JP2012208261A - Multilayer film, retardation film, and manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multilayer film including a B layer comprising a resin b with a negative intrinsic birefringence value, and A layers formed on both surfaces of the B layer and comprising a resin (a) with a positive intrinsic birefringence value, in which the resin b includes a styrene polymer, and the resin (a) includes polycarbonate, and capable of measuring the film thickness of the A layer and the B layer.SOLUTION: The multilayer film includes: the B layer comprising the resin b with the negative intrinsic birefringence value; a C layer formed on both surfaces of the B layer and comprising a resin c; and the A layer formed on the surfaces of the C layers at the far sides from the B layer, and comprising the resin (a) with the positive intrinsic birefringence value. The resin b includes the styrene polymer, the resin c includes a thermoplastic acrylic resin, and the resin (a) includes the polycarbonate. The thickness of each of the C layers is 5 μm or more.

Description

  The present invention relates to a multilayer film, a retardation film using the same, and a method for producing them.

  For example, a retardation film used for optical compensation of a liquid crystal display device is required to be capable of reducing a change in color tone of the display device depending on an observation angle, and various techniques have been developed conventionally. For example, Patent Document 1 proposes a retardation film in which a film made of a resin having a positive intrinsic birefringence value and a film made of a resin having a negative intrinsic birefringence value are bonded together. However, a resin having a negative intrinsic birefringence value usually has low strength and is brittle. Therefore, if a layer made of a resin having a negative intrinsic birefringence value is exposed on the surface of the retardation film, the layer made of a resin having a negative intrinsic birefringence value is easily damaged when the retardation film is handled.

  In order to prevent damage to a layer made of a resin having a negative intrinsic birefringence value, in Patent Document 2, a layer made of a resin having a negative intrinsic birefringence value is formed by a pair of layers made of a resin having a positive intrinsic birefringence value. It has been proposed to pinch. According to the configuration described in Patent Document 2, a layer made of a resin having a negative intrinsic birefringence value can be protected by a pair of layers made of a resin having a positive intrinsic birefringence value. Damage to the layer can be prevented.

JP 2008-216998 A JP 2009-192845 A

  When a layer made of a resin having a negative intrinsic birefringence value is sandwiched between a pair of layers made of a resin having a positive intrinsic birefringence value as described in Patent Document 2, a layer made of a resin having a positive intrinsic birefringence value Becomes a plurality of layers, and the control of retardation (phase difference) becomes complicated. Therefore, in order to precisely control the retardation of the retardation film, it is required to precisely control the thickness of each layer of the retardation film.

  In order to precisely control the film thickness of each layer of the retardation film, it is preferable to accurately measure the film thickness of each layer during the production and appropriately set the production conditions based on the measured film thickness. However, with the conventional retardation film described in Patent Document 2, it is difficult to accurately measure the film thickness of each layer during production. If it is after production, for example, it is conceivable to measure the film thickness by observing the cross section of the film with a scanning electron microscope (SEM). It was difficult. In particular, when a resin containing polycarbonate is used as a resin having a positive intrinsic birefringence value and a resin containing a styrene polymer is used as a resin having a negative intrinsic birefringence value, the refractive index of the resin forming each layer is Because of the same degree, it was particularly difficult to optically measure the film thickness of each layer.

  Further, for example, as described in Patent Document 1, when there are only one layer made of a resin having a positive intrinsic birefringence value and one layer made of a resin having a negative intrinsic birefringence value, an infrared film thickness meter It is also conceivable to measure the film thickness from the absorption of infrared rays in each layer. However, when there are a plurality of layers made of a resin having a positive intrinsic birefringence value as described in Patent Document 2, the thickness of each layer made of a resin having a positive intrinsic birefringence value is measured with an infrared film thickness meter. could not.

