JP5396318B2 - Optical film, reflective polarizing plate and brightness enhancement film using the same - Google Patents

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, a method of displaying an image using polarized light obtained by separating a backlight light source is known.
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, it is possible to effectively use the light of the backlight light source.

As the reflection type polarizing plate described above, 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

On the other hand, 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 if a dispersed phase and a continuous phase are formed within the disclosed refractive index difference range, the degree of polarization is low, so that a good contrast cannot be obtained, and luminance and There is a problem that sufficient image characteristics cannot be obtained in balance with contrast.
Patent Document 5 discloses a reflective polarizing plate made of a copolymer comprising a polyester resin, a styrene monomer, a (meth) acrylate monomer, and a monomer having a cyclic structure. However, since this reflective polarizing plate has a large particle size of the dispersed phase, sufficient luminance cannot be obtained, and sufficient image characteristics cannot be obtained in balance between luminance and contrast. Yes.

  Therefore, in the present invention, an optical film that does not complicate the manufacturing process, has good productivity and can be manufactured at low cost, has excellent polarization characteristics, exhibits high brightness enhancement performance, and provides excellent contrast is provided. The purpose is to do.

As a result of intensive studies to solve the problems of the prior art, the inventors of the present invention, in an optical film having a sea-island structure composed of a dispersed phase (I) and a continuous phase (II), the orientation of the dispersed phase (I) Solving the problems of the prior art described above by specifying the refractive index of the dispersed phase (I) and the continuous phase (II) in an axis perpendicular to the direction and parallel to the optical film surface within a predetermined numerical range The present inventors have found that this can be done and have completed the present invention.
That is, the present invention is as follows.

[1]
An optical film having a sea-island structure composed of a dispersed phase (I) and a continuous phase (II),
The dispersed phase (I) contains a polyethylene naphthalate resin (P) as a main component,
The continuous phase (II) contains acrylic resin (S) and rubber (R) as main components,
The refractive index difference between the disperse phase (I) and the continuous phase (II) of the axis perpendicular to the orientation direction of the disperse phase (I) and parallel to the optical film surface is greater than 0.05 and greater than 0.1 A small optical film.

[2]
The refractive index difference between the dispersed phase (I) and the continuous phase (II) is 0.1 or more in the axis parallel to the orientation direction of the dispersed phase (I) and parallel to the optical film surface [1 ] The optical film as described in.

[3]
The optical phase according to [1] or [2], wherein the continuous phase (II) contains 10 to 60 parts by mass of the rubber (R) with respect to 100 parts by mass of the acrylic resin (S). the film.

[4]
The said rubber (R) is an optical film as described in any one of said [1] thru | or [3] whose diameter is 400 nm or less.

[5]
The acrylic resin (S) is
Any one of [1] to [4], which is a copolymer comprising a styrene monomer, a (meth) acrylic acid ester monomer, and a monomer represented by the following formula (3): The optical film as described.

  In the formula (3), X represents O or N—R, O represents an oxygen atom, N represents a nitrogen atom, R represents a hydrogen atom, an alkyl group, an aryl group, or a cycloalkyl group. .

[6]
The optical film according to any one of [1] to [5], wherein the aspect ratio of the dispersed phase (I) is 4 or more.

[7]
The optical film according to any one of [1] to [6], wherein the optical film is uniaxially stretched at a stretch ratio of 4 times or more.

[8]
A reflective polarizing plate comprising the optical film according to any one of [1] to [7].

[9]
A brightness enhancement film comprising the optical film according to any one of [1] to [7].

  According to the present invention, an optical film that does not complicate the manufacturing process, has good productivity, can be manufactured at low cost, has excellent polarization characteristics, exhibits high brightness enhancement performance, and has excellent contrast. Can be provided.

The relationship between the orientation degree and refractive index of the disperse phase (I) and continuous phase (II) which comprise an optical film is shown. The schematic sectional drawing of the principal part of the apparatus for brightness | luminance evaluation is shown.

Hereinafter, modes 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 of this embodiment is
It has a sea-island structure consisting of a dispersed phase (I) and a continuous phase (II).
The dispersed phase (I) contains a polyethylene naphthalate resin (P) as a main component.
The continuous phase (II) contains acrylic resin (S) and rubber (R) as main components.

Here, “main component” means that the amount of the resin contained in the phase exceeds 50% by volume, preferably 80% by volume or more, more preferably 90% by volume or more. To do.
In the continuous phase (II), it means that the total amount of the acrylic resin (S) + rubber (R) is more than 50% by volume in the continuous phase (II), preferably 80 volumes. % Or more, more preferably 90% by volume or more.

  The optical film of the present embodiment has a structure in which the rubber (R) is contained in the continuous phase (II), so that the particle size of the dispersed phase (I) is reduced, and good polarization characteristics, brightness enhancement characteristics, and contrast are obtained. Can be obtained.

The optical film of the present embodiment includes the dispersed phase (I) and the continuous phase having an axis perpendicular to the orientation direction of the dispersed phase (I) given a predetermined orientation by a method described later and parallel to the film surface. The refractive index difference from (II) is larger than 0.05 and smaller than 0.1.
Furthermore, the optical film has a refractive index difference of 0.1 or more between the dispersed phase (I) and the continuous phase (II) having an axis parallel to the orientation direction of the dispersed phase (I) and parallel to the film surface. It is preferable.
The difference in refractive index between the dispersed phase (I) and the continuous phase (II) on a predetermined axis will be described later.

The rubber (R) contained in the continuous phase (II) preferably has a diameter of 400 nm or less.
When the diameter of the rubber (R) is 400 nm or less, the light transmittance of the continuous phase (II) is improved, and the optical film of the present embodiment has excellent luminance enhancement characteristics. The diameter of the rubber (R) is more preferably 200 nm or less.
The diameter of the rubber (R) is obtained by photographing the morphology of the optical film of the present embodiment with a transmission electron microscope, measuring the short diameter and the long diameter of the rubber (R) from the obtained photograph, and calculating the average of the short diameter and the long diameter. The value is defined as the diameter of one rubber (R), and the average value of the diameters of 100 rubbers (R) is obtained, which is defined as the diameter of the rubber (R).

As described above, the continuous phase (II) contains the acrylic resin (S) and the rubber (R) as main components. However, the acrylic resin (S) and the rubber (R) in the continuous phase (II) The content ratio of R) is preferably 10 to 60 parts by mass of the rubber (R) with respect to 100 parts by mass of the acrylic resin (S).
When the content ratio of the rubber (R) is 60 parts by mass or less with respect to 100 parts by mass of the acrylic resin (S), the light transmittance of the continuous phase (II) is improved, and the optical film of this embodiment is , Excellent in polarization characteristics and brightness enhancement characteristics. More preferably, it is 50 parts by mass or less.

