured by Wakayama Seika Kogyo Co., Ltd.), and 46.86 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) were added as diamine components, and the mixture was stirred at a system temperature of 70°C under a nitrogen atmosphere at a rotation speed of 200 rpm to obtain a solution.

To this solution, 16.47 g (0.07 mol) of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (manufactured by Mitsubishi Gas Chemical Co., Ltd.) and 11.72 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Co., Ltd.) were added as a tetracarboxylic acid component, and then 3.54 g of triethylamine (manufactured by Kanto Chemical Co., Ltd.) was added as an imidization catalyst. The mixture was heated with a mantle heater, and the temperature in the reaction system was raised to 190°C over approximately 20 minutes. The components that were distilled off were collected, and the temperature in the reaction system was maintained at 190°C and refluxed for 5 hours while adjusting the rotation speed according to the increase in viscosity.

After that, 97.62 g of γ-butyrolactone (manufactured by Mitsubishi Chemical Corporation) was added, and the temperature in the reaction system was cooled to 120°C. After that, the mixture was further stirred for about 3 hours to homogenize, and a polyimide 2 solution with a solid content concentration of 20% by mass was obtained. Next, the obtained polyimide 2 solution was applied onto the support using a die by the backcoat method in the same manner as in the preparation of the substrate film 101, and then the substrate film was dried in the drying step described below to form a substrate film with a thickness of 5 μm, and substrate film 114 was obtained.

(Initial drying)
1st step: 10 minutes at 40°C 2nd step: 10 minutes at 70°C 3rd step: 30 minutes at 100°C 4th step: 30 minutes at 130°C (post-drying)
5th step: 250°C for 30 minutes

<Preparation of Base Film 115>
(Support)
As a support, a polyethylene terephthalate film (PET film): (TN100 manufactured by Toyobo Co., Ltd., with a release layer containing a non-silicone based release agent, film thickness 38 μm) was used.

(Preparation of solution for substrate film 115)
Methyl ethyl ketone (MEK) 90 parts by weight Rubber particles R1 10 parts by weight The mixture was stirred with a magnetic stirrer to obtain a solution for substrate film 115.

(Preparation of Base Film 115)
The solution for the base film 115 was applied onto the release layer of the support using a die by a backcoat method, and then the base film was dried in the drying step described below to form a base film with a thickness of 5 μm, thereby obtaining the base film 115.

(Initial drying)
1st step: 1 minute at 40°C 2nd step: 1 minute at 70°C 3rd step: 1 minute at 100°C 4th step: 2 minutes at 130°C (post-drying)
5th step: 110°C for 15 minutes

<Preparation of Base Film 116>
A substrate film 116 was prepared in the same manner as in the preparation of the substrate film 102, except that the film thickness was adjusted to 25 μm.

Evaluation
The obtained substrate film was subjected to the following evaluations.

<Stress-strain curve>
The substrate film was cut into a size of 100 mm (MD direction: longitudinal direction) x 10 mm (TD direction: width direction) to obtain a sample film. The sample film was conditioned for 24 hours in an environment of 23°C and 55% RH, and the sample film after the conditioned humidity was pulled in the MD direction with a chuck distance of 50 mm using a Tensilon RTC-1225A manufactured by Orientec Co., Ltd. to obtain a stress-strain curve until breakage in accordance with JIS K7127:1999. The stress-strain curve is represented by the vertical axis as stress (MPa) and the horizontal axis as tensile elongation at break (%). The stress-strain curve was measured under conditions of 23°C, 55% RH, and a pulling speed of 50 mm/min.

The slope of the straight line according to the present invention was determined from the stress-strain curve of the base film obtained by the above measurements.

As shown in Figure 2, when the substrate film is subjected to a tensile test, it is elongated to a breaking point X. When a straight line Y is drawn between the breaking point X and the origin (0 point), the slope α of this line is defined as "the slope of the line connecting the origin and the breaking point in the stress-strain curve of the substrate film" in the present invention.

