JP5312077B2 - Optical film, reflective polarizing plate and brightness enhancement film using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical film excellent in polarization characteristics and luminance improving performance. <P>SOLUTION: The optical film includes polyethylenenaphthalate based resin (P), thermoplastic resin (S) containing (a) vinylcyanide based monomer, (b) aromatic vinyl-based monomer, (c) monomer copolymerizable with (a) and (b), where the weight-average molecular weight is 150,000-230,000 and the average refractive index is 1.545-1.575, and polymer (M) produced by block-binding or graft-binding copolymer (m1) obtained from ethylene and (meth) acrylic ester-based monomer to copolymer (m2) consisting of (d=a), (e=b) and (f=c). The content of the polymer (M) is 0.1-6 mass part based on (P)+(S)=100 mass part. The optical film includes morphology having a sea-island structure consisting of a phase (I) mainly containing (P) and a phase (II) mainly containing (S), and one of the phase (I) and phase (II) is dispersed phase. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an optical film, and more particularly to an optical film that can be used as a reflective polarizing plate or a brightness enhancement film.

In recent years, with the expansion of the display market, in particular in the field of liquid crystal displays, there has been a demand for optical members having better polarization characteristics.
Further, since the liquid crystal display is not a self-luminous type, there is a demand for improving the light use efficiency to improve the screen brightness and to form a more beautiful image.
Furthermore, due to the recent increase in awareness of environmental problems, there is also a demand for securing desired luminance with low power consumption.

In a liquid crystal display, image display is performed using polarized light from which a backlight source is separated.
As a polarizing plate used for polarization separation, a so-called absorption polarizing plate is generally known which is obtained by stretching and orienting a film in which iodine or a dye is absorbed in a polyvinyl alcohol resin.
Such an absorption-type polarizing plate has very high polarization characteristics, but only transmits light in the direction of the transmission axis and absorbs the rest. Therefore, the light transmittance is about 50% at the maximum, and the use of light The efficiency is low, and there is a problem that sufficient performance cannot be obtained as a brightness enhancement film.

  As a polarizing plate other than the absorptive polarizing plate, a so-called reflective polarizing plate that performs polarization separation by reflecting a polarized light component orthogonal to the transmission axis is known, and this reflective polarizing plate is used as a brightness enhancement film. By using it, the light of the backlight source is effectively used.

As the above-described reflection type polarizing plate, a reflection type polarizing plate having a multilayer laminated structure in which two kinds of materials are laminated in multiple layers and stretched has been proposed (for example, see Patent Documents 1 to 3). These reflective polarizing plates perform polarization separation by utilizing reflection at the interface between layers. A single layer has low polarization characteristics, but a multilayer structure can provide high polarization characteristics as a whole. It has the advantage that it can exhibit sufficient brightness enhancement performance as a brightness enhancement film by precisely controlling the thickness of each layer.
However, the reflective polarizing plate having such a multilayer laminated structure requires uniform multilayer lamination and precise control of the thickness of each layer, so that the manufacturing process is complicated, the productivity is poor, and the yield is reduced. Have the problem of

Conventionally, a reflective polarizing plate using a morphology having a sea-island structure has been proposed (see, for example, Patent Documents 4 and 5).
These reflective polarizing plates perform polarized light separation using reflection at the interface between the continuous phase and the dispersed phase. Compared with the above-mentioned multilayer laminated structure type, multilayer reflection and precise control of the thickness of each layer are possible. Therefore, there is an advantage that the manufacturing process is not complicated, the productivity is high, and the yield is good.

JP-T 9-507308 Japanese National Patent Publication No. 9-506985 Japanese National Patent Publication No. 9-506984 JP 2000-506990 A JP 2008-164929 A

However, in the reflective polarizing plate of Patent Document 4, even when the disclosed polymers are combined to form a continuous phase and a dispersed phase, the refractive indexes are mismatched, and as a result, sufficient polarization characteristics cannot be obtained. In addition, there is a problem that excellent brightness improvement performance cannot be obtained.
In addition, in the reflective polarizing plate of Patent Document 5, it is described that polymers having a specific molecular weight are combined with each other. However, since the refractive indexes of the polymers are mismatched, sufficient polarization characteristics cannot be obtained. There is a problem that excellent brightness improvement performance cannot be obtained.

  In addition, in the reflection type polarizing plate using the so-called sea-island structure, there is a relationship that the polarization characteristics are enhanced by reducing the compatibility between the continuous phase (sea) and the dispersed phase (island). When material selection is performed with emphasis on characteristics, there is a problem that the interface strength between the continuous phase and the dispersed phase is insufficient, and a practically sufficient film strength cannot be secured.

  Furthermore, in the reflection type polarizing plate using the morphology having a sea-island structure, when the compatibility between the continuous phase (the sea) and the dispersed phase (the island) is lowered, the morphology becomes unstable. There is a problem that the fluctuation of the resin pressure increases and the uniformity of the film thickness is impaired.

  In Patent Documents 4 and 5, a proposal is made to increase the affinity between the continuous phase and the dispersed phase by using a compatibilizing agent containing an epoxy group, and to improve the film strength and thickness uniformity finally obtained. However, compatibilizers containing epoxy groups are very reactive, and the amount of epoxy used in the compatibilizer is very large, which makes it difficult to control the reaction. There is a problem of being invited.

  Therefore, in the present invention, the production process is not complicated, the productivity is good, the production can be performed at low cost, the polarization property is excellent, the brightness enhancement performance is high, and the practically sufficient film strength and thickness uniformity are obtained. It aims at providing the optical film which has.

As a result of intensive studies to solve the problems of the prior art, the present inventors have obtained a polyethylene naphthalate resin,
A thermoplastic resin obtained from a vinyl cyanide monomer, aromatic vinyl monomer having a specific composition range, and other monomers copolymerizable therewith, having a weight average molecular weight in a specific range and an average refraction in a specific range. A thermoplastic resin with a rate,
Select a specific amount of block or graft polymer with a specific structure so that these resins form a disperse phase and a continuous layer, and have a sea-island structure with a volume ratio within a specific range. The present inventors have found that an optical film in which the aspect ratio of the dispersed phase is controlled to a specific value or more can realize the above-mentioned problems, and the present invention has been completed.

[1]
An optical film comprising a polyethylene naphthalate resin (P), a thermoplastic resin (S), and a polymer (M),
The thermoplastic resin (S) is
(A) 20-40% by mass of vinyl cyanide monomer,
(B) 40-80% by mass of an aromatic vinyl monomer,
(C) 0 to 20% by mass of other monomers copolymerizable with (a) and (b) ((a) + (b) + (c) = 100% by mass)
A thermoplastic resin having a weight average molecular weight of 150,000 to 230,000 and an average refractive index of 1.545 to 1.575 ,
The polymer (M) is a polymer in which the copolymer (m1) and the copolymer (m2) are block-bonded or graft-bonded, and the content of the epoxy group is 1 with respect to the total mass of the polymer (M). Mass% or less,
The copolymer (m1) is a copolymer obtained from ethylene and a (meth) acrylic acid ester monomer ,
The copolymer (m2) is
(D) 20 to 40% by mass of vinyl cyanide monomer,
(E) 40-80% by mass of an aromatic vinyl monomer,
(F) the (d), (e) and other copolymerizable monomer 0-20 wt% ((d) + (e ) + (f) = 100 wt%) consisting a a copolymer Yes,
With respect to the polyethylene naphthalate resin (P) + the thermoplastic resin (S) = 100 parts by weight, the polymer (M) is 0.1 to 6 parts by weight,
Having a morphology having a sea-island structure composed of a phase (I) mainly containing the polyethylene naphthalate resin (P) and a phase (II) mainly containing the thermoplastic resin (S),
Either the phase (I) or the phase (II) is a dispersed phase, and the aspect ratio of the dispersed phase is 2 or more,
The volume ratio ((I) / (II)) between the phase (I) and the phase (II) is 10/90 to 90/10.
An optical film is provided.

[2]
Wherein the copolymer constituting the front Symbol polymer (M) (m1) and the copolymer (m2) and the mass ratio of ((m1) / (m2)) is a 50 / 50-90 / 10 [ An optical film as described in 1) is provided.

[3]
The optical film according to [1] or [2], wherein the thermoplastic resin (S) is a polymer obtained by radical polymerization using an organic peroxide as a polymerization initiator.
[4]
Providing an optical film according to any one of the aspect ratio of prior SL dispersed phase is at least 4 [1] to [3].

[5]
Providing an optical film according to any one of the which is uniaxially oriented in extension Shin magnification 3 times or more [1] to [4].

[6]
Provided is a reflective polarizing plate comprising the optical film described in any one of [1] to [5] .

[7]
A brightness enhancement film comprising the optical film according to any one of [1] to [5] is provided.