[Refractive index of optical film]
(Maximum refractive index and minimum refractive index of dispersed phase (I) and continuous phase (II))
Regarding the refractive index of the optical film of the present embodiment, the refractive index in the direction in which the refractive index of the dispersed phase (I) or continuous phase (II) is maximum is the maximum refractive index and the direction in which the refractive index is minimum. Was defined as the minimum refractive index.
In the same plane of the optical film, 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 separately stretching the resin materials constituting the dispersed phase (I) and the continuous phase (II) 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 are measured and calculated, and these are set as the maximum refractive index and the minimum refractive index of the dispersed phase (I) and the continuous phase (II).

(Average refractive index of resin constituting optical film)
The average refractive index of the resin such as polyethylene naphthalate resin (P), acrylic resin (S) and rubber (R) constituting the optical film in this embodiment is as follows: polyethylene naphthalate resin (P), acrylic resin When each of (S) + rubber (R) is used to form an unstretched film, the maximum refractive index, the minimum refractive index, and the refractive index in the thickness direction at 23 ° C. and 550 nm wavelength light are used. Average value.
However, the above-mentioned maximum refractive index, minimum refractive index, and refractive index in the thickness direction at 23 ° C. and 550 nm wavelength light are obtained by measuring each refractive index at 532 nm, 633 nm, and 838 nm wavelength light under the condition of 23 ° C. Then, a chromatic dispersion curve is created by the Cauchy equation defined by the following equation (1), 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]
The resin constituting the optical film of the present embodiment has a physical property called “inherent birefringence”.
This “intrinsic birefringence” is a value representing the magnitude of birefringence depending on the orientation, and is defined by the following equation (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 dispersed phase (I) and the continuous phase (II) constituting the optical film in this embodiment.
"Polyethylene naphthalate resin (P)" which is the main component of the dispersed phase (I) constituting the optical film, "Acrylic resin (S) + rubber (R)" which is the main component of the continuous phase (II) Intrinsic birefringence 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 dispersed phase (I) mainly containing the polyethylene naphthalate resin (P), that is, the dispersed phase (I) The axis parallel to the orientation direction of the optical film and the axis parallel to the optical film surface is the X direction, the direction of the minimum refractive index ny (1), that is, the axis perpendicular to the orientation direction of the dispersed phase (I) and parallel to the film surface In the Y direction, the continuous phase (II) mainly comprising an acrylic resin (S) and a rubber (R) having a different intrinsic birefringence sign from 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 continuous phase (II) mainly containing the acrylic resin (S) and the rubber (R) are as shown in FIG.

Also, the absolute value | nx (1) −nx (2) | of the difference between the maximum refractive index nx (1) of the dispersed phase (I) in the X direction and the minimum refractive index nx (2) of the continuous phase (II). Δnx, and the absolute value of the difference between the minimum refractive index ny (1) of the dispersed phase (I) in the Y direction and the maximum refractive index ny (2) of the continuous phase (II) | ny (1) −ny (2) If | is Δny, the result is as shown in FIG.
In the optical film in the present embodiment, it is preferable to increase Δnx as much as possible in order to realize excellent polarization characteristics and enhance the luminance improvement performance.
For that purpose, the uniaxial orientation of the polyethylene naphthalate resin (P) and the acrylic resin (S) constituting the dispersed phase (I) and the continuous phase (II), respectively, is enhanced as much as possible by means such as stretching. is necessary. Thereby, the difference in refractive index between the orientation direction of each resin and the direction perpendicular to the orientation direction can be maximized.

By setting Δnx shown in FIG. 1 to 0.1 or more, the light component whose electric field vibrates in the X direction is reflected, and excellent polarization characteristics can be obtained, and the effect of improving the contrast can be obtained.
Further, in the past, it was considered that Δny should be as close to 0 as possible. However, from the results of simulation, the dispersed phase diameter in the Y direction is made fine by using the material specified in the optical film of this embodiment. By controlling Δny to a value larger than 0.05 and smaller than 0.1, the transmittance of light whose electric field vibrates in the Y direction is not lowered, and the light component whose electric field vibrates in the X direction is further reduced. It has been found that reflection at a high level results in excellent balance of brightness and contrast.
In the optical film of the present embodiment, Δny is preferably less than 0.09, more preferably less than 0.08, in order to achieve excellent polarization characteristics and obtain high brightness enhancement performance and contrast. More preferably, it is smaller than 0.07.
Further, Δnx is preferably 0.15 or more, more preferably 0.22 or more, and further preferably 0.27 or more.

  In addition, in the main component single material of the dispersed phase (I), in addition to the maximum refractive index nx (1) of the dispersed phase (I) in the X direction and the minimum refractive index ny (1) of the dispersed phase (I) in the Y direction. When the refractive index nz (1) in the thickness direction of the optical film is measured, the average refractive index = (nx (1) + ny (1) + nz (1)) / 3.

[Polyethylene naphthalate resin (P)]
The polyethylene naphthalate resin (P) that constitutes the optical film in this embodiment and is the main component of the dispersed phase (I) is any one of a polyethylene naphthalate homopolymer and a copolymer of polyethylene naphthalate and polyethylene terephthalate. However, in order to obtain an optical film having excellent polarization characteristics and brightness enhancement performance, it is preferable to use a homopolymer alone.
In the optical film of the present embodiment, a polyethylene naphthalate system obtained by the calculation method described in the above item (average refractive index of resin constituting the optical film) in order to realize excellent polarization characteristics and brightness improvement performance The average refractive index of the resin (P) is preferably 1.60 or more, more preferably 1.63 or more, and further 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 can be obtained in the optical film and the polarization characteristics can be improved.

[Acrylic resin (S)]
The acrylic resin (S) contained as a main component in the continuous phase (II) of the optical film of the present embodiment is a single amount of at least a methacrylic acid ester monomer or an acrylic acid ester monomer. A polymer contained as a body component.
A plurality of types of acrylic resins (S) having different compositions and molecular weights can be used in combination.
A (meth) acrylic ester monomer is a methacrylic ester monomer or an acrylate ester monomer.
Methacrylic acid ester monomers or acrylic acid ester monomers are methacrylic acid esters such as cyclohexyl methacrylate, t-butyl cyclohexyl methacrylate, 2,2,2-trifluoroethyl methacrylate, and methyl methacrylate; It refers to acrylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, isopropyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, and the like. The above methacrylic acid ester monomers or acrylic acid ester monomers can be used alone or in combination.

The acrylic resin (S) containing an acrylic acid ester monomer or a methacrylic acid ester monomer as a monomer component includes other than the methacrylic acid ester monomer and the acrylic acid ester monomer. Those in which monomers are copolymerized are also included.
Examples of monomers other than acrylic acid esters, methacrylic acid esters copolymerizable with methacrylic acid esters, and acrylic acid esters include aromatic vinyl compounds such as styrene, vinyltoluene, and α-methylstyrene; acrylonitrile, methacrylonitrile And vinyl cyanides such as N-phenylmaleimide and N-cyclohexylmaleimide; unsaturated carboxylic acid anhydrides such as maleic anhydride; unsaturated acids such as acrylic acid, methacrylic acid, and maleic acid It is done. These may be used alone or in combination of two or more.