<Measurement of density of sides A and B of base film>
The density of the surface (side A and side B) of the substrate film was measured using X-ray reflectivity (XRR). When X-rays are incident on the film surface at a very shallow angle, they are totally reflected, and when the angle of the incident X-rays is equal to or greater than the critical angle of total reflection, the X-rays penetrate into the film and the reflectivity decreases. The reflectivity profile measured by the XRR method can be analyzed using dedicated reflectivity analysis software. In the present invention, when the angle at which the reflectivity starts to decrease is defined as θa, the density at which the fitting error between the measurement result and the calculation result is the smallest in the range of 2θ from 2θa to 2θa+0.1° is defined as the surface density. At that time, the fitting was performed with the surface roughness in the range of 0 to 1 nm.

The substrate film was cut into a size of 30 mm x 30 mm, fixed to a sample stage, and measured under the following measurement conditions.
(Measurement conditions)
・Apparatus: Thin film X-ray diffraction apparatus (ATX-G manufactured by Rigaku Corporation)
・Sample size: 30mm x 30mm
・Incidence X-ray wavelength: 1.5405Å
Measurement range (θ): 0 to 6°
・Analysis software: Reflectance analysis software GXRR (Rigaku Corporation)

<Measurement of Elastic Modulus of Base Film>
The conditions for measuring the elastic modulus (also called the tensile elastic modulus) were set as follows, and the elastic modulus was determined by linear regression between strains of 0.05 and 0.25%.

The tensile modulus in the MD direction was measured in accordance with JIS K7127 (1999) by the following method.
1) The substrate film was cut into a size of 100 mm (MD) x 10 mm (TD) to prepare a test piece.
2) The test piece was pulled in the longitudinal direction (MD direction) of the test piece at a pulling speed of 50 mm/min using a Tensilon RTC-1225A manufactured by Orientec Co., Ltd., with a chuck distance of 50 mm, to measure the tensile modulus in the MD direction. The measurement was carried out at 23°C and 55% RH. The modulus of elasticity of the transparent substrate described later was also measured in the same manner.

<Indentation strength>
The A and B sides of the substrate film were measured according to the indentation test procedure specified in ISO 14577. The measurements were performed in an environment of 23°C and 55% RH using an ultra-microhardness tester (manufactured by Fisher Instruments, product name "Fisherscope 100C") as a tester and a pyramidal diamond indenter with a square base and a facing angle of 136° as an indenter.

Measurements were performed by pressing an indenter into the substrate film at a constant speed, applying a load of 10 mN. Martens hardness was calculated by applying a load (10 mN) to the film and dividing the load by the surface area of the indenter that penetrated beyond the zero point of contact.

Regarding the indentation strength of the base film of the present invention, the Martens hardness was measured according to the above-mentioned method, and the rank of the surface hardness (Martens hardness) was determined for the average value of sides A and B according to the following criteria.

: Martens hardness is 200 N/ ^mm2 or more ◯: Martens hardness is 50 N/ ^mm2 or more and less than 200 N/ ^mm2 △: Martens hardness is 25 N/ ^mm2 or more and less than 50 N/ ^mm2 ×: Martens hardness is less than 25 N/ ^mm2 When the Martens hardness evaluation rank is △ or more, the indentation strength of the base film is improved, and when the base film is provided in a cover member, cracking of the transparent base material is prevented and the handleability of the cover member is improved. Desirably, it is ◯ to ◎.

The composition of the base film and the evaluation results are shown in Table I below.

<Preparation of transparent substrate 1: In the table, indicated as glass.>
A glass substrate having a dimension of 12 inches was prepared according to the following steps (see FIG. 4).

(Step 1) A step of forming a glass substrate so that a first surface of the glass substrate is in contact with a glass carrier substrate having a bonding surface, and attaching a contact film (also called a "contact film") having adhesive properties to a second surface opposite the first surface.

(Step 2) Next, the glass substrate is peeled off from the carrier substrate by the highly adhesive contact film.
(Step 3) Removing the contact film from the second surface of the glass substrate peeled off from the carrier substrate by a weakening treatment (electromagnetic radiation exposure) that weakens the adhesive strength of the contact film.

In step 1, a glass substrate 1 was formed to a predetermined thickness so as to contact a carrier substrate having a thickness of 500 μm, and then the contact film described below was attached. Next, the glass substrate was peeled off together with the contact film from the carrier substrate in 30 seconds to remove the carrier substrate (step 2). The contact film used was a commercially available product named "NDS4150-20" that contains polyolefin (PO) as a substrate having a thickness of 150 μm and a 10 μm adhesive layer.