  According to the present invention, the production process is not complicated, the productivity is good, the production can be performed at low cost, the polarization property is excellent, the brightness enhancement performance is high, and the practically sufficient film strength and thickness uniformity. An optical film that can be used as a reflective polarizing plate or a brightness enhancement film can be provided.

The relationship between the degree of orientation of the phase (I) and the phase (II) constituting the optical film and the refractive index is shown. (A)-(d) The schematic diagram of the structure where the copolymer (m1) and the copolymer (m2) block-bonded is shown. In the polymer (M), a schematic diagram of a structure in which a copolymer (m1) and a copolymer (m2) are graft-bonded is shown. The schematic sectional drawing of the principal part of the apparatus for brightness | luminance evaluation is shown.

  Hereinafter, the best mode for carrying out the present invention (hereinafter referred to as the present embodiment) will be described in detail. The present invention is not limited to the following embodiments, and can be implemented with various modifications within the scope of the gist.

[Optical film]
The optical film in this embodiment consists of a polyethylene naphthalate resin (P), a thermoplastic resin (S), and a polymer (M).
The thermoplastic resin (S) comprises (a) 20 to 40% by mass of vinyl cyanide monomer, (b) 40 to 80% by mass of aromatic vinyl monomer, (c) (a) and (b ) And other monomers copolymerizable with 0 to 20% by mass ((a) + (b) + (c) = 100% by mass), weight average molecular weight 150,000 to 230,000, average refractive index Is 1.545 to 1.575.
The polymer (M) comprises a copolymer (m1) obtained from ethylene and a (meth) acrylic acid ester monomer, (d) a vinyl cyanide monomer 20 to 40% by mass, (e) an aromatic Obtained from 40 to 80% by mass of vinyl-based monomer, (f) 0 to 20% by mass of other monomers copolymerizable therewith ((d) + (e) + (f) = 100% by mass) The resulting copolymer (m2) is block-bonded or graft-bonded.
Polyethylene naphthalate resin (P) + thermoplastic resin (S) = 100 parts by mass of polymer (M) is 0.1 to 6 parts by mass, and the polyethylene naphthalate resin (P) is mainly used. The phase (I) and the phase (II) mainly containing the thermoplastic resin (S), and have a morphology with a sea-island structure, and any one of the phase (I) and the phase (II) It is a dispersed phase, the aspect ratio of the dispersed phase is 2 or more, and the volume ratio ((I) / (II)) between the phase (I) and the phase (II) is 10/90 to 90/10. It is.

Here, “mainly containing phase” means that the amount of the resin contained in the phase exceeds 50% by volume, more preferably 80% by volume or more, and still more preferably 90% by volume or more. It shall be.
Polyethylene naphthalate resin (P) + thermoplastic resin (S) = 100 parts by mass and polyethylene naphthalate resin (P) + thermoplastic resin (S) = 100% by volume mean equivalent amounts of substances. Is.

[Refractive index of optical film]
Regarding the refractive index of the optical film in the present embodiment, the phase (I) or phase (II) is in the film plane, the refractive index in the direction in which the refractive index of each phase is maximum, the maximum refractive index, and the refractive index is minimum. The refractive index in the direction is defined as the minimum refractive index.
In the same plane, the angle between the direction axis indicating the maximum refractive index and the direction axis indicating the minimum refractive index is 90 degrees.
The refractive index is obtained by stretching the single resin constituting each phase under the same stretching conditions as in the case of obtaining the optical film in the present embodiment, and the maximum refractive index and the minimum refractive index of the stretched film of the obtained resin carrier. Were measured and calculated, and these were taken as the maximum refractive index and the minimum refractive index of each phase.

The average refractive index of the resin such as the polyethylene naphthalate resin (P) and the thermoplastic resin (S) constituting the optical film in the present embodiment is independent of each other when the optical film is unstretched. The average value of the three refractive indexes of the maximum refractive index, the minimum refractive index, and the refractive index in the thickness direction at 550 nm and 550 nm wavelength light.
However, each refractive index in the wavelength light of 23 ° C. and 550 nm is defined by the following formula (1) by measuring each refractive index in the wavelength light of 532 nm, 633 nm, and 838 nm under the condition of 23 ° C. A chromatic dispersion curve is created by Cauchy's equation, and the values at 550 nm obtained from the curve are taken as the maximum refractive index, the minimum refractive index, and the refractive index in the thickness direction, respectively.
n = A + B / λ ² + C / λ ⁴ (1)
In the above formula (1), “n” represents the refractive index, “A”, “B” and “C” represent constants, and “λ” represents the wavelength of light.

[Intrinsic birefringence]
Regarding the resin constituting the optical film in the present embodiment, “inherent birefringence” described later is a value representing the magnitude of birefringence depending on the orientation, and is defined by the following formula (2).
Intrinsic birefringence = npr−nvt (2)
In the formula (2), “npr” indicates a refractive index in a direction parallel to the alignment direction of the polymer aligned in a uniaxial order, and “nvt” indicates a refractive index in a direction perpendicular to the alignment direction. .

  That is, the resin having a positive “intrinsic birefringence” means that when light is incident on a layer formed by aligning the resin in a uniaxial order, the refractive index of light in the alignment direction is in the alignment direction. A resin that is larger than the refractive index of light in the orthogonal direction, and a resin that has a negative intrinsic birefringence means a resin that becomes smaller.

FIG. 1 illustrates the relationship between the degree of orientation and the refractive index of the phase (I) and the phase (II) constituting the optical film in this embodiment.
The intrinsic birefringence of the “polyethylene naphthalate resin (P)” and “thermoplastic resin (S)” constituting the optical film is positive and negative, respectively.
Therefore, when the optical film has a uniaxial orientation, the direction showing the maximum refractive index nx (1) of the phase (I) mainly containing the polyethylene naphthalate resin (P) is the X direction, and the minimum refractive index ny. When the direction of (1) is the Y direction, the phase (II) mainly containing a thermoplastic resin (S) having a different sign of intrinsic birefringence from that of the polyethylene naphthalate resin (P) exhibits a maximum refractive index. The direction is the Y direction, and the direction showing the minimum refractive index is the X direction.
That is, the maximum refractive index (ny (2)) and the minimum refractive index (nx (2)) of the phase (II) mainly containing the thermoplastic resin (S) are as illustrated.

  Also, the absolute value | nx (1) −nx (2) | of the difference between the maximum refractive index nx (1) of the phase (I) in the X direction and the minimum refractive index nx (2) of the phase (II) is Δnx. The absolute value | ny (1) −ny (2) | of the difference between the minimum refractive index ny (1) of the phase (I) in the Y direction and the maximum refractive index ny (2) of the phase (II) is Δny Then, it becomes as shown in the figure.

In the optical film in the present embodiment, Δny is set to 0 or as close to 0 as possible and Δnx is increased as much as possible in order to realize excellent polarization characteristics and enhance the luminance improvement performance.
For this purpose, the uniaxial orientation of the polyethylene naphthalate resin (P) and the thermoplastic resin (S) constituting the phase (I) and the phase (II), respectively, is enhanced as much as possible by means such as stretching. As a result, the refractive index difference between the orientation direction of each resin and the direction perpendicular to the orientation direction is increased as much as possible. Further, the minimum refractive index ny (() of the phase (I) mainly containing the polyethylene naphthalate resin (P)). The absolute value Δny of the difference between 1) and the maximum refractive index ny (2) of the phase (II) mainly containing the thermoplastic resin (S) is set to 0 or as close to 0 as possible.

  Increasing Δnx shown in FIG. 1 reflects light components whose electric field oscillates in the X direction, and decreasing Δny increases the transmittance of light whose electric field oscillates in the Y direction. Is obtained.

In the optical film of the present embodiment, Δny is preferably 0.01 or less, and more preferably 0.005 or less, in order to achieve excellent polarization characteristics and obtain high luminance improvement performance. 0.002 or less is more preferable, and 0 is most preferable.
Δnx is preferably 0.17 or more, more preferably 0.22 or more, and further preferably 0.27 or more.

[Polyethylene naphthalate resin (P)]
As the polyethylene naphthalate resin (P) constituting the optical film in the present embodiment, any of a polyethylene naphthalate homopolymer and a copolymer of polyethylene naphthalate and polyethylene terephthalate can be used. In order to obtain an optical film excellent in improving performance, it is preferable to use a homopolymer alone.
In the optical film of the present embodiment, the average refractive index of the polyethylene naphthalate-based resin (P) is preferably 1.60 or more in order to realize excellent polarization characteristics and brightness enhancement performance, and 1.63 More preferably, it is more preferably 1.65 or more.
Further, the difference between the maximum refractive index and the average refractive index of the polyethylene naphthalate resin (P) is preferably 0.1 or more, and more preferably 0.15 or more.
Furthermore, the difference between the maximum refractive index and the minimum refractive index of the polyethylene naphthalate resin (P) is preferably 0.15 or more, more preferably 0.20 or more, and 0.25 or more. More preferably.
Furthermore, the intrinsic viscosity of the polyethylene naphthalate resin (P) is preferably 0.4 to 0.9 dl / g as a value at 35 ° C. in an o-chlorophenol solution, and 0.6 to 0.8 dl / g. Is more preferable. By setting the intrinsic viscosity within the above range, high uniformity is obtained in the optical film, and the polarization characteristics are improved.