  When copolymerizing monomer components other than methacrylic acid ester and acrylic acid ester, the copolymerization ratio is preferably less than 70% by mass with respect to methacrylic acid ester and acrylic acid ester. More preferably, it is 60 mass% or less, Most preferably, it is 50 mass% or less. Less than 70% by mass is preferable because optical properties such as total light transmittance are excellent.

Among polymers containing acrylic acid esters or methacrylic acid esters as monomer components, methyl methacrylate homopolymers or copolymers of methyl methacrylate and other monomers are heat resistant, transparent, etc. Since it has the characteristic calculated | required by material, it is preferable.
As a monomer to be copolymerized with methyl methacrylate, acrylic acid esters are particularly preferable because they are excellent in thermal decomposition resistance and have high fluidity during molding of a methacrylic resin obtained by copolymerizing them. The amount of acrylic ester used when methyl methacrylate is copolymerized with methyl methacrylate is preferably 0.1% by mass or more from the viewpoint of thermal decomposition resistance, and 15% by mass or less from the viewpoint of heat resistance. It is more preferable that The content is more preferably 0.2% by mass or more and 14% by mass or less, and still more preferably 1% by mass or more and 12% by mass or less.
Among the acrylate esters, methyl acrylate and ethyl acrylate are preferable because the effect of improving the fluidity during the molding process described above can be remarkably obtained only by copolymerizing with a small amount of methyl methacrylate.
Moreover, as said methacrylic acid ester, isotactic polymethacrylic acid ester and syndiotactic polymethacrylic acid ester can also be used simultaneously.

<Weight average molecular weight of acrylic resin (S)>
The weight average molecular weight (Mw) of the acrylic resin (S) is preferably 50,000 to 200,000.
A weight average molecular weight (Mw) is calculated | required by polystyrene conversion using a gel permeation chromatography (GPC).
The weight average molecular weight is preferably 50,000 or more from the viewpoint of the strength of the optical film of the present embodiment, and preferably 200,000 or less from the viewpoint of molding processability and fluidity. A more preferable range is 70,000 to 150,000.

<Acrylic resin (S) made of copolymer>
As the acrylic resin (S) as the main component of the continuous phase (II), a copolymer comprising an alkyl (meth) acrylate monomer, a styrene monomer, and a monomer represented by the following general formula (3): Use of a polymer is preferable from the viewpoints of the heat resistance, optical characteristics, refractive index, compatibility with the dispersed phase (I), and the like of the optical film of the present embodiment.

  In the formula (3), X represents O or N—R, O is an oxygen atom, N is a nitrogen atom, R is any one selected from the group consisting of a hydrogen atom, an alkyl group, an aryl group, and a cycloalkyl group. Indicates.

Among the units represented by the formula (3), those in which X is O are unsaturated dicarboxylic acids which are anhydrides such as maleic anhydride, itaconic acid, ethylmaleic acid, methylitaconic acid, chloromaleic acid and the like. An anhydride monomer unit is mentioned. Among these, maleic anhydride is more preferable from the viewpoint of heat resistance and optical characteristics.
Examples of X being N—R include maleimide monomer units such as N-phenylmaleimide and N-cyclohexylmaleimide.
The copolymerization ratio is 40% by mass or more and 90% by mass or less of the methacrylic acid ester and / or acrylic ester unit, and 5% by mass or more and 40% by mass or less of the aromatic vinyl compound unit from the viewpoint of heat resistance and the like. The compound unit represented by the formula (3) is preferably 5% by mass or more and 20% by mass or less, more preferably 42% by mass or more and 83% by mass or less of methacrylic acid ester and / or acrylic acid ester unit. The group vinyl compound unit is 12% by mass or more and 40% by mass or less, and the compound unit represented by the formula (3) is 5% by mass or more and 18% by mass or less, more preferably a methacrylic acid ester and / or an acrylic acid ester. The unit is 45 mass% to 78 mass%, the aromatic vinyl compound unit is 16 mass% to 40 mass%, the compound unit represented by the formula (3) 6 wt% or more and 15 mass% or less.

<Method for producing acrylic resin (S)>
Examples of the method for producing the acrylic resin (S) that is the main component of the continuous phase (II) include, for example, cast polymerization, bulk polymerization, suspension polymerization, solution polymerization, emulsion polymerization, and anion polymerization. The method can be used.
Since acrylic resin (S) is a constituent material of optical films for optical applications, it is preferable to avoid the introduction of minute foreign substances as much as possible. From this viewpoint, bulk polymerization and solution polymerization without using suspending agents and emulsifiers are preferable. preferable.
When solution polymerization is performed, a solution prepared by dissolving a mixture of monomers in an aromatic hydrocarbon solvent such as toluene or ethylbenzene can be used.
In the case of polymerization by bulk polymerization, the polymerization can be started by irradiation with free radicals generated by heating or ionizing radiation as is usually done.

As the initiator used in the polymerization reaction, any initiator used in radical polymerization can be used. Examples thereof include azo compounds such as azobisisobutylnitrile, and organic peroxides such as benzoyl peroxide, lauroyl peroxide, and t-butylperoxy-2-ethylhexanoate.
In particular, when polymerization is carried out at a high temperature of 90 ° C. or higher, solution polymerization is generally used. Therefore, a peroxide, azobis, which has a 10-hour half-life temperature of 80 ° C. or higher and is soluble in the organic solvent used. Initiators and the like are preferred. Specifically, 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, cyclohexane peroxide, 2,5-dimethyl-2,5-di (benzoylperoxy) hexane, 1, Examples thereof include 1-azobis (1-cyclohexanecarbonitrile) and 2- (carbamoylazo) isobutyronitrile.
These initiators are preferably used in the range of 0.005 to 5% by mass when the entire material used for polymerization is 100% by mass.

In the polymerization reaction of the acrylic resin (S), a molecular weight regulator may be used as necessary.
As the molecular weight regulator, any one used in radical polymerization can be used, and for example, mercaptan compounds such as butyl mercaptan, octyl mercaptan, dodecyl mercaptan, 2-ethylhexyl thioglycolate are particularly preferable.
These molecular weight regulators are added in a concentration range such that the polymerization degree of the acrylic resin (S) is controlled within a preferable range.

[Rubber (R)]
The rubber (R) contained as a main component in the continuous phase (II) of the optical film of the present embodiment can be selected within a range that does not decrease the transmittance of the optical film.
Examples of the rubber (R) include acrylic rubber, nitrile rubber, isoprene rubber, urethane rubber, ethylene-propylene rubber, epichlorohydrin rubber, silicone rubber, styrene-butadiene rubber, butadiene rubber, fluorine rubber, and polyisobutylene rubber. . Acrylic rubber having a multilayer structure is preferred from the viewpoint of affinity with the above-mentioned acrylic resin (S), impact resistance, and weather resistance.