Next, the exposed contact film was subjected to a weakening treatment to reduce the adhesive force. In the weakening treatment, the contact film was irradiated with 365 nm ultraviolet light for 10 seconds. The ultraviolet light irradiation power was about 500 mW/cm ² , and the total irradiation energy was 500 mJ/cm ^2. At that time, the adhesive force before the weakening treatment was about 11 N/25 mm, and after the weakening treatment, the adhesive force was reduced to 0.4 N/25 mm. As a result, the contact film could be easily peeled off from the glass substrate, and a single glass substrate with a thickness of 28 μm was obtained (step 3).

The thickness of the transparent substrate was adjusted to 8 μm, 28 μm, 45 μm, and 54 μm, and the elastic modulus measured by the above-mentioned measurement method was as shown in Table II.

<Preparation of transparent substrate 2: In the table, indicated as polyimide.>
In the preparation of the substrate film 114, the obtained polyimide 2 solution was applied onto a glass plate and held on a hot plate at 100°C for 60 minutes to volatilize the solvent, thereby obtaining a colorless and transparent primary dried film having self-supporting properties. This film was fixed onto a stainless steel frame and heated in a hot air dryer at 250°C for 2 hours to evaporate the solvent, thereby obtaining a transparent substrate 2 having a thickness of 28 μm. The elastic modulus measured by the above-mentioned measurement method was 10 GPa.

<Preparation of Base Film 2>
(Preparation of polyethylene terephthalate pellets (a))
As an esterification reaction apparatus, a continuous esterification reaction apparatus consisting of a three-stage complete mixing tank having an agitator, a partial condenser, a raw material inlet, and a product outlet was used. Terephthalic acid (TPA) was supplied at 2 tons/hr, ethylene glycol (EG) was supplied at 2 moles per mole of TPA, and antimony trioxide was supplied at an amount such that the antimony (Sb) atom was 160 ppm relative to the produced PET. These slurries were continuously supplied to a first esterification reaction vessel of the esterification reaction apparatus and reacted at normal pressure for an average residence time of 4 hours at 255°C.

Next, the reaction product in the first esterification reactor was continuously taken out of the system and fed to the second esterification reactor, and EG distilled from the first esterification reactor was fed into the second esterification reactor at 8% by mass relative to the produced polymer (produced PET), and an EG solution containing magnesium acetate in an amount such that the produced PET had 65 ppm magnesium (Mg) atoms, and an EG solution containing tetramethylphthalic acid (TMPA) in an amount such that the produced PET had 20 ppm P atoms, were added, and reacted at normal pressure for an average residence time of 1.5 hours at 260°C. Next, the reaction product in the second esterification reactor was continuously taken out of the system and fed to the third esterification reactor, and an EG solution containing TMPA in an amount such that the produced PET had 20 ppm P atoms, were added, and reacted at normal pressure for an average residence time of 0.5 hours at 260°C. The esterification reaction product produced in the third esterification reactor was continuously fed to a three-stage continuous polycondensation reactor to carry out polycondensation, and was then filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 90% cut of 5 μm particles) to obtain polyethylene terephthalate pellets (a) with an intrinsic viscosity of 0.580 dl/g.

(Preparation of polyethylene terephthalate film)
Pellets of polyethylene terephthalate (a) were fed to an extruder and melted at 285°C. The polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy 10μm particles 95% cut), extruded from a die in the form of a sheet, and then contacted with a casting drum with a surface temperature of 30°C using an electrostatic casting method to cool and solidify, producing an unstretched film. The unstretched film was uniformly heated to 75°C using a heating roll, heated to 100°C with a non-contact heater, and roll-stretched (longitudinal stretching) 1.5 times. The obtained uniaxially stretched film was introduced into a tenter, heated to 125°C, stretched laterally 5.5 times, fixed in width, heat-treated at 190°C for 5 seconds, and further relaxed in the width direction by 4% at 100°C to obtain a substrate film 2, which is a polyethylene terephthalate film with a thickness of 50μm (represented as PET in the table).