[Thermoplastic resin (S)]
The thermoplastic resin (S) constituting the optical film in the present embodiment includes (a) vinyl cyanide monomer, (b) aromatic vinyl monomer, (c) (a), ( It is obtained from a monomer copolymerizable with b).
The ratio of the monomers (a) to (c) is as follows: (a) vinyl cyanide when the total mass of the monomers ((a) + (b) + (c)) is 100% by mass 20 to 40% by mass of the monomer based on the monomer, (b) 40 to 80% by mass of the aromatic vinyl monomer, and (c) 0 to 20% of the monomer copolymerizable therewith. It shall be mass%.
When the thermoplastic resin (S) contains no component (a) obtained from the vinyl cyanide monomer, the thermoplastic resin (S) and the polyethylene naphthalate resin (P) Since the compatibility is very low, the dispersed phase diameter is enlarged, and the number of dispersed phases is decreased. In the optical film finally obtained, the polarization characteristics and the luminance enhancement performance are lowered.

By including the component obtained from the vinyl cyanide monomer in the thermoplastic resin (S), the interaction between the thermoplastic resin (S) and the polyethylene naphthalate resin (P) increases. When the component obtained from the vinyl cyanide monomer (a) in 100% by mass of the thermoplastic resin (S) is less than 20% by mass, the thermoplastic resin (S) and the polyethylene naphthalate resin (P) Since the effect of the interaction between them is not sufficient, the finally obtained optical film is inferior in polarization characteristics.
On the other hand, when the component obtained from (a) vinyl cyanide monomer in 100% by mass of the thermoplastic resin (S) is more than 40% by mass, the thermoplastic resin (S) and the polyethylene naphthalate resin (P) Thus, the optical film finally obtained is inferior in polarization characteristics and brightness enhancement performance.

The thermoplastic resin (S) has an average refractive index of 1.545 to 1.575 under a temperature condition of 23 ° C. If the average refractive index of the thermoplastic resin (S) is outside the above range, sufficient polarization characteristics are exhibited in the finally obtained optical film due to a mismatch in refractive index with the polyethylene naphthalate resin (P). In addition, it cannot exhibit excellent brightness enhancement characteristics.
In the thermoplastic resin (S), (a) a vinyl cyanide monomer, (b) an aromatic vinyl monomer, (c) a monomer copolymerizable with (a) and (b). The mass ratio and the average refractive index are not constituent requirements that can be controlled independently, but in practice, (a) a vinyl cyanide monomer and (b) an aromatic vinyl type so as to satisfy both constituent requirements. It is necessary to determine the mass ratio of the monomer and (c) the monomer copolymerizable therewith.
Further, in order for the mass ratio and the average refractive index to satisfy the above-described conditions and to exhibit more excellent polarization characteristics and brightness enhancement performance in the optical film, the ratio of (a) vinyl cyanide monomer is 30. It is preferable that it is -40 mass%.

<(A) Vinyl cyanide monomer>
Examples of the vinyl cyanide monomer (a) used for obtaining the thermoplastic resin (S) include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and acrylonitrile is particularly preferable. These vinyl cyanide monomers may be used alone or in combination of two or more.

<(B) Aromatic vinyl monomer>
Examples of the (b) aromatic vinyl monomer used for obtaining the thermoplastic resin (S) include styrene, α-methylstyrene, vinyltoluene, p-methylstyrene, o-methylstyrene, and m-methylstyrene. , Ethyl styrene, vinyl xylene, bromo styrene, vinyl benzyl chloride, pt-butyl styrene, chlorostyrene, alkyl styrene, divinyl benzene, trivinyl benzene and the like, and styrene is particularly preferable. These aromatic vinyl monomers may be used alone or in combination of two or more.

<(C) Other monomer copolymerizable with (a) and (b)>
Examples of other monomers copolymerizable with (c) (a) and (b) used to obtain the thermoplastic resin (S) include (meth) acrylic acid alkyl ester monomers, ( (Meth) acrylamide monomers, vinyl acetates such as vinyl acetate; vinyl halides such as vinyl chloride; amino group-containing ethylenic monomers such as aminoethyl acrylate and dimethylaminoethyl acrylate; conjugated dienes Monomers, ethylenically unsaturated carboxylic acid monomers, ethylenically unsaturated carboxylic acid anhydride monomers, ethylenically unsaturated carboxylic acid imidized monomers, sodium styrenesulfonate, and the like. These monomers copolymerizable with the above (a) and (b) may be used alone or in combination of two or more.

  Examples of the (meth) acrylic acid alkyl ester monomer include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, n-butyl (meth) acrylate, t-butyl (meth) acrylate, Isobutyl (meth) acrylate, n-amyl (meth) acrylate, isoamylhexyl (meth) acrylate, octyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, dodecyl (meth) acrylate, octadecyl (meth) acrylate , Cyclohexyl (meth) acrylate, phenyl (meth) acrylate, benzyl (meth) acrylate, 2-ethyl-hexyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, glycidyl (meth) acrylate Relate, ethylene glycol di (meth) acrylate, 1,3-butylene glycol di (meth) acrylate, 1,4-butylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, 1,5-pentanediol di ( (Meth) acrylate, neopentyl glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, diethylene glycol di (meth) acrylate, diethylene glycol ethoxy acrylate, triethylene glycol di (meth) acrylate, tetraethylene glycol di ( (Meth) acrylate, polyethylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, pentaerythritol tri (meth) acrylate, trimer Roll propane tri (meth) acrylate, tetramethylolmethane tetra (meth) acrylate, allyl (meth) acrylate, bis (4-acryloxypolyethoxyphenyl) propane, methoxypolyethylene glycol (meth) acrylate, stearyl (meth) acrylate, phenoxy Ethyl (meth) acrylate, phenoxypolyethylene glycol (meth) acrylate, 2,2-bis [4-((meth) acryloxyethoxy) phenyl] propane, 2,2-bis [4-((meth) acryloxy diethoxy) Phenyl] propane, 2,2-bis [4-((meth) acryloxy-polyethoxy) phenyl] propane, isobornyl (meth) acrylate, and the like.

  Examples of the (meth) acrylamide monomer include N-monoalkyl (meth) acrylamides such as (meth) acrylamide, N-methylol (meth) acrylamide, N-methyl (meth) acrylamide, and N, N-dimethyl. N, N-dialkyl (meth) acrylamide such as (meth) acrylamide, glycidylmethacrylamide, N-alkoxy (meth) acrylamide, 2-acrylamido-2-methylpropanesulfonic acid and the like can be mentioned.

  Examples of the conjugated diene monomer include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 2-methyl-1,3- Examples include butadiene, 1,3-pentadiene, chloroprene, 2-chloro-1,3-butadiene, and cyclopentadiene.

  Examples of the ethylenically unsaturated carboxylic acid monomer include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, and crotonic acid.

  Examples of the ethylenically unsaturated carboxylic acid anhydride monomer include maleic anhydride, and examples of the ethylenically unsaturated carboxylic acid imidized monomer include N-phenylmaleimide. .

<Weight average molecular weight of thermoplastic resin (S)>
The weight average molecular weight of the thermoplastic resin (S) is 150,000 to 230,000.
A weight average molecular weight is calculated | required by polystyrene conversion by gel permeation chromatography (GPC).
When the weight average molecular weight of the thermoplastic resin (S) is out of the above range, the polyethylene naphthalate-based resin (P) and the thermoplastic resin (S) are kneaded using, for example, an extruder and formed into a film. In the film morphology, the diameter of the dispersed phase increases and the number decreases, and as a result, good polarization characteristics cannot be exhibited in the optical film.
Moreover, when the weight average molecular weight of a thermoplastic resin (S) is smaller than 150,000, in the optical film finally obtained, intensity | strength will fall and handleability will deteriorate.