Among the acrylic rubbers, acrylic rubber particles having a multilayer structure of three or more layers are more preferable.
By using rubber particles having a multilayer structure of three or more layers as the rubber (R), deformation of the rubber particles due to heating is suppressed, and the glass transition temperature (Tg) and transparency of the optical film tend to be maintained. is there.
The rubber particle having a multilayer structure of three or more layers is a rubber particle having a multilayer structure in which a soft layer made of a rubber-like polymer and a hard layer made of a glass-like polymer are laminated. It is a particle having a three-layer structure formed in the order of soft layer-hard layer.
By having a hard layer in the innermost layer and the outermost layer, deformation of the rubber particles tends to be suppressed, and by having a soft component in the central layer, good toughness tends to be imparted.

When the rubber (R) is a rubber particle having a three-layer structure, the innermost layer of the rubber particle having a three-layer structure is 65 to 96% by mass of methyl methacrylate, and other copolymerizable monomers that can be copolymerized therewith. It is preferable that it is formed with the copolymer containing 4 to 35 mass% of bodies.
From the viewpoint of appropriately controlling the refractive index, the other copolymerizable monomer includes 0.1 to 5% by mass of an acrylate monomer, 5 to 35% by mass of an aromatic vinyl compound monomer, It is preferable that it contains 0.01 to 5% by mass of a copolymerizable polyfunctional monomer.
As the acrylate monomer in the copolymer that forms the innermost layer of rubber particles having a three-layer structure, n-butyl acrylate and 2-hexyl acrylate are preferred.
As the aromatic vinyl compound monomer, the same monomers as those used for the acrylic resin (S) described above can be used, but the present invention is carried out by adjusting the refractive index of the innermost layer of the rubber particles. From the viewpoint of improving the transparency of the optical film in the form, styrene or a derivative thereof is preferable. Although it does not specifically limit as a copolymerizable polyfunctional monomer, Preferably, ethylene glycol di (meth) acrylate, polyethyleneglycol di (meth) acrylate, 1,3-butylene glycol di (meth) acrylate, 1,4 -Butanediol di (meth) acrylate, allyl (meth) acrylate, triallyl isocyanurate, diallyl maleate, divinylbenzene and the like may be used alone or in combination of two or more. Good. Among these, allyl (meth) acrylate is more preferable.

When the rubber (R) is a rubber particle having a three-layer structure, the central layer of the rubber particle having a three-layer structure is 55 to 80% by mass of an acrylate ester and another copolymerizable monomer copolymerizable therewith. It is preferable that it is formed with the copolymer containing 20-45 mass% of bodies.
As said other copolymerizable monomer, when the said other copolymer monomer is 100 mass%, an aromatic vinyl compound monomer, copolymerizable polyfunctional monomer 0.1. What contains 5 mass% can be used.
The copolymer forming the central layer of rubber particles having a three-layer structure is preferably a copolymer exhibiting soft rubber elasticity from the viewpoint of imparting excellent toughness to the optical film.
The acrylic ester constituting the central layer of the rubber particles is not particularly limited, but one or two or more of methyl acrylate, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, etc. are used in combination. And preferably used. In particular, n-butyl acrylate and 2-hexyl acrylate are more preferable.
Of the other copolymerizable monomers copolymerizable with the acrylate ester, the aromatic vinyl compound monomer is the same as the monomer used for the acrylic resin (S) described above. Although styrene or a derivative thereof is preferably used from the viewpoint of adjusting the refractive index of the central layer to improve the transparency of the optical film.
Furthermore, among the other copolymerizable monomers copolymerizable with the acrylic ester, the copolymerizable polyfunctional monomer is the same as the copolymerizable polyfunctional monomer used in the innermost layer described above. Can be used. The content thereof is 0.1% by mass or more and 5% by mass or less when the other copolymerizable monomer copolymerizable with the acrylate ester is 100% by mass, and has a good crosslinking effect. In addition, since the crosslinking is moderate and the rubber elastic effect is increased, the toughness of the optical film tends to be improved, which is preferable.

When rubber (R) is a rubber particle having a three-layer structure, the outermost layer is preferably 70 to 100% by mass of methyl methacrylate and 0 to 30% by mass of other copolymerizable monomers copolymerizable therewith. % Is preferable.
Although it does not specifically limit as another copolymerizable monomer copolymerizable with methyl methacrylate of the outermost layer of the rubber particle which has a three-layer structure, N-butyl acrylate and 2-hexyl acrylate are preferable. .

When the rubber (R) is a rubber particle, the method for producing the rubber particle is not particularly limited, and can be obtained by a known polymerization method such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. In particular, it is preferably obtained by emulsion polymerization.
In this case, in the presence of an emulsifier and an initiator, the innermost monomer mixture is first added to complete the polymerization, and then the central monomer mixture is added to complete the polymerization, and then the outermost layer. When the monomer mixture is added to complete the polymerization, the multilayer structure particles can be easily obtained as a latex.
The rubber particles can be recovered from the latex as a powder by a known method such as salting out, spray drying, freeze drying and the like.
The rubber particles that are rubber (R) preferably have a refractive index difference of 0.015 or less, more preferably 0.012 or less, and 0.01 or less, with respect to the acrylic resin (S) described above. More preferably it is.
When the difference in refractive index between the rubber particles (R) and the acrylic resin (S) is 0.015 or less, an optical film excellent in transparency can be obtained.
As a method for satisfying the refractive index condition, a method for adjusting the unit composition ratio of each monomer of the acrylic resin (S), a composition ratio of the polymer or monomer in each layer used for the rubber particles, The method of adjusting etc. are mentioned.

[Structure of optical film]
The optical film of this embodiment has a morphology with a sea-island structure composed of a dispersed phase (I) mainly containing a polyethylene naphthalate resin (P) and a continuous phase (II) mainly containing an acrylic resin (S). Have. By setting it as such a structure, the optical film excellent in the polarization characteristic and the brightness improvement performance is obtained.
In the optical film of the present embodiment, the aspect ratio of the dispersed phase (I) corresponding to the “island” of the sea-island structure composed of the dispersed phase (I) and the continuous phase (II) is preferably 4 or more. 8 or more is more preferable.
By setting the aspect ratio of the dispersed phase to 4 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 (I) was measured by taking the morphology of the optical film of the present embodiment with a transmission electron microscope, measuring the minor axis and major axis of the dispersed phase (I) with this photograph, ).
Aspect ratio = Dispersed phase major axis / Dispersed phase minor axis (4)
In the optical film of this embodiment, the average minor axis of the disperse phase (I) corresponding to the “islands” of the sea-island structure composed of the disperse phase (I) and the continuous phase (II) is 10 to 400 nm. Preferably, it is 25-300 nm.
When the average minor axis of the dispersed phase (I) is in the range of 10 to 400 nm, excellent polarization characteristics can be obtained in the optical film, and the luminance improvement performance is enhanced.