<Preparation of Cover Member>
<Production of Cover Member 201>
The substrate film 101 prepared in Example 1, the transparent substrate 1 prepared above, and the substrate film 2 prepared above were laminated together via the following pressure-sensitive adhesive layer to prepare a cover member 201 shown in Table II.

<Materials for Adhesive Layer>
(Preparation of Acrylic Polymer)
A monomer mixture containing 100 parts of n-butyl acrylate and 5 parts of acrylic acid was charged into a four-neck flask equipped with a stirring blade, a thermometer, a nitrogen gas inlet tube, and a cooler. Furthermore, 0.1 parts of 2,2'-azobisisobutyronitrile as a polymerization initiator was charged together with 100 parts of ethyl acetate for 100 parts of the monomer mixture (solid content), and nitrogen gas was introduced while gently stirring to replace the atmosphere with nitrogen. The liquid temperature in the flask was kept at about 55°C, and a polymerization reaction was carried out for 8 hours to prepare a solution of an acrylic polymer having a weight average molecular weight (Mw) of 1.6 million.

(Preparation of Pressure-Sensitive Adhesive Composition Solution)
0.45 parts of an isocyanate crosslinking agent (Coronate L, trimethylolpropane tolylene diisocyanate, manufactured by Tosoh Corporation) was mixed with 100 parts of the solid content of the acrylic polymer solution obtained above to prepare a solution of an acrylic pressure-sensitive adhesive composition.

(Formation of adhesive layer and preparation of cover member)
Next, the A-side of the base film 101 was subjected to a corona discharge treatment with a corona output intensity of 2.0 kW and a line speed of 18 m/min. The corona discharge-treated surface was coated with the above-prepared adhesive composition solution using a bar coater so that the film thickness after drying would be approximately 3 μm. The film was then dried at 50° C., 60° C., and 70° C. in that order for 60 seconds each to form an adhesive layer. After lamination, the support of the base film 101 was peeled off to obtain a cover member 201 (see FIG. 5B ).

<Production of Cover Members 202 to 222>
Cover members 202 to 222 were produced in the same manner as in the production of cover member 201, except that the combinations of the types of base film 1, transparent base material, and base film 2 were changed as shown in Table II.

In the production of cover member 202, cover member 214 used the substrate film 102 produced above as substrate film 2. The substrate film 2 was also bonded so that surface A of substrate film 102 was positioned on the glass substrate side. In the production of cover member 202, cover member 215 did not have anything bonded to the substrate film 2 side.

Evaluation
<Folding evaluation>
A cover member sample with a size of 20 mm in the TD direction x 110 mm in the MD direction is prepared. Using a no-load U-shaped stretch tester (Yuasa System Co., Ltd., DLDMLH-FS), the bending radius is set to 1 mm, and the sample is bent 100 times at a speed of 1 time/second. At that time, the sample is fixed at a position 10 mm from both ends on the MD side, and the bending portion is 20 mm x 90 mm. After the bending process is completed, the sample is placed on a flat surface with the inside of the bend facing down, and the bent portion is measured with a commercially available haze meter to determine the haze value before evaluation as A and the haze value after evaluation as B. The difference between the values (B-A) was obtained and the following ranking was performed.

⊚: Less than 0.2 ◯: 0.2 or more and less than 0.5 △: 0.5 or more and less than 1.0 ×: 1.1 or more △ or more is desirable.

The configuration of the cover member and the above evaluation results are shown in Table II below.

The results of Tables I and II show that cover members 201 to 217 using base films 101 to 112 of the present invention achieved superior results in both the indentation strength of the base film and the folding marks of the cover members compared to cover members 218 to 221 using base films 113 to 116 of the comparative examples and cover member 222, which has a thick transparent base material.

The cover member of the present invention has the characteristic that, in addition to improving the ease of handling, the base film provided on the cover member does not leave fold marks when repeatedly bent, so that the image is not distorted at the folded portion of the display, and the cover member-attached display device has excellent visibility and can be suitably used in a foldable display using an organic electroluminescence element.