<Method for producing thermoplastic resin (S)>
As the thermoplastic resin (S), a commercially available product may be used as it is, or may be produced from a commercially available monomer by a known method.
As a method for producing the thermoplastic resin (S), generally used polymerization methods such as cast polymerization, bulk polymerization, suspension polymerization, solution polymerization, emulsion polymerization, anionic polymerization and the like can be applied. In particular, when the optical film finally obtained is applied to optical applications such as a reflective polarizing plate and a brightness enhancement film, bulk polymerization or solution polymerization without using a suspending agent or an emulsifier that causes a foreign matter claim Is preferred.
When the thermoplastic resin (S) is produced by solution polymerization, a solution prepared by dissolving a mixture of monomers in an aromatic hydrocarbon solvent such as toluene or ethylbenzene can be used.
When the thermoplastic resin (S) is produced by bulk polymerization, the polymerization can be initiated by irradiation with free radicals generated by heating or ionizing radiation by a known method.

As an initiator for the polymerization reaction of the thermoplastic resin (S), any initiator generally used in radical polymerization can be used. Examples thereof include azo compounds such as azobisisobutylnitrile; organic peroxides such as benzoyl peroxide, lauroyl peroxide, and t-butylperoxy-2-ethylhexanoate.
In particular, when polymerization is performed under a high temperature condition of 90 ° C. or higher, solution polymerization is generally used, and therefore, a peroxide having a 10-hour half-life temperature of 80 ° C. or higher and soluble in the applied organic solvent. Azobis initiator and the like are preferable. Specifically, 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, cyclohexane peroxide, 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, , 1-azobis (1-cyclohexanecarbonitrile), 2- (carbamoylazo) isobutyronitrile and the like. These initiators are preferably used, for example, in the range of 0.005 to 5 parts by mass with respect to 100 parts by mass of the whole monomer.

In the polymerization reaction of the thermoplastic resin (S), a molecular weight regulator may be used as necessary.
As the molecular weight regulator, any one generally used in radical polymerization can be used. Examples thereof include mercaptan compounds such as butyl mercaptan, octyl mercaptan, dodecyl mercaptan, and 2-ethylhexyl thioglycolate. These molecular weight regulators are preferably added in such an amount that the weight average molecular weight of the thermoplastic resin (S) can be controlled within the above-described range.

[Polymer (M)]
The polymer (M) comprises a copolymer (m1) obtained from ethylene and a (meth) acrylic acid ester monomer, (d) a vinyl cyanide monomer 20 to 40% by mass, (e) an aromatic 40 to 80% by mass of vinyl monomer, (f) 0 to 20% by mass of other monomer copolymerizable with (d) and (e) ((d) + (e) + (f) = 100% by mass) and a copolymer (m2) obtained from the block bond or graft bond.
When these copolymers (m1) and (m2) have a random structure, the effect of obtaining the affinity described later is insufficient, and a practically sufficient strength is obtained in the finally obtained optical film. In addition, the film thickness uniformity deteriorates.
Note that (d) the vinyl cyanide monomer is (a) described above, (e) the aromatic vinyl monomer is (b), (f) (d), (e) and As other copolymerizable monomers, the same monomers as those described above for (c) can be used.

Copolymer obtained from ethylene and (meth) acrylate monomer in polymer (M) by adding polymer (M) to polyethylene naphthalate resin (P) and thermoplastic resin (S) (M1) part is polyethylene naphthalate resin (P), (d) vinyl cyanide monomer, (e) aromatic vinyl monomer, (f) other monomer copolymerizable with these Since the copolymer (m2) portion obtained from the body has an affinity with the thermoplastic resin (S), respectively, the effect of improving the strength of the finally obtained optical film can be obtained. The effect of improving the thickness uniformity is also obtained.
Further, by adding the polymer (M), a short dispersion phase of the phase (I) mainly containing the polyethylene naphthalate resin (P) and the phase (II) mainly containing the thermoplastic resin (S) is obtained. The aspect ratio when stretched under the same stretching conditions (same stretching ratio, stretching temperature, stretching speed, etc.) becomes smaller, resulting in superior polarization characteristics and improved brightness in the final optical film. Performance is obtained.

  In the polymer (M), a copolymer (m1) obtained from ethylene and a (meth) acrylic acid ester monomer, (d) a vinyl cyanide monomer, (e) an aromatic vinyl monomer (F) As a structural example in which a copolymer (m2) obtained from other monomers copolymerizable with these is block-bonded, FIGS. 2 (a) to 2 (d) show the linear block copolymer. A schematic diagram is shown.

In the polymer (M), a copolymer (m1) obtained from ethylene and a (meth) acrylic acid ester monomer, (d) a vinyl cyanide monomer, (e) an aromatic vinyl monomer FIG. 3 shows a schematic diagram of a structural example in which (f) a copolymer (m2) obtained from another monomer copolymerizable with these is graft-bonded.
The graft bond of polymer (M) means a graft-like bond between a polymer chain (main chain) having a certain structure and a polymer chain (side chain) having a different type of structure.

  In the polymer (M), a copolymer (m1) obtained from ethylene and a (meth) acrylic acid ester monomer, (d) a vinyl cyanide monomer, (e) an aromatic vinyl monomer (F) When the copolymer (m2) obtained from other monomers copolymerizable with these has a graft-bonded structure, the copolymer (m1) is copolymerized in the main chain. Even if the polymer (m2) is a side chain, the copolymer (m2) may be a main chain and the copolymer (m1) may be a side chain. The coalescence (m2) is preferably a side chain.

About the content rate of a polymer (M), it is 0.1 to 6 mass parts with respect to 100 mass parts of polyethylene naphthalate type resin (P) + thermoplastic resin (S), and 0.5 mass The amount is preferably 4 parts by mass or more and 4 parts by mass or less.
When the polymer (M) is less than 0.1 parts by mass with respect to 100 parts by mass of polyethylene naphthalate resin (P) + thermoplastic resin (S), it is practically used in an optical film finally obtained. Sufficient strength cannot be obtained, and thickness uniformity deteriorates. On the other hand, if it exceeds 6 parts by mass, the light transmittance of the finally obtained optical film is lowered, and the polarization property is deteriorated.

The copolymer (m1) constituting the polymer (M) is a copolymer composed of ethylene and a (meth) acrylic acid ester monomer, but is composed of ethylene and a (meth) acrylic acid ester monomer. The mass ratio is preferably 5/95 to 95/5, more preferably 50/50 to 90/10, and still more preferably 60/40 to 85/15.
By setting these mass ratios to 5/95 to 95/5, good affinity can be secured in the polymer (M) and the polyethylene naphthalate resin (P).

Mass of (a) vinyl cyanide monomer component when the total of all monomer components ((a) + (b) + (c)) in the thermoplastic resin (S) described above is 100 mass% % Is Wsa, (b) the mass% of the aromatic vinyl monomer component is Wsb,
(D) vinyl cyanide when the sum of all monomer components ((d) + (e) + (f)) constituting the copolymer (m2) in the polymer (M) is 100% by mass When the mass% of the monomeric monomer component is Wmd and the mass% of the (e) aromatic vinyl monomer component is Wme, 0.90 ≦ Wsa / Wmd ≦ 1.1 and 0.90 ≦ Wsb / Wme ≦ 1.1, more preferably 0.95 ≦ Wsa / Wmd ≦ 1.05 and 0.95 ≦ Wsb / Wme ≦ 1.05.
When Wsa / Wmd and Wsb / Wme are in the above ranges, good affinity can be secured in the thermoplastic resin (S) and the polymer (M).

  Copolymer (m1) obtained from ethylene and (meth) acrylic acid ester monomer constituting polymer (M) used in the present invention, (d) vinyl cyanide monomer, (e) aromatic The mass ratio (m1) / (m2) of the polymer (m2) obtained from the vinyl monomer and (f) other monomer copolymerizable with these is preferably 10/90 to 90/10, More preferably, it is 50 / 50-90 / 10, More preferably, it is 60 / 40-80 / 20. Thereby, compatibility of the affinity with respect to the phase (I) and phase (II) which the polymer (M) mentioned above is achieved.

The content of the epoxy group in the polymer (M) is preferably 1% by mass or less, more preferably 0.1% by mass or less, and 0.01% by mass with respect to the total mass of the polymer (M). More preferably, it is even more preferable that it is not contained at all.
Epoxy groups have very high reactivity with other functional groups, so that even a small amount can improve the strength of the optical film and the film thickness uniformity, but the polyethylene naphthalate resin (P) and the polymer ( Since it is very difficult to control the reaction with the epoxy group in M), when the epoxy content exceeds 1% by mass with respect to the total mass of the polymer (M), a large number of gels are generated by the reaction. This is not preferable because it may cause a hole in the finally obtained optical film or cause a decrease in polarization characteristics.