In addition, the polarization characteristic of the optical film can be evaluated by the polarization degree PE and the average transmittance Tsp represented by the following formula.
In the following formula, Tp represents the transmittance (%) of polarized light whose electric field vibrates in parallel with the orientation direction of the polymer main chain.
In the following formula, Tv represents the transmittance (%) of polarized light whose electric field vibrates 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 this 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 this embodiment, in order to make 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 acrylic 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 of the dispersed phase (I) mainly containing the polyethylene naphthalate resin (P) and the continuous phase (II) mainly containing the acrylic resin (S) and the rubber (R). The ratio ((I) / (II)) is preferably 5/95 to 50/50. Thereby, an excellent polarization characteristic is obtained in the optical film, and the luminance improvement performance is enhanced. Volume ratio ((I) / (II) of dispersed phase (I) mainly containing polyethylene naphthalate resin (P) and continuous phase (II) mainly containing acrylic resin (S) and rubber (R) ) Is more preferably in the range of 10/90 to 40/60.

[Other materials composing optical film]
(Refractive index control agent)
In addition to the polyethylene naphthalate resin (P), the acrylic resin (S), and the rubber (R) constituting the dispersed phase (I) and the continuous phase (II), the optical film of this embodiment includes these 2 A predetermined refractive index control agent for controlling the refractive index of the phase may be added.
(Other resins)
In addition to the polyethylene naphthalate resin (P), the acrylic resin (S), and the rubber (R), 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 resins may be used alone or in combination of two or more.
The content of the other resins 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 acrylic resin (S), and the rubber (R). 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.
(Additive)
In the optical film in this 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 compounding amount of the additive is the total value of the polymers constituting the optical film (polyethylene naphthalate resin (P), acrylic resin (S), rubber (R), and other resins mixed as necessary). Is 100 parts by mass, 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 polyethylene naphthalate resin (P), acrylic resin (S), rubber (R), and other resins and additives as necessary, in a film form. It can be manufactured at high productivity and at low cost by forming into a (sheet shape) and imparting orientation by stretching treatment or the like.
(Molding process)
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), acrylic resin (S), and rubber (R) in advance may be used. These materials may be molded by melt-kneading.
In addition, the method of melt kneading the polyethylene naphthalate resin (P), the acrylic resin (S), and the rubber (R) in advance before the extrusion molding step is not particularly limited, and a conventionally known method is used. Applicable. For example, melt kneaders such as single-screw extruders, twin-screw extruders, Banbury mixers, Brabenders, and various kneaders can be applied.

  Further, the optical film of the present embodiment is prepared by using a solvent in which the polyethylene naphthalate resin (P), the acrylic resin (S), and the rubber (R) are all soluble, and then drying by casting. An unstretched film is obtained by solidified cast molding.

  In addition, about the shaping | molding method of the optical film of this embodiment, since the combination of polyethylene naphthalate type-resin (P) and acrylic resin (S) is poor in compatibility, a sea-island structure is formed reliably and a dispersed phase ( From the viewpoint of ensuring morphological stability, such as ensuring good dispersibility of I), extrusion molding and inflation molding are preferable, extrusion molding is more preferable, and in view of thickness uniformity of the optical film, extrusion by T-die Molding is more preferred.

(Orientation imparting step)
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 applied to the entire film, and a polyethylene naphthalate resin (P) and an acrylic resin (S) contained in the optical 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 acrylic resin (S) is preferable because of high productivity and low cost.

(Stretching process)
As a stretching method for the optical film of this embodiment, a method of stretching an unstretched film in the longitudinal direction in the mechanical flow direction (MD) and in the lateral direction in the direction perpendicular to the mechanical flow direction (TD) 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 dispersed 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 continuous phase (II) mainly containing acrylic resin (S) and rubber (R) is larger than 0.05 and smaller than 0.1, Maximum refractive index nx (1) of dispersed phase (I) mainly containing polyethylene naphthalate resin (P) and minimum refractive index of continuous phase (II) mainly containing acrylic resin (S) and rubber (R) It is preferable to select a draw ratio at which the absolute value Δnx of the difference of nx (2) is 0.1 or more and the optical film is not damaged during stretching.
Such a draw ratio depends on the types and ratios of polyethylene naphthalate resin (P), acrylic resin (S) and rubber (R), but in order to increase Δnx in FIG. 1 as much as possible. The uniaxial stretching is preferably performed at a stretching ratio of 4 times or more, more preferably uniaxial stretching at 4 times or more and 10 times or less, and further preferably uniaxial stretching at 5 times or more and 10 times or less.
In addition, a draw ratio is calculated | required by following formula (5).
Stretch ratio (times) = Length after stretching / Length before stretching (5)

As the stretching temperature when producing the optical film in the present embodiment, the minimum refractive index ny (1) of the dispersed phase (I) mainly containing the polyethylene naphthalate resin (P) shown in FIG. The absolute value Δny of the difference between the maximum refractive index ny (2) of the continuous phase (II) mainly containing the resin (S) and the rubber (R) is larger than 0.05 and smaller than 0.1, The difference between the maximum refractive index nx (1) of the dispersed phase (I) mainly containing the phthalate resin (P) and the minimum refractive index nx (2) of the continuous phase (II) mainly containing the acrylic resin (S). It is preferable to select a stretching temperature at which the absolute value Δnx is 0.1 or more and the optical film is not damaged during stretching.
Such a stretching temperature depends on the types and ratios of the polyethylene naphthalate resin (P) and the acrylic resin (S), but the polyethylene naphthalate resin (P) and the acrylic resin (S) + Among the glass transition temperatures with the rubber (R), it is preferable to select the higher glass transition temperature +0 to +20 (° C.), more preferably the glass transition temperature −10 to +20 of the polyethylene naphthalate resin (P). The glass transition temperature of the polyethylene naphthalate resin (P) is preferably −10 to + 10 ° C.

As the stretching speed when producing the optical film in the present embodiment, the minimum refractive index ny (1) of the dispersed 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 continuous phase (II) mainly containing the resin (S) rubber (R) is larger than 0.05 and smaller than 0.1, and polyethylene naphthalate Absolute difference between the maximum refractive index nx (1) of the dispersed phase (I) mainly containing the resin (P) and the minimum refractive index nx (2) of the continuous phase (II) mainly containing the acrylic resin (S) It is preferable to select a stretching speed at which the value Δnx is 0.1 or more and the optical film is not damaged during stretching.
Such a stretching speed depends on the types of the polyethylene naphthalate resin (P) and the acrylic 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.

  In addition, about the extending | stretching temperature mentioned above, an extending | stretching speed, and an extending | stretching magnification, the minimum refractive index ny (1) of the disperse phase (I) mainly containing a polyethylene naphthalate type resin (P), acrylic resin (S), and rubber | gum The absolute value Δny of the difference from the maximum refractive index ny (2) of the continuous phase (II) mainly containing (R) is made larger than 0.05 and smaller than 0.1, and the polyethylene naphthalate resin (P) Absolute difference between the maximum refractive index nx (1) of the dispersed phase (I) mainly containing and the minimum refractive index nx (2) of the continuous phase (II) mainly containing the acrylic resin (S) and the rubber (R) Since the value Δnx is 0.1 or more and they are related to each other as a requirement for preventing the optical film from being damaged, it is appropriately selected within these suitable ranges.