  The present invention was devised in view of the above-described problems, and includes a B layer made of a resin b having a negative intrinsic birefringence value, and a resin a having a positive intrinsic birefringence value formed on both sides of the B layer. A multilayer film and a retardation film that can measure the film thickness of each of the A layer and the B layer, and the resin b includes a styrene polymer, the resin a includes a polycarbonate, and It aims at providing these manufacturing methods.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have additionally provided a layer between the resin a and the resin b that can increase the refractive index difference between them, thereby producing a multilayer film. Even in the middle, the thickness of each of the A layer and the B layer is measured with an interference type film thickness meter using the reflection of light at the interface between the A layer and the other layer and the interface between the B layer and the other layer. It was found that it can be measured. Furthermore, when the multilayer film is a retardation film, the feedback control is performed according to the film thicknesses of the A layer, the B layer, and the other layers measured during the manufacturing, and the manufacturing conditions are appropriately adjusted. It has been found that the retardation of the layer film can be precisely controlled.
Based on these findings, the present inventor has completed the present invention.
That is, the gist of the present invention is the following [1] to [8].

[1] A B layer made of a resin b having a negative intrinsic birefringence value;
C layer made of resin c formed on both surfaces of the B layer;
A multilayer film comprising the C layer and an A layer formed of a resin a having a positive intrinsic birefringence value formed on a surface far from the B layer;
The resin b includes a styrene polymer,
The resin c includes a thermoplastic acrylic resin;
The resin a includes polycarbonate;
The multilayer film whose thickness of each said C layer is 5 micrometers or more.
[2] The multilayer film according to [1], wherein the styrenic polymer is a copolymer containing a repeating unit derived from maleic anhydride.
[3] A retardation film obtained by stretching the multilayer film according to [1] or [2].
[4] The phase difference according to [3], wherein retardation Re at an incident angle of 0 ° and retardation R ₄₀ at an incident angle of 40 ° satisfy a relationship of 0.92 ≦ R ₄₀ /Re≦1.08. the film.
[5] A method for producing a multilayer film according to [1] or [2],
A resin b containing a styrenic polymer and having a negative intrinsic birefringence value;
A resin c containing a thermoplastic acrylic resin;
A resin a containing polycarbonate and having a positive intrinsic birefringence value;
The manufacturing method including co-extrusion.
[6] A resin b containing a styrene-based polymer and having a negative intrinsic birefringence value;
A resin c containing a thermoplastic acrylic resin;
A resin a containing polycarbonate and having a positive intrinsic birefringence value;
And extrude
When the uniaxial stretching direction is the X axis, the direction perpendicular to the uniaxial stretching direction in the film plane is the Y axis, and the film thickness direction is the Z axis, the plane of incidence is perpendicular to the film plane and the vibration plane of the electric vector is The phase of linearly polarized light in the XZ plane that is perpendicularly incident on the film surface and the plane of vibration of the electric vector in the YZ plane is delayed when uniaxially stretched in the X-axis direction at the temperature T1 and is different from the temperature T1. A coextrusion step for obtaining a pre-stretched film, which proceeds when uniaxially stretched in the X-axis direction at a temperature T2,
A first stretching step for subjecting the pre-stretched film to a uniaxial stretching treatment in one direction at a temperature T1 or T2.
A second stretching step in which a uniaxial stretching treatment is performed at a temperature T2 or T1 different from the above in a direction orthogonal to the direction in which the uniaxial stretching treatment is performed in the first stretching step;
The film before stretching is
A B layer made of the resin b;
C layer composed of the resin c formed on both surfaces of the B layer, each having a thickness of 5 μm or more;
The manufacturing method of retardation film which is a multilayer film provided with the A layer which consists of the said resin a formed in the surface on the side far from the said B layer of the said C layer.
[7] The co-extrusion step includes extruding one or more of the resin a, the resin b, and the resin c from the opening of a die having an opening whose size can be adjusted.
The manufacturing method further includes:
A measuring step of measuring the film thickness of each of the layer consisting of the resin a, the layer consisting of the resin b, and the layer consisting of the resin c of the pre-stretch film with an interference film thickness meter;
The manufacturing method according to [6], further including an opening adjusting step of adjusting the size of the opening of the die in accordance with the measured film thickness of each layer.
[8] The production method according to [7], further including a speed adjustment step of adjusting an extrusion speed of one or more of the resin a, the resin b, and the resin c according to the measured film thickness of each layer. .