[Structure of optical film]
The optical film in this embodiment has a morphology having a sea-island structure composed of a phase (I) mainly containing a polyethylene naphthalate resin (P) and a phase (II) mainly containing a thermoplastic resin (S). doing.
In this morphology, the phase (I) and the phase (II) may be present in the optical film in a state of being separated (having an interface). That is, in the case of a sea-island structure where phase (I) is the sea (continuous phase) and phase (II) is the island (dispersed phase), and phase (II) is the sea (continuous phase) and phase (I) is the island (dispersed phase) ) Sea-island structure. The optical film of the present embodiment may have any configuration from the viewpoints of phase stability, polarization characteristics, and brightness enhancement performance.
However, when phase (I) and phase (II) have a co-continuous morphology, in the region where the sea-island-structured continuous phase and the dispersed phase are phase-inverted, the phase structure may change even if there is a slight change in conditions. This is not preferable because it tends to be partially unstable and the polarization characteristics and brightness enhancement performance become non-uniform in the plane.

In the optical film of this embodiment, the aspect ratio of the dispersed phase corresponding to the “island” in the sea-island structure composed of the phase (I) and the phase (II) is 2 or more, and 4 or more. It is preferable that it is 8 or more.
By setting the aspect ratio of the dispersed phase to 2 or more, excellent polarization characteristics can be obtained in the optical film, and high luminance improvement performance can be obtained.
The aspect ratio of the dispersed phase can be calculated by the following formula (3) by photographing the morphology of the optical film with a transmission electron microscope, measuring the minor axis and major axis of the dispersed phase from this photograph.
Aspect ratio = Dispersed phase major axis / Dispersed phase minor axis (3)

In the optical film of this embodiment, the average minor axis of the disperse phase corresponding to the “island” in the sea-island structure composed of the phase (I) and the phase (II) is preferably 0.01 to 10 μm. More preferably, it is 0.025-5 micrometers.
When the average minor axis of the dispersed phase is in the range of 0.01 to 10 μm, excellent polarization characteristics can be obtained in the optical film, and the luminance improvement performance is enhanced.

The polarization characteristics can be evaluated by the degree of polarization PE and the average transmittance Tsp represented by the following formula.
In the following formula, Tp is the transmittance of polarized light whose electric field oscillates in parallel with the orientation direction of the polymer main chain (%), and Tv is the transmittance of polarized light whose electric field oscillates in the direction perpendicular to the orientation direction of the polymer main chain and in the film plane. (%).

The closer the polarization degree PE and the average transmittance Tsp are to 100% and 50%, respectively, the better the reflective polarizing plate.
In the optical film of the present embodiment, the degree of polarization PE is preferably 75% or more, more preferably 80% or more, and further preferably 90% or more.
In the optical film of the present embodiment, the average transmittance Tsp is preferably 40 to 60%, and more preferably 45 to 55%.
In the optical film of the present embodiment, when the degree of polarization PE and the average transmittance Tsp are within the above suitable ranges, they can be suitably used as a reflective polarizing plate and a brightness enhancement film.
In the optical film of the present embodiment, in order to set the degree of polarization PE and the average transmittance Tsp within the preferable ranges as described above, as the polyethylene naphthalate resin (P) and the thermoplastic resin (S), There is a tendency that it is preferable to select one having a large absolute value of intrinsic birefringence.

  In the optical film of the present embodiment, the volume ratio ((I) / () of the phase (I) mainly containing the polyethylene naphthalate resin (P) and the phase (II) mainly containing the thermoplastic resin (S). II)) is preferably 10/90 to 90/10. As a result, excellent polarization characteristics can be obtained in the optical film, and the luminance improvement performance is enhanced. The volume ratio ((I) / (II)) of the phase (I) mainly containing the polyethylene naphthalate resin (P) and the phase (II) mainly containing the thermoplastic resin (S) is 20/80 to A range of 80/20 is more preferred.

In the optical film of the present embodiment, the volume ratio of the phase (I) is 25% or more and 44% or less, the phase (I) is a continuous phase, and the phase (II) has a sea-island structure that is a dispersed phase. Morphology having a sea-island structure in which the volume ratio of phase (I) is 15% to 40%, phase (I) is a dispersed phase, and phase (II) is a continuous phase. It is preferable to control so that.
When the volume ratio of the phase (I) is 25% or more and 44% or less, the phase (I) is a continuous phase, and the phase (II) is a morphology having a sea-island structure that is a dispersed phase, contrary to the normal state. Since the volume of the dispersed phase becomes larger than the volume of the continuous phase, the distance between the dispersed phases becomes very small, and the polarization characteristics and the brightness enhancement performance are greatly improved.
When the volume ratio of the phase (I) is 15% or more and 40% or less, the phase (I) is a dispersed phase, and the phase (II) has a sea-island structure in which the phase (II) is a continuous phase, The smoothness of the surface is improved, and the polarization characteristics and the brightness improvement performance are dramatically improved.

[Other materials constituting optical film]
The optical film in the present embodiment includes the phase (I), the polyethylene naphthalate resin (P) constituting the phase (II), the thermoplastic resin (S), and the polymer (M), and these two phases. A predetermined refractive index controlling agent for controlling the refractive index of the liquid may be added.
In addition to the polyethylene naphthalate resin (P), the thermoplastic resin (S), and the polymer (M), other resins may be mixed in the optical film as long as the function is not impaired. Examples of other resins include polyolefin resins such as polymethyl methacrylate resin, polyethylene and polypropylene; polyamide resins; polyphenylene sulfide resins; polyether ether ketone resins; polyesters, polysulfones, polyphenylene oxides, polyimides, polyether imides, polyacetals and the like. Thermosetting resins such as phenol resins, melamine resins, silicone resins, and epoxy resins. These may be used alone or in combination of two or more.
The content of the other resin is preferably 10 parts by mass or less with respect to a total of 100 parts by mass of the polyethylene naphthalate resin (P), the thermoplastic resin (S), and the polymer (M). The amount is more preferably no greater than part by mass, and even more preferably no greater than 2 parts by mass. In addition, it is preferable even if it does not contain the said other resin.

In the optical film in the present embodiment, arbitrary additives may be blended according to various purposes within a range not impairing the function.
As an additive, a conventionally well-known thing can be applied as a material mix | blended with an optical material. For example, inorganic fillers such as silicon dioxide; pigments such as iron oxide; lubricants such as stearic acid, behenic acid, zinc stearate, calcium stearate, magnesium stearate, ethylene bisstearamide; mold release agent; paraffinic process oil Softeners and plasticizers such as naphthenic process oil, aromatic process oil, paraffin, organic polysiloxane and mineral oil; hindered phenolic antioxidants; antioxidants such as phosphorus thermal stabilizers; hindered amine light Stabilizer; Flame retardant; Antistatic agent; Reinforcing agent such as organic fiber, glass fiber, carbon fiber and metal whisker; Colorant; Benzotriazole compound, benzotriazine compound, benzoate compound, benzophenone compound, phenol compound , Oxazole compounds, malonic esters Compounds, ultraviolet absorbers such as lactone compounds; Other additives. These can be used in combination of two or more.
The amount of additive added is the total of the polymers constituting the optical film (polyethylene naphthalate resin (P), thermoplastic resin (S), polymer (M), and other resins mixed as necessary). When the value is 100 parts by mass, it is preferably 10 parts by mass or less, and more preferably 5 parts by mass or less.

[Method for producing optical film]
The optical film of this embodiment is obtained by melt-kneading a polyethylene naphthalate resin (P), a thermoplastic resin (S), a polymer (M), and other resins and additives as necessary. It can be manufactured with high productivity and low cost by forming it into a sheet (sheet form) and subjecting it to a stretching treatment as necessary.
The molding method is not particularly limited, and for example, known methods such as injection molding, blow molding, injection blow molding, inflation molding, extrusion molding, foam molding, etc. can be applied, and further, compression molding, vacuum molding, etc. The secondary processing molding method can also be applied.
In the case of producing by extrusion molding, a material obtained by melt-kneading polyethylene naphthalate resin (P), thermoplastic resin (S), and polymer (M) in advance may be used. You may shape | mold by melt-kneading these materials.
The method of melt kneading the polyethylene naphthalate resin (P), the thermoplastic resin (S), and the polymer (M) before the extrusion molding process is not particularly limited, and a conventionally known method is applied. it can. For example, melt kneaders such as single-screw extruders, twin-screw extruders, Banbury mixers, Brabenders, and various kneaders can be applied.
The optical film should be cast-dried and solidified after dissolving these using a solvent in which the polyethylene naphthalate resin (P), the thermoplastic resin (S) and the polymer (M) are all soluble. Thus, an unstretched film can be cast.

  However, for the method of forming the optical film of the present embodiment, since the combination of the polyethylene naphthalate resin (P) and the thermoplastic resin (S) is poorly compatible, the dispersibility of the sea-island structure and the dispersed phase, etc. From the viewpoint of ensuring morphological stability, extrusion molding and inflation molding are preferred, extrusion molding is more preferred, and in view of thickness uniformity of the optical film, extrusion molding with a T-die is more preferred.