The morphology of the optical film in the present embodiment has a sea-island structure in which the dispersed phase (I) and the continuous phase (II) are separated from each other, so that the maximum refraction of the dispersed phase (I) and the continuous phase (II) is The refractive index, the minimum refractive index, the average refractive index, etc. are respectively the maximum refractive index and the minimum refractive index of polyethylene naphthalate resin (P) and acrylic resin (S) + rubber (R) constituting these phases. Assuming that the refractive index of the optical film is almost the same as the average refractive index, the stretching conditions of the optical film can be determined from the independent stretching conditions of polyethylene naphthalate resin (P) and acrylic resin (S) + rubber (R). it can.
Specifically, for each single film of polyethylene naphthalate resin (P) and acrylic resin (S) + rubber (R), for example, stretching conditions (stretching ratio, stretching temperature, It can be determined by measuring the relationship between the stretching speed or the like) and the refractive index (stretching direction, stretching vertical direction, thickness direction).

The thickness of the optical film in this embodiment is preferably 1 μm to 10 mm, more preferably 5 μm to 5 mm, and still more preferably 20 μm to 1 mm.
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.

[Use]
The optical film of the present embodiment has a sea-island structure composed of a dispersed phase (I) and a continuous phase (II), and the dispersed phase (I) includes a polyethylene naphthalate resin (P) as a main component, The continuous phase (II) contains acrylic resin (S) and rubber (R) as main components, and the dispersed phase (axis perpendicular to the orientation direction of the dispersed phase (I) and parallel to the optical film surface ( By specifying the refractive index difference between I) and the continuous phase (II) to be larger than 0.05 and smaller than 0.1, excellent change characteristics and high brightness enhancement performance can be exhibited.
Thereby, the outstanding contrast is obtained and it can utilize suitably especially as a reflection type polarizing plate or a brightness improvement 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.

The measuring method of the physical property regarding an optical film is shown below.
[(1) Measuring method]
(I) Determination of positive / negative of intrinsic birefringence Main component (for example, polyethylene naphthalate (P)) of dispersed phase (I) alone or main component (for example, acrylic resin (S) + rubber (R) of continuous phase (II) )) A single optical film is formed, stretched while applying an extensional stress within the temperature range of the glass transition temperature to the glass transition temperature + 50 ° C., rapidly cooled and solidified, and the intrinsic birefringence (npr-nvt at 23 ° C.) ) 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 <Maximum refractive index and minimum refractive index of each phase of optical film>
The maximum refractive index and the minimum refractive index of each phase of the optical film are obtained by stretching the resin constituting each phase under the same stretching conditions as in the case of obtaining the optical film in this example. The maximum refractive index and the minimum refractive index at a wavelength of 550 nm were obtained under the temperature condition of ° C., and the maximum refractive index and the minimum refractive index of each phase were obtained.
Specifically, the maximum refractive index and the minimum refractive index of the polyethylene naphthalate resin (P) constituting the dispersed phase (I) and the acrylic resin (S) + rubber (R) constituting the continuous phase (II) are Using a model 2010 prism coupler manufactured by METRICON, the maximum refractive index and the minimum refractive index of each stretched film at a wavelength of 532 nm, 633 nm, and 838 nm under a temperature condition of 23 ° C. were measured, respectively, and the following formula (6 ), A chromatic dispersion curve was created, and the value at 550 nm obtained from the curve was defined as the maximum refractive index and the minimum refractive index.
n = A + B / λ ² + C / λ ⁴ (6)
In the above formula (6), “n” represents the refractive index, “A”, “B” and “C” represent constants, and “λ” represents the wavelength of light.
The value of Δny is the minimum refractive index ny (1) of the single stretched film of the main component (for example, polyethylene naphthalate (P)) of the dispersed phase (I), the main component of the continuous phase (II) (for example, the acrylic resin (S)) + Rubber (R)) The maximum refractive index ny (2) of a single stretched film is measured, and is obtained from Δny = ny (1) −ny (2).
<Average refractive index>
The average refractive index of the polyethylene naphthalate-based resin (P) is a maximum refractive index in a wavelength light of 532 nm, 633 nm, and 838 nm of a non-stretched film under a temperature condition of 23 ° C. using a model 2010 prism coupler manufactured by METRICON. Measure the minimum refractive index and the refractive index in the thickness direction, create a chromatic dispersion curve according to the Cauchy equation defined by the above equation (6), and calculate the value at 550 nm from the curve as the maximum refractive index, The minimum refractive index and the refractive index in the thickness direction were used, and the average value of these three refractive indexes was obtained.

(Iii) Minor axis and aspect ratio of dispersed phase The morphology of the optical film was photographed with a transmission electron microscope, and the minor axis and major axis of the dispersed phase were measured from the obtained photographs.
The average value of the minor axis of 100 dispersed phases was determined and used as the average minor axis of the dispersed phase.
Further, the average value of 100 dispersed phases was obtained as the aspect ratio of the dispersed phase = (dispersed phase major axis / dispersed phase minor axis), and this was used as the aspect ratio.
As an evaluation standard of the dispersed phase minor axis, it was judged as ◯ (good) when the dispersed phase minor axis was 400 nm or more, and x (bad) when it was less than 400 nm.
As an aspect ratio evaluation standard of the dispersed phase, when the aspect ratio was 4 or more, it was judged as ◎ (good), and when it was less than 4, it was judged as × (bad).

(Iv) Diameter of rubber (R) The morphology of the optical film was photographed with a transmission electron microscope, the short diameter and the long diameter of rubber (R) were measured from the obtained photograph, and the average value of the short diameter and the long diameter was determined as rubber. (R) One diameter was obtained, and an average value of 100 rubber (R) diameters was obtained, and this was taken as the diameter of rubber (R).
The diameter of the rubber (R) is shown in Table 2 below.

(V) Measurement of polarization degree 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.
The light passing through the optical film was collected and measured, including the light diffused by the integrating sphere.
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 PE 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 Tsp was 40 to 60% was judged as ◯ (good), and a case outside this range was judged as × (bad).

(Vi) Average brightness, brightness increase rate, contrast In place of the brightness enhancement film inside the Aquos (registered trademark) 15-inch liquid crystal television LC-15SX7A manufactured by Sharp Corporation did.
A schematic cross-sectional view of the main part of the luminance evaluation apparatus is shown in FIG.
The luminance evaluation apparatus in FIG. 2 has a configuration in which a reflector 1, a light guide plate 3, a diffusion sheet 6, and a prism sheet 7 are sequentially provided, and a luminance enhancement film 4 and a liquid crystal unit 5 are further provided thereon. doing.
Light emitted from the edge light 2 as a light source is emitted from the surface of the light guide plate 3.
The arrangement direction of the sample film was such that the transmission axis of the original brightness enhancement film and the transmission axis of the sample film were arranged in the same direction.
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 and used for white luminance measurement. Further, in order to measure the contrast, a black signal was sent and used for black luminance measurement.
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.
And the average brightness is compared with the brightness of 277 cd / m ² measured in the absence of the sample film and the brightness enhancement film inserted inside the Aquos (registered trademark) 15-inch liquid crystal television LC-15SX7A manufactured by Sharp Corporation. Then, the numerical value of the average brightness of the sample film when the brightness when the brightness enhancement film originally inserted in the sample film or the television is not set to 1 is defined as the brightness increase rate.
The contrast was calculated by contrast = white luminance / black luminance.
A brightness increase rate of 1.10 or higher and a contrast of 390 or higher were judged to be practically good.