According to the multilayer film of the present invention, for example, even during the production of the multilayer film, the film thickness of each layer in the film including the A layer and the B layer can be accurately measured.
According to the method for producing a multilayer film of the present invention, the retardation film of the present invention having a desired retardation can be produced.

FIG. 1 is a cross-sectional view schematically showing a cross section of an example of the multilayer film of the present invention taken along a plane perpendicular to the film main surface. FIG. 2 shows that when the glass transition temperature Tg _A of the resin a forming the A layer is high and the glass transition temperature Tg _B of the resin b forming the B layer is low, the A layer and the B layer of the film before stretching. The temperature dependence of the retardation of each of the A layer and the B layer when the film is stretched, and the temperature dependence of the retardation Δ of the film before stretching when the film before stretching (here, A layer + B layer) is stretched. It is a figure which shows an example. FIG. 3 is a diagram schematically showing an outline of a retardation film manufacturing apparatus according to an embodiment of the method for manufacturing a multilayer film of the present invention.

  Hereinafter, the present invention will be described in detail with reference to embodiments and examples, but the present invention is not limited to the following embodiments and examples, and the gist of the present invention and its equivalent scope are described below. You may implement arbitrarily changing in the range which does not deviate.

[1. (Multilayer film)
The multilayer film of the present invention includes a B layer composed of a resin b having a negative intrinsic birefringence value, a C layer composed of a resin c formed on both surfaces of the B layer, and a surface of the C layer far from the B layer. And an A layer made of resin a having a positive intrinsic birefringence value. That is, the multilayer film of the present invention includes an A layer, a C layer, a B layer, a C layer, and an A layer in this order.
Here, a positive intrinsic birefringence value means that the refractive index in the stretching direction is larger than the refractive index in the direction orthogonal to the stretching direction. Further, the negative intrinsic birefringence value means that the refractive index in the stretching direction is smaller than the refractive index in the direction orthogonal to the stretching direction. The intrinsic birefringence value can also be calculated from the dielectric constant distribution.

[1-1. B layer]
The B layer is made of a resin b having a negative intrinsic birefringence value. The resin b having a negative intrinsic birefringence value includes at least a styrene polymer. In addition, a styrenic polymer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The styrenic polymer is a polymer containing a repeating unit derived from a styrenic monomer (hereinafter referred to as “styrenic monomer unit” as appropriate). The aforementioned styrene monomer refers to styrene and styrene derivatives. Examples of the styrene derivative include α-methylstyrene, o-methylstyrene, p-methylstyrene, p-chlorostyrene, p-nitrostyrene, p-aminostyrene, p-carboxystyrene, p-phenylstyrene, and the like. . In addition, a styrenic monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. Therefore, the styrenic polymer may contain one type of styrenic monomer unit alone, or may contain two or more types of styrenic monomer units in combination at any ratio. .

  The styrene polymer may be a homopolymer or copolymer containing only a styrene monomer, or may be a copolymer of a styrene monomer and another monomer. Good. Examples of monomers that can be copolymerized with styrene monomers include, for example, ethylene, propylene, butadiene, isoprene, acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, N-phenylmaleimide, methyl acrylate, methyl methacrylate, Examples include ethyl acrylate, ethyl methacrylate, maleic anhydride, acrylic acid, methacrylic acid, and vinyl acetate. In addition, these monomers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  In particular, when the styrenic polymer is a copolymer, the styrenic polymer is a copolymer containing a repeating unit derived from maleic anhydride (hereinafter referred to as “maleic anhydride unit” as appropriate). It is preferable. When the copolymer contains maleic anhydride units, the heat resistance of the styrenic polymer can be improved. The amount of maleic anhydride units is preferably 5 parts by weight or more, more preferably 10 parts by weight or more, particularly preferably 15 parts by weight or more, preferably 30 parts by weight or less, based on 100 parts by weight of the styrenic polymer. More preferably, it is 28 parts by weight or less, and particularly preferably 26 parts by weight or less.

  The styrenic polymer may contain a polymer unit derived from an acrylic compound in order to adjust the average refractive index difference. Specific examples of the acrylic compound can be the same as the specific examples of the material of the resin c.