  The method for imparting orientation to the optical film of the present embodiment is not particularly limited. For example, an external force or an electromagnetic field is imparted to the entire film, and a polyethylene naphthalate resin (P) and a thermoplastic resin (S) contained in the film. Can be simultaneously aligned under the same conditions. In particular, a method of stretching under an appropriate temperature condition and simultaneously orienting the polyethylene naphthalate resin (P) and the thermoplastic resin (S) is preferable because of high productivity and low cost.

As a stretching method for the optical film of the present embodiment, a method in which an unstretched film is longitudinally uniaxially stretched in the mechanical flow direction (MD) and laterally uniaxially stretched in a direction (TD) orthogonal to the mechanical flow direction can be applied.
Also, uniaxial stretching method by roll stretching, tenter stretching, or hot air furnace stretching, combination of roll stretching and tenter stretching, sequential biaxial stretching method by combination of hot air furnace stretching and tenter stretching, simultaneous biaxial stretching by tenter stretching Stretched film can be obtained by the biaxial stretching method using a tubular stretching method, etc., but from the viewpoint of increasing the aspect ratio of the dispersed phase and ensuring the polarization property and brightness enhancement performance, roll stretching, tenter stretching or hot-air furnace stretching Is preferable, and a method of longitudinally uniaxially stretching in the mechanical flow direction (MD) is more preferable.

As a draw ratio applied when manufacturing the optical film in the present embodiment, the minimum refractive index ny (1) of the phase (I) mainly containing the polyethylene naphthalate resin (P) shown and described in FIG. And the absolute value Δny of the difference between the maximum refractive index ny (2) of the phase (II) mainly containing the thermoplastic resin (S) is close to 0 or 0 as much as possible, and mainly contains the polyethylene naphthalate resin (P). The absolute value Δnx of the difference between the maximum refractive index nx (1) of the phase (I) and the minimum refractive index nx (2) of the phase (II) mainly containing the thermoplastic resin (S) is maximized, and the aspect ratio of the dispersed phase It is necessary to select a draw ratio at which the optical film becomes 2 or more and the optical film is not damaged during stretching. Such a draw ratio depends on the types and ratios of the polyethylene naphthalate resin (P) and the thermoplastic resin (S), but is preferably uniaxially drawn at a draw ratio of 3 times or more, and more than 3 times. It is more preferable to uniaxially stretch 10 times or less, and it is more preferable to uniaxially stretch 5 times to 10 times. In addition, a draw ratio is calculated | required by following formula (4).
Stretch ratio (times) = Length after stretching / Length before stretching (4)

  As the stretching temperature at the time of producing the optical film in the present embodiment, the minimum refractive index ny (1) of the phase (I) mainly containing the polyethylene naphthalate resin (P) shown in FIG. The absolute value Δny of the difference from the maximum refractive index ny (2) of the phase (II) mainly containing the plastic resin (S) is close to 0 or 0 as much as possible, and the phase mainly containing the polyethylene naphthalate resin (P) ( The absolute value Δnx of the difference between the maximum refractive index nx (1) of I) and the minimum refractive index nx (2) of the phase (II) mainly containing the thermoplastic resin (S) is maximized, and the aspect ratio of the dispersed phase is 2 Thus, it is necessary to select a stretching temperature at which the optical film does not break during stretching. Such a stretching temperature depends on the types and ratios of the polyethylene naphthalate resin (P) and the thermoplastic resin (S), but the polyethylene naphthalate resin (P) and the thermoplastic resin (S) Of the glass transition temperatures, it is preferable to select the higher glass transition temperature +0 to +20 (° C.), more preferably the glass transition temperature −10 to + 20 ° C. of the polyethylene naphthalate resin (P), more preferably The glass transition temperature of the polyethylene naphthalate resin (P) is −10 to + 10 ° C.

  As the stretching speed at the time of producing the optical film in the present embodiment, the minimum refractive index ny (1) of the phase (I) mainly containing the polyethylene naphthalate resin (P) shown in FIG. The absolute value Δny of the difference from the maximum refractive index ny (2) of the phase (II) mainly containing the plastic resin (S) is close to 0 or 0 as much as possible, and the phase mainly containing the polyethylene naphthalate resin (P) ( The absolute value Δnx of the difference between the maximum refractive index nx (1) of I) and the minimum refractive index nx (2) of the phase (II) mainly containing the thermoplastic resin (S) is maximized, and the aspect ratio of the dispersed phase is 2 Thus, it is necessary to select a stretching speed at which the optical film is not damaged during stretching. Such a stretching speed depends on the types of the polyethylene naphthalate resin (P) and the thermoplastic resin (S), the ratio thereof, and the like, but the stretching speed is preferably 100% / min or more and 50000% / min or less, More preferably, it is 2000% / min or more and 50000% / min or less, more preferably 3000% / min or more and 20000% / min or less.

  However, the stretching temperature, stretching speed, and stretching ratio described above mainly include the minimum refractive index ny (1) of the phase (I) mainly including the polyethylene naphthalate resin (P) and the thermoplastic resin (S). The absolute value Δny of the difference from the maximum refractive index ny (2) of the phase (II) is close to 0 or as much as possible, and the maximum refractive index nx (1) of the phase (I) mainly containing the polyethylene naphthalate resin (P) ) And the minimum refractive index nx (2) of the phase (II) mainly containing the thermoplastic resin (S) is the maximum, the aspect ratio of the dispersed phase is 2 or more, and the optical film Since these are related to each other as a requirement for preventing damage, they are appropriately selected within these preferable ranges.

In addition, since the morphology of the optical film in the present embodiment has a sea-island structure in which the phase (I) and the phase (II) are separated from each other, the maximum refractive index of the phase (I) and the phase (II), the minimum Refractive index, average refractive index, etc. are almost the same as the single maximum refractive index, minimum refractive index, average refractive index, etc. of polyethylene naphthalate resin (P) and thermoplastic resin (S) constituting these phases, respectively. Therefore, the stretching conditions of the optical film can be determined from the stretching conditions of the polyethylene naphthalate resin (P) and the thermoplastic resin (S).
Specifically, for each single film of polyethylene naphthalate resin (P) and thermoplastic resin (S), for example, stretching conditions (stretching ratio, stretching temperature, stretching speed, etc.) when uniaxially stretching It can be determined by measuring the relationship with the refractive index (stretching direction, stretching vertical direction, thickness direction).

The thickness of the optical film in the present embodiment is preferably 1 μm or more and 10 mm or less, more preferably 5 μm or more and 5 mm or less, and further preferably 20 μm or more and 1 mm or less.
In a narrow sense, a case where the thickness is 300 μm or less is referred to as an optical film, and a case where the thickness exceeds 300 μm may be distinguished as an optical sheet.

About the intensity | strength of the optical film of this embodiment, it is preferable that the frequency | count by a repeated bending test is 50 times or more, More preferably, it is 100 times or more, Especially preferably, it is 200 times or more. If the number of repeated bending tests is 50 times or more, practically good strength can be exhibited as an optical film.
The thickness uniformity of the optical film of the present embodiment is preferably less than ± 10%, more preferably less than ± 5%. If the film thickness uniformity is less than ± 10%, it can be suitably used as an optical film.

  Hereinafter, the present invention will be described in detail with reference to specific examples and comparative examples, but the present invention is not limited to the following examples.

[(1) Measuring method]
Measuring methods for each physical property and the like are shown below.
(I) Judgment of Intrinsic Birefringence Positive / Negative Birefringence The optical film is stretched while applying an extensional stress within the temperature range from the glass transition temperature to the glass transition temperature + 50 ° C., and then rapidly cooled and solidified. Intrinsic birefringence (npr-nvt) was measured.
npr represents a refractive index in a direction parallel to the alignment direction of the polymer aligned in a uniaxial order, and nvt represents a refractive index in a direction perpendicular to the alignment direction.
Note that a birefringence measuring device RETS-100 manufactured by Otsuka Electronics Co., Ltd. was used as a measuring device, the sample was measured so that the measurement surface was perpendicular to the measuring light, and measurement was performed by the rotating analyzer method.
When npr-nvt is negative, the intrinsic birefringence is negative, and when npr-nvt is positive, the intrinsic birefringence is positive.