(Vii) 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.

[(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) Acrylic resin (S-1)
An acrylic resin (S-1) which is a methyl methacrylate-maleic anhydride-styrene copolymer was obtained by the method described in JP-B-63-1964.
The composition of the obtained copolymer was 72% by weight of methyl methacrylate, 12% by weight of maleic anhydride, and 16% by weight of styrene, and the melt flow rate value of the copolymer (ASTM-D1238; 230 ° C., 3.8 kg). Load) was 1.6 g / 10 min. Intrinsic birefringence was negative.

(Iii) Acrylic resins (S-2) and (S-3)
Acrylic resins (S-2) and (S-3) were produced in the same manner as the acrylic resin (S-1) described above, by changing the charged composition of the monomers. The compositions of the obtained (S-1) to (S-3) are shown in Table 1 below.

(Iv) Acrylic resin (S-4)
1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane was added to a monomer mixture consisting of 89.2 parts by weight of methyl methacrylate, 5.8 parts by weight of methyl acrylate and 5 parts by weight of xylene. 0.0294 parts by mass and 0.115 parts by mass of n-octyl mercaptan were added and mixed uniformly. This solution is continuously supplied to a sealed pressure resistant reactor having an internal volume of 10 L, polymerized with stirring at an average temperature of 130 ° C. and an average residence time of 2 hours, and then continuously supplied to a storage tank with a deaeration facility connected to the reactor. To remove volatiles. Furthermore, it transferred to the extruder in the molten state continuously, and the pellet of acrylic resin (S-4) which is a methyl methacrylate-methyl acrylate copolymer was obtained. The resulting acrylic resin (S-4) had a methyl acrylate content of 6.0% by mass and a melt flow rate value (ASTM-D1238; 230 ° C., 3.8 kg load) of 2.0 g / 10 min. there were.

(V) Rubber (R-1)
A reactor with an internal volume of 10 L equipped with a reflux condenser was charged with 4600 mL of ion-exchanged water and 24 g of sodium dioctylsulfosuccinate as an emulsifier, and the temperature was raised to 80 ° C. in a nitrogen atmosphere while stirring at a rotational speed of 250 rpm. There was virtually no state. Subsequently, 1.2 g of Rongalite was added as a reducing agent and dissolved uniformly.
As a first layer, a monomer mixture of MMA 150 g, BA 2.5 g, St 40 g, ALMA 0.2 g, and DPBHP 0.2 g was added and polymerized at 80 ° C. The reaction was complete in about 15 minutes.
Subsequently, as a second layer, a monomer mixture of BA 1110 g, St572 g, PDEGA 40 g, ALMA 7.0 g, DPBHP 3.5 g and Rongalite 2.0 g was added dropwise over 90 minutes. The reaction was completed 60 minutes after the completion of the dropwise addition.
Next, a monomer mixture of 190 g of MMA, 2.0 g of BA, 0.2 g of DPBHP, and 0.1 g of n-OM was dropped over 5 minutes as the third layer, and after completion of the dropping, the reaction at this stage was completed in about 15 minutes. .
Finally, a monomer mixture of MMA 380 g, BA 2.5 g, DPBHP 0.4 g, and n-OM 1.2 g was added as the third layer two steps over 10 minutes. This step was complete in about 15 minutes.
The temperature is raised to 95 ° C. and held for 1 hour, and the resulting emulsion is poured into a 0.5% aluminum chloride aqueous solution to agglomerate the polymer, washed 5 times with warm water, dried and dried in a white floc form. Obtained material. The average particle size of the obtained rubber (R-1) was 0.085 μm.
Note that MMA: methyl methacrylate, BA: butyl acrylate, ALMA: allyl methacrylate, DPBHP: diisopropylbenzene hydroperoxide, PDEGA: polyethylene glycol diacrylate, n-OM: n-octyl mercaptan.

(Vi) Rubber (R-2)
A reactor with an internal volume of 10 L equipped with a reflux condenser was charged with 4600 mL of ion-exchanged water and 24 g of sodium dioctylsulfosuccinate as an emulsifier, and the temperature was raised to 80 ° C. in a nitrogen atmosphere while stirring at a rotational speed of 250 rpm. There was virtually no state. Subsequently, 1.2 g of Rongalite was added as a reducing agent and dissolved uniformly.
As a first layer, a monomer mixture of MMA 140 g, BA 2.5 g, St 50 g, ALMA 0.2 g and DPBHP 0.2 g was added and polymerized at 80 ° C. The reaction was complete in about 15 minutes.
Next, as a second layer, a monomer mixture of BA 982 g, St 700 g, PDEGA 40 g, ALMA 7.0 g, DPBHP 3.5 g and Rongalite 2.0 g was added dropwise over 90 minutes. The reaction was completed 60 minutes after the completion of the dropwise addition.
Next, a monomer mixture of 190 g of MMA, 2.0 g of BA, 0.2 g of DPBHP, and 0.1 g of n-OM was dropped over 5 minutes as the third layer, and after completion of the dropping, the reaction at this stage was completed in about 15 minutes. .
Finally, a monomer mixture of MMA 380 g, BA 2.5 g, DPBHP 0.4 g, and n-OM 1.2 g was added as the third layer two steps over 10 minutes. This step was complete in about 15 minutes.
The temperature is raised to 95 ° C. and held for 1 hour, and the resulting emulsion is poured into a 0.5% aluminum chloride aqueous solution to agglomerate the polymer, washed 5 times with warm water, dried and dried in a white floc form. Obtained material. The average particle size of the obtained rubber (R-2) was 0.085 μm.

(Vii) Rubber (R-3)
A reactor with an internal volume of 10 L equipped with a reflux condenser was charged with 4600 mL of ion-exchanged water and 24 g of sodium dioctylsulfosuccinate as an emulsifier, and the temperature was raised to 80 ° C. in a nitrogen atmosphere while stirring at a rotational speed of 250 rpm. There was virtually no state. Subsequently, 1.2 g of Rongalite was added as a reducing agent and dissolved uniformly.
As a first layer, a monomer mixture of MMA 130 g, BA 2.5 g, St 60 g, ALMA 0.2 g, and DPBHP 0.2 g was added and polymerized at 80 ° C. The reaction was complete in about 15 minutes.
Next, as a second layer, a monomer mixture of BA 932 g, St 750 g, PDEGA 40 g, ALMA 7.0 g, DPBHP 3.5 g and Rongalite 2.0 g was added dropwise over 90 minutes. The reaction was completed 60 minutes after the completion of the dropwise addition.
Next, a monomer mixture of 190 g of MMA, 2.0 g of BA, 0.2 g of DPBHP, and 0.1 g of n-OM was dropped over 5 minutes as the third layer, and after completion of the dropping, the reaction at this stage was completed in about 15 minutes. .
Finally, a monomer mixture of MMA 380 g, BA 2.5 g, DPBHP 0.4 g, and n-OM 1.2 g was added as the third layer two steps over 10 minutes. This step was complete in about 15 minutes.
The temperature is raised to 95 ° C. and held for 1 hour, and the resulting emulsion is poured into a 0.5% aluminum chloride aqueous solution to agglomerate the polymer, washed 5 times with warm water, dried and dried in a white floc form. Obtained material. The average particle size of the obtained rubber (R-3) was 0.085 μm.