  In addition, the resin b having a negative intrinsic birefringence value may contain components other than the styrene polymer as long as the effects of the present invention are not significantly impaired. For example, the resin b having a negative intrinsic birefringence value may contain a polymer other than a styrene polymer, a compounding agent, and the like.

  From the viewpoint of making the intrinsic birefringence value of the resin b negative, the polymer other than the styrenic polymer is preferably a polymer having a negative intrinsic birefringence value. Specific examples thereof include cellulose ester polymers, or multi-component copolymers thereof. Moreover, the structural component of these polymers may be contained as a repeating unit in a part of styrene-type polymer. Furthermore, these may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. However, from the viewpoint of remarkably exhibiting the advantages of the present invention, the amount of the polymer other than the styrene polymer in the resin b is preferably small. For example, 10 parts by weight or less with respect to 100 parts by weight of the styrene polymer. Is preferably 5 parts by weight or less, more preferably 3 parts by weight or less. Among these, it is particularly preferable that no polymer other than the styrene polymer is contained.

The resin b may also include a polymer containing a repeating unit derived from an acrylic compound (hereinafter, referred to as “acrylic polymer” as appropriate) as a polymer other than the styrene-based polymer. Thereby, the difference of the average refractive index of resin b and resin c can be adjusted, the affinity of C layer and B layer can be improved, and adhesive strength can also be improved. Furthermore, since the acrylic polymer has high strength and is hard, the strength of the B layer can be increased, and consequently the strength of the multilayer film of the present invention can be increased.
Specific examples of the acrylic polymer can be the same as the specific examples of the material of the resin c.

When the resin b includes a repeating unit derived from an acrylic compound as a copolymer component of the styrene polymer or a component of the polymer other than the styrene polymer, the amount is the sum of all the polymers included in the resin b. Is 100% by weight, preferably 45% by weight or less, more preferably 40% by weight or less, and particularly preferably 35% by weight or less. The lower limit may be 0% by weight.
By setting the amount of the repeating unit derived from the acrylic compound to be equal to or less than the upper limit of the above range, the average refractive index difference between the C layer and the B layer can be sufficiently expressed.

  Examples of compounding agents include: lubricants; layered crystal compounds; inorganic fine particles; stabilizers such as antioxidants, heat stabilizers, light stabilizers, weathering stabilizers, ultraviolet absorbers, near infrared absorbers; plasticizers; dyes And coloring agents such as pigments and pigments; antistatic agents; and the like. Among these, a lubricant and an ultraviolet absorber are preferable because they can improve flexibility and weather resistance. In addition, a compounding agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. Further, the amount of the compounding agent can be appropriately determined within a range that does not significantly impair the effects of the present invention. For example, the total light transmittance in terms of 1 mm thickness of the multilayer film of the present invention can be maintained at 80% or more. It is good.

  Examples of the lubricant include inorganic particles such as silicon dioxide, titanium dioxide, magnesium oxide, calcium carbonate, magnesium carbonate, barium sulfate, strontium sulfate; polymethyl acrylate, polymethyl methacrylate, polyacrylonitrile, polystyrene, cellulose acetate, cellulose acetate pro Organic particles such as pionate can be mentioned. Among these, organic particles are preferable as the lubricant.

  Examples of ultraviolet absorbers include oxybenzophenone compounds, benzotriazole compounds, salicylic acid ester compounds, benzophenone ultraviolet absorbers, benzotriazole ultraviolet absorbers, acrylonitrile ultraviolet absorbers, triazine compounds, nickel complex compounds. And inorganic powders. Specific examples of suitable UV absorbers include 2,2′-methylenebis (4- (1,1,3,3-tetramethylbutyl) -6- (2H-benzotriazol-2-yl) phenol), 2- (2′-hydroxy-3′-tert-butyl-5′-methylphenyl) -5-chlorobenzotriazole, 2,4-di-tert-butyl-6- (5-chlorobenzotriazol-2-yl) ) Phenol, 2,2′-dihydroxy-4,4′-dimethoxybenzophenone, 2,2 ′, 4,4′-tetrahydroxybenzophenone, and the like