(Ii) Measurement of refractive index The average refractive index of resins such as polyethylene naphthalate resin (P) and thermoplastic resin (S) is 23 ° C of each unstretched film using a model 2010 prism coupler manufactured by METRICON. The maximum refractive index, the minimum refractive index, and the refractive index in the thickness direction are measured under the temperature conditions of 532 nm, 633 nm, and 838 nm, respectively, and according to the Cauchy equation defined by the following equation (5), A curve was created, and the value at 550 nm obtained from the curve was defined as the maximum refractive index, the minimum refractive index, and the refractive index in the thickness direction, and the average value of these three refractive indexes was determined.
The maximum refractive index and the minimum refractive index of a single resin such as polyethylene naphthalate resin (P) and thermoplastic resin (S) are 23 ° C. temperature conditions when the resin is made into a single film when unstretched or stretched. The maximum refractive index and the minimum refractive index in 550 nm wavelength light were obtained by the same method as described above.
The maximum refractive index and the minimum refractive index of each phase of the optical film in the present embodiment were obtained by stretching a single resin constituting each phase under the same stretching conditions as in the case of obtaining the optical film in the present embodiment. The maximum refractive index and the minimum refractive index of a stretched film of a single resin at a wavelength of 550 nm under a temperature condition of 23 ° C. were determined in the same manner as described above, and the maximum refractive index and the minimum refractive index of each phase were obtained.
n = A + B / λ ² + C / λ ⁴ (5)
In the above formula (5), “n” represents the refractive index, “A”, “B” and “C” represent constants, and “λ” represents the wavelength of light.

(Iii) Acrylonitrile content Thermoplastic resin (S) (styrene-acrylonitrile copolymer) was formed into a film using a hot press machine, and 1603 cm of the film using FT-410 manufactured by JASCO Corporation. ^-1 and absorbance at 2245 cm ^-1 were measured.
The acrylonitrile content in the styrene-acrylonitrile copolymer was quantified using the relationship between the amount of acrylonitrile in the known styrene-acrylonitrile copolymer and the absorbance ratio of 1603 cm ⁻¹ and 2245 cm ⁻¹ .
As the thermoplastic resin (S), in addition to styrene and acrylonitrile, for example, when butyl acrylate is copolymerized, the absorbance at 1727 cm ⁻¹ characteristic of butyl acrylate is measured. The acrylonitrile content was quantified using the relationship of the absorbance ratio with the polymer.

(Iv) Weight average molecular weight (Mw)
It calculated | required by the gel permeation chromatography method.
Specifically, GPC-8020 manufactured by Tosoh Co., Ltd. as a measuring device, a differential thermal refractometer as a detector, Shodex K-805, 801 manufactured by Showa Denko Co., Ltd. as a column, and a solvent as a solvent Tetrahydrofuran was used, the measurement temperature was 40 ° C., and the weight average molecular weight was calculated in terms of polystyrene using commercially available standard polystyrene.

(V) Aspect ratio of disperse phase The morphology of the optical film was photographed with a transmission electron microscope, and the short diameter and long diameter of the disperse phase were measured from the obtained photograph. As an aspect ratio, an average value of 100 dispersed phases was obtained and used as an aspect ratio.
As an evaluation standard, when the aspect ratio was 4 or more, it was judged as ◎ (good), when 2 or more, it was judged as ◯ (normal), and when it was less than 2, it was judged as x (bad).

(Vi) Measurement of degree of polarization PE and transmittance Tsp As a measuring apparatus, an ultraviolet-visible near-infrared spectrophotometer V-670 manufactured by JASCO Corporation with an integrating sphere having a diameter of 150 mm was used.
Apply liquid paraffin thinly on both sides of the optical film, set it on the sample holder with both sides sandwiched by slide glass with a thickness of 1 mm, and light that passes through the optical film is diffused by the integrating sphere. It was collected and measured.
A polarizing plate was set between the light source and the optical film, and the transmission axis of the polarizing plate was set to 0 ° or 90 ° with respect to the stretching direction of the optical film.
The transmittance (%) of 550 nm wavelength light at 0 ° was Tp, and the transmittance (%) of 550 nm wavelength light at 90 ° was Tv.
From the values of Tp and Tv, the degree of polarization PE and the average transmittance Tsp were calculated by the following formula.
The case where the degree of polarization was 75% or more was evaluated as ◯ (good), and the case where it was less than 75% was evaluated as x (bad).
A case where the average transmittance was 48 to 58% was judged as ◯ (good), and a case outside this range was judged as × (bad).

(Vii) Average brightness, brightness improvement rate In place of the brightness enhancement film inside the Aquos (registered trademark) 37 type liquid crystal television LC-37XJ1 manufactured by Sharp Corporation, a sample film described later was inserted to obtain a brightness evaluation apparatus.
FIG. 4 shows a schematic cross-sectional view of the main part of the luminance evaluation apparatus.
The brightness evaluation apparatus in FIG. 4 includes a reflection sheet, a backlight, a diffusion plate, a prism sheet, and four diffusion sheets, which are sequentially provided, and further, a brightness enhancement film and a liquid crystal unit. have.
The arrangement direction of the sample film was such that the long side of the luminance evaluation apparatus and the transmission axis of the sample film were parallel.
A multi-video signal generator manufactured by Techio Co., Ltd. was connected to this luminance evaluation apparatus, and a white (100%) signal was sent to display white on the screen.
In this state, a two-dimensional high-speed color luminance meter ICAM manufactured by Toyo Corporation was installed in the front direction of the screen, and the surface luminance was measured.
The measurement range is a rectangular area of 300 mm length x 400 mm width in the center of the display, and the luminance of each pixel is measured for a total of 3072 pixels obtained by dividing the rectangular area into 48 pixels x 64 pixels. The average value was defined as the average luminance.
Then, the average brightness is compared with the brightness of 362 cd / m ² measured in the absence of the sample film or the brightness enhancement film inserted in the Aquos (registered trademark) 37 type liquid crystal television LC-37XJ1 manufactured by Sharp Corporation. And the numerical value of the average brightness of the sample film when the brightness when there is no brightness enhancement film originally inserted in the sample film or the television is 1 was defined as the brightness improvement rate.
When the luminance improvement rate was 1.20 or more, it was judged to be practically good.

(Viii) Measurement of glass transition temperature (Tg) Using a Pyris 1 DSC manufactured by PERKIN ELMER, the glass transition temperature (Tg) was measured at a heating rate of 20 ° C./min.

(Ix) Evaluation of optical film thickness and film thickness uniformity In a square measurement sample with a side of 10 cm and a side centered on the center of the optical film, the film is divided at intervals of 1 cm in length and width, and a film at a total of 121 measurement points. The thickness was measured with a micrometer, and the average value of these values was calculated as the thickness of the optical film.
The case where the difference between the thickness measurement value and the average thickness at each measurement point is less than ± 10% with respect to the average thickness of the optical film at all measurement points is evaluated as ○, and the case where it is ± 10% or more is evaluated as ×. did.

(X) Gel evaluation of unstretched film About the unstretched film obtained by extrusion-molding the resin material, the presence or absence of gel generation was confirmed visually and evaluated.
The case where no gel was generated was evaluated as ◯ (good), and the case where gel was generated was evaluated as x (bad).

(Xi) Evaluation of optical film strength As a measurement test piece, a rectangular film having a width of 15 mm from the center of the optical film in the stretching direction and a width of 130 mm in the direction perpendicular to the stretching was used.
Using a Toyo Seiki MIT Fatigue Fatigue Tester Model DA, the load is 0.1 kgf, the folding angle is 45 degrees, and the folding speed is 90 times / second. The test was repeatedly bent vertically.
The number of times until the test piece broke at the bending position was measured three times to obtain an average value.

[(2) Type and preparation of polymer]
(I) Polyethylene naphthalate resin (P)
As the polyethylene naphthalate resin, Teonex (registered trademark) TN8065S manufactured by Teijin Chemicals Ltd., which is a homopolymer, was used.
Tg was 122 ° C., the average refractive index was 1.654, and the intrinsic birefringence was positive.

(Ii) Thermoplastic resin (S: S-1)
A mixed liquid consisting of 28.4 parts by mass of acrylonitrile, 42.6 parts by mass of styrene, 29.0 parts by mass of ethylbenzene, and 0.02 parts by mass of t-butylperoxy-isopropyl carbonate as a polymerization initiator is 2.5 liters per hour. Was continuously fed to a fully mixed reactor having a capacity of 5 liters and polymerization was carried out at 150 ° C.
The polymerization solution was continuously led to an extruder with a vent, and unreacted monomers and solvent were removed under conditions of 260 ° C. and 40 Torr. The polymer was continuously cooled and solidified, and chopped to form particulate acrylonitrile-styrene copolymer. A polymer was obtained.
The composition of this acrylonitrile-styrene copolymer was 34% by mass of acrylonitrile units and 66% by mass of styrene units, the weight average molecular weight was 214,000, the average refractive index was 1.56, and the intrinsic birefringence was negative. . This acrylonitrile copolymer was designated as a thermoplastic resin (S-1). The properties of the thermoplastic resin (S-1) are shown in Table 1 below.