(Viii) Rubber (R-4)
A reactor with an internal volume of 10 L equipped with a reflux condenser was charged with 4600 mL of ion-exchanged water and 24 g of sodium dioctylsulfosuccinate as an emulsifier, and the temperature was raised to 80 ° C. in a nitrogen atmosphere while stirring at a rotational speed of 250 rpm. There was virtually no state. Subsequently, 1.2 g of Rongalite was added as a reducing agent and dissolved uniformly.
As a first layer, a monomer mixture of MMA 184.5 g, BA 6 g, St2 g, ALMA 0.2 g, and DPBHP 0.2 g was added and polymerized at 80 ° C. The reaction was complete in about 15 minutes.
Next, as a second layer, a monomer mixture of BA 1345 g, St 337 g, PDEGA 40 g, ALMA 7.0 g, DPBHP 3.5 g and Rongalite 2.0 g was added dropwise over 90 minutes. The reaction was completed 60 minutes after the completion of the dropwise addition.
Next, a monomer mixture of 190 g of MMA, 2.0 g of BA, 0.2 g of DPBHP, and 0.1 g of n-OM was dropped over 5 minutes as the third layer, and after completion of the dropping, the reaction at this stage was completed in about 15 minutes. .
Finally, a monomer mixture of MMA 380 g, BA 2.5 g, DPBHP 0.4 g, and n-OM 1.2 g was added as the third layer two steps over 10 minutes. This step was complete in about 15 minutes.
The temperature is raised to 95 ° C. and held for 1 hour, and the resulting emulsion is poured into a 0.5% aluminum chloride aqueous solution to agglomerate the polymer, washed 5 times with warm water, dried and dried in a white floc form. Obtained material. The average particle size of the obtained rubber (R-4) was 0.085 μm.

(Ix) Resin composition (B-1)
Acrylic resin (S) + rubber (R) = 100% by weight, acrylic resin (S-1) is 55% by weight and rubber (R-1) is 45% by weight. The resin composition (B-1) in which (R-1) was uniformly dispersed in (S-1) was obtained by kneading with a shaft extruder.
The composition and properties of the resin composition (B-1) are shown in Table 2 below.

(X) Resin compositions (B-2) to (B-5)
Resin compositions (B-2) to (B-5) were produced by changing the type of rubber (R) and the charged composition in the same manner as the resin composition (B-1) described above.
All intrinsic birefringence was negative. The properties of the resin compositions (B-2) to (B-5) are shown in Table 2 below.

(Xi) Thermoplastic resin (AS-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 used as a thermoplastic resin (AS-1).

[(3)] Production of optical film]
(Examples 1-6), (Comparative Examples 1-4)
Dry resin pellets were introduced into the hopper of a Ikegai twin-screw extruder (PCM-30) so as to have the blending ratio shown in Table 3 below.
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 3, 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 3, molding conditions), the extrusion amount, 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 3 below shows the evaluation results of the resin composition, extrusion conditions, molding conditions, unstretched film characteristics, stretching conditions, and post-stretching film characteristics.
The evaluation results were judged by polarization characteristics (when the degree of polarization and average transmittance are both good), the luminance increase rate, and contrast.

In each of 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, a luminance improvement rate of 1.10 or more, and 390 or more. A sufficiently high luminance characteristic of contrast was obtained in practical use.
In Comparative Example 1, since rubber (R) was not used, the particle size of the dispersed phase was increased, and good polarization characteristics, brightness enhancement characteristics, and contrast were not obtained.
In Comparative Example 2, the draw ratio was as small as 3 and Δny was 0.1 or more, and good polarization characteristics, brightness enhancement characteristics and contrast were not obtained.
In Comparative Example 3, since the refractive index of the continuous phase was high, Δny was 0.05 or less, and the polarization degree was lowered, so that good polarization characteristics and contrast were not obtained.
In Comparative Example 4, since the refractive index of the continuous phase was high, Δny was 0.05 or less, and the polarization degree was lowered, so that good polarization characteristics and contrast 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, or a rear projection television, and a luminance. Improvement 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, and solar cells As the transparent substrate used, etc., there are industrial applicability as waveguides, lenses, optical fibers, etc. in the fields of other optical communication systems, optical switching systems, and optical measurement systems.

DESCRIPTION OF SYMBOLS 1 Reflector 2 Edge light 3 Light guide plate 4 Sample film 5 Liquid crystal unit 6 Diffusion sheet 7 Prism sheet

Claims (9)

An optical film having a sea-island structure composed of a dispersed phase (I) and a continuous phase (II),
The dispersed phase (I) contains a polyethylene naphthalate resin (P) as a main component,
The continuous phase (II) contains acrylic resin (S) and rubber (R) as main components,
The refractive index difference between the disperse phase (I) and the continuous phase (II) of the axis perpendicular to the orientation direction of the disperse phase (I) and parallel to the optical film surface is greater than 0.05 and greater than 0.1 A small optical film.   The refractive index difference between the dispersed phase (I) and the continuous phase (II) of an axis parallel to the orientation direction of the dispersed phase (I) and parallel to the optical film surface is 0.1 or more. The optical film described in 1. The continuous phase (II) is
The optical film according to claim 1 or 2, wherein the rubber (R) is contained in an amount of 10 to 60 parts by mass with respect to 100 parts by mass of the acrylic resin (S).   The optical film according to any one of claims 1 to 3, wherein the rubber (R) has a diameter of 400 nm or less. The acrylic resin (S) is
The copolymer according to any one of claims 1 to 4, which is a copolymer comprising a styrene monomer, a (meth) acrylic acid ester monomer, and a monomer represented by the following formula (3). Optical film.

(In the formula (3), X represents O or N—R, O represents an oxygen atom, N represents a nitrogen atom, and R represents a hydrogen atom, an alkyl group, an aryl group, or a cycloalkyl group. )   The optical film according to any one of claims 1 to 5, wherein the aspect ratio of the dispersed phase (I) is 4 or more.   The optical film according to any one of claims 1 to 6, wherein the optical film is uniaxially stretched at a stretch ratio of 4 times or more.   A reflective polarizing plate comprising the optical film according to claim 1.   The brightness improvement film which consists of an optical film as described in any one of Claims 1 thru | or 7.

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