1 Base film 1
2 Base film 2
REFERENCE SIGNS LIST 3 transparent substrate 4 adhesive layer 5 support 10 cover member 21 carrier substrate 22 glass substrate 23 contact film 24 electromagnetic radiation 100 display device 101 organic EL layer 102 polarizing plate B110 support B120 substrate film B200 manufacturing device B210 supply section B220 coating section B230 drying section B240 cooling section B250 winding section R bending radius

Claims (12)

A cover member for a foldable flexible display having a substrate film and a transparent substrate ,
The thickness of the base film is in the range of 1 μm or more and less than 15 μm,
The thickness of the transparent substrate is in the range of 5 μm or more and less than 50 μm,
the substrate film contains a cellulose ester-based resin, a cycloolefin-based resin, a fumaric acid diester-based resin, a polypropylene-based resin, a (meth)acrylic resin, a polyester-based resin, a polyarylate-based resin, a polyimide-based resin, a styrene-based resin, or a composite resin thereof;
A cover member for a foldable flexible display, characterized in that, when the relationship between tensile stress and tensile elongation at break of the base film is measured under conditions of 23°C, 55% RH, and a pulling speed of 50 mm/min, the slope of a straight line connecting the origin and the breaking point in a stress-strain curve , with the vertical axis representing stress (MPa) and the horizontal axis representing tensile elongation at break (%), is within a range of 1.1 MPa/% to 25.0 MPa/% . When the surface of the base film that is attached to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B,
The cover member of a foldable flexible display according to claim 1 , wherein the film density (ρ _A ) of the A side is smaller than the film density (ρ _B ) of the B side. When the surface of the base film that is attached to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B,
The cover member for a foldable flexible display according to claim 1 or 2, characterized in that the ratio (ρ _A /ρ _B ) of the film density (ρ _B ) of the B side to the film density ( _ρ A ) of the A side is within a range of 0.80 to 0.95. The cover member for a foldable flexible display according to claim 1 or 2 , characterized in that the base film contains rubber particles in a range of 40 to 85 mass %. A cover member for a foldable flexible display as described in claim 1 or claim 2, characterized in that the elastic modulus of the transparent substrate is within the range of 55 to 80 GPa, and the ratio of the elastic modulus of the transparent substrate to the elastic modulus of the substrate film (elastic modulus of transparent substrate/elastic modulus of substrate film) is 30 or more. 3. The cover member for a foldable flexible display according to claim 1, wherein the base film contains at least a polyimide resin. 3. The cover member for a foldable flexible display according to claim 1, wherein the base film does not contain rubber particles. The thickness of the base film is in the range of 1 μm or more and less than 15 μm,
the substrate film contains a cellulose ester-based resin, a cycloolefin-based resin, a fumaric acid diester-based resin, a polypropylene-based resin, a (meth)acrylic resin, a polyester-based resin, a polyarylate-based resin, a polyimide-based resin, a styrene-based resin, or a composite resin thereof;
When the relationship between tensile stress and tensile elongation at break of the substrate film is measured under conditions of 23°C, 55% RH, and a pulling speed of 50 mm/min, the slope of a straight line connecting the origin and the breaking point in a stress-strain curve, with the vertical axis representing stress (MPa) and the horizontal axis representing tensile elongation at break (%) , is within a range of 1.1 MPa/% to 25.0 MPa/% . The substrate film is attached to a transparent substrate,
When the surface of the base film that is attached to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B,
The substrate film for a cover member of a foldable flexible display according to claim 8 , wherein the film density (ρ _A ) of the A side is smaller than the film density (ρ _B ) of the B side. The substrate film is attached to a transparent substrate,
When the surface of the base film that is attached to the transparent base material is designated as surface A and the back surface of the base film opposite surface A is designated as surface B,
The substrate film for _a cover member of a foldable flexible display according to claim 8 or 9, characterized in that the ratio (ρ _A /ρ _B ) of the film density (ρ _B ) of the B side to the film density (ρ A ) of the A side is within a range of 0.80 to 0.95 . The substrate film for a cover member of a foldable flexible display according to claim 8 or claim 9 , characterized in that the substrate film contains rubber particles in a range of 40 to 85 mass%. A display device comprising: a cover member for a foldable flexible display according to claim 1 or 2 ; or a base film for a cover member for a foldable flexible display according to claim 8 or 9 .

Images