(Iii) Thermoplastic resins (S-2) to (S-7)
The thermoplastic resins (S-2) to (S-7) were produced in the same manner as the thermoplastic resin (S-1) described above by changing the monomer charge composition and the polymerization temperature.
All intrinsic birefringence was negative. The characteristics of the thermoplastic resins (S-2) to (S-7) are shown in Table 1 below.

(Iv) Polymer (M: M-1)
Modifier A5400 (EEA-g-AS) manufactured by NOF Corporation was used.
In the polymer (M-1), the copolymer (m1) is an EEA (ethylene-ethyl acrylate) copolymer, and the copolymer (m2) is an AS (styrene-acrylonitrile) copolymer. did.
The polymer (M-1) has a structure in which AS is grafted to EEA, EEA / AS = 70/30 (wt / wt), the ethyl acrylate content in EEA is 20% by mass, and the acrylonitrile content in AS is 30% by mass.

(V) Polymer (M-2)
Modifier A4400 (EGMA-g-AS) manufactured by NOF Corporation was used.
In the polymer (M-2), the copolymer (m1) was an EGMA (ethylene-glycidyl methacrylate) copolymer, and the copolymer (m2) was an AS (styrene-acrylonitrile) copolymer.
The polymer (M-2) has a structure in which AS is grafted to EGMA, EGMA / AS = 70/30 (wt / wt), and the acrylonitrile content in AS is 30% by mass.

[(3)] Production of optical film]
(Examples 1-6), (Comparative Examples 1-8)
In the hopper of the Ikegai twin screw extruder (PCM-30), the dried polyethylene naphthalate resin (P), the thermoplastic resin (S), and the polymer (M) so as to have the blending ratio shown in Table 1 below. Of pellets.
Extrusion conditions consisting of the resin temperature in the cylinder of the extruder and the extrusion amount (screw rotation speed, discharge amount) were adjusted (in Table 1, extrusion conditions) to obtain compound pellets.
The compound pellets obtained as described above were further mixed with a φ30 different direction twin screw extruder (BT-30-C-36-L type) manufactured by Plastic Engineering Laboratory, a gear pump HTD1-20-5 × 2 manufactured by Kyowa Finetech. Using a single-layer T die (T die width 400 mm, lip width 800 μm), the resin temperature in the cylinder of the extruder, the temperature of the T die (in Table 1, molding conditions), the amount of extrusion, and the winding speed are adjusted. To obtain an unstretched film.
Next, the unstretched film obtained as described above was cut into a width of 20 cm in the film length direction (MD direction). Using this cut unstretched film, Ichikin Kogyo Co., Ltd. lateral stretching apparatus SF-625, the unstretched film has a mechanical flow direction (MD direction) in the stretching direction and the end perpendicular to the stretching direction is a free end. Various evaluations were carried out on the obtained uniaxially stretched film.
Table 1 below shows the evaluation results of the resin composition, the properties of the thermoplastic resins (S-1) to (S-7), the extrusion conditions, the molding conditions, the unstretched film characteristics, the stretching conditions, and the film characteristics after stretching. .

  In the optical films of Examples 1 to 6 having the constitutional requirements of the present invention, a degree of polarization of 75% or more is obtained, the film has good polarization characteristics, and a practically sufficiently high brightness of 1.20 or more. The improvement rate was obtained, the film had practically sufficient film strength, and the film thickness uniformity was also good.

In Comparative Example 1, since the polymer (M-2) was used, a gel was generated in the film during film formation, and a hole was formed in the stretching process, so that an optical film could not be obtained.
In Comparative Example 2, since the polymer (M) was not used, the affinity between the phase (I) and the phase (II) was insufficient, and practically sufficient optical film strength could not be obtained. Uniformity deteriorated.
In Comparative Example 3, since the ratio of the polymer (M) was too high, the light transmittance of the optical film was lowered, and good polarization characteristics and brightness enhancement characteristics were not obtained.
In Comparative Example 4, the draw ratio was as small as 2 and the aspect ratio of the dispersed phase of the optical film was less than 2, and good polarization characteristics and brightness enhancement performance could not be obtained.
In Comparative Example 5, the ratio of components obtained from the vinyl cyanide monomer in the thermoplastic resin (S-4) was too low, and the average refractive index of the thermoplastic resin (S-4) was too high. A mismatch between the minimum refractive index of the polyethylene naphthalate resin (P) and the maximum refractive index of the thermoplastic resin (S-4) occurred, and good polarization characteristics could not be obtained. Moreover, the component ratio obtained from the vinyl cyanide monomer in the thermoplastic resin (S-4) is too low, and the affinity between the phase (I) and the phase (II) becomes insufficient, which is practically used. Sufficient film strength and film thickness uniformity were not obtained.
In Comparative Example 6, the component ratio obtained from the vinyl cyanide monomer in the thermoplastic resin (S-5) is too high, and the minimum refractive index of the polyethylene naphthalate resin (P) and the thermoplastic resin. A mismatch with the maximum refractive index of (S-5) occurred, and good polarization characteristics and brightness enhancement performance could not be obtained. Moreover, since the component ratio obtained from the vinyl cyanide monomer in the thermoplastic resin (S-5) was too high, the affinity between the phase (I) and the phase (II) was insufficient, and practical use In addition, sufficient film strength and film thickness uniformity were not obtained.
In Comparative Example 7, the weight average molecular weight of the thermoplastic resin (S-6) was too high. In Comparative Example 8, the weight average molecular weight of the thermoplastic resin (S-7) was too low. Enlargement and a decrease in the number of dispersed phases occurred, and good polarization characteristics and brightness enhancement performance could not be obtained, and practically sufficient film strength and film thickness uniformity could not be obtained.

  The optical film of the present invention can be used as a reflective polarizing plate or a brightness enhancement film, and in particular, a polarizing plate used in a display or projector such as a liquid crystal display, a plasma display, an organic EL display, a field emission display, a rear projection television, Brightness-enhancing films, retardation plates such as quarter-wave plates and half-wave plates, liquid crystal optical compensation films such as viewing angle control films, display front plates, display substrates, touch panels, lenses, projector screens, solar cells As a transparent substrate or the like used in the field, there is industrial applicability as a waveguide, a lens, an optical fiber, etc. in the fields of other optical communication systems, optical switching systems, and optical measurement systems.

Claims (7)

An optical film comprising a polyethylene naphthalate resin (P), a thermoplastic resin (S), and a polymer (M),
The thermoplastic resin (S) is
(A) 20-40% by mass of vinyl cyanide monomer,
(B) 40-80% by mass of an aromatic vinyl monomer,
(C) 0 to 20% by mass of other monomers copolymerizable with (a) and (b) ((a) + (b) + (c) = 100% by mass),
A thermoplastic resin having a weight average molecular weight of 150,000 to 230,000 and an average refractive index of 1.545 to 1.575 ,
The polymer (M) is a polymer in which the copolymer (m1) and the copolymer (m2) are block-bonded or graft-bonded, and the content of the epoxy group is 1 with respect to the total mass of the polymer (M). Mass% or less,
The copolymer (m1) is a copolymer obtained from ethylene and a (meth) acrylic acid ester monomer ,
The copolymer (m2) is
(D) 20 to 40% by mass of vinyl cyanide monomer,
(E) 40-80% by mass of an aromatic vinyl monomer,
(F) 0 to 20% by mass of other monomers copolymerizable with (d) and (e) ((d) + (e) + (f) = 100% by mass)
A copolymer consisting of,
With respect to the polyethylene naphthalate resin (P) + the thermoplastic resin (S) = 100 parts by weight, the polymer (M) is 0.1 to 6 parts by weight,
Having a morphology having a sea-island structure composed of a phase (I) mainly containing the polyethylene naphthalate resin (P) and a phase (II) mainly containing the thermoplastic resin (S),
Either the phase (I) or the phase (II) is a dispersed phase, and the aspect ratio of the dispersed phase is 2 or more,
The volume ratio ((I) / (II)) between the phase (I) and the phase (II) is 10/90 to 90/10.
An optical film. The mass ratio ((m1) / (m2)) of the copolymer (m1) and the copolymer (m2) constituting the polymer (M) is 50/50 to 90/10. An optical film as described in 1. The optical film according to claim 1 or 2, wherein the thermoplastic resin (S) is a polymer obtained by radical polymerization using an organic peroxide as a polymerization initiator. The optical film according to claim 1, wherein an aspect ratio of the dispersed phase is 4 or more. The optical film according to any one of claims 1 to 4 , which is uniaxially stretched at a stretch ratio of 3 times or more. Reflective polarizer formed of the optical film according to any one of claims 1 to 5. A brightness enhancement film comprising the optical film according to any one of claims 1 to 5 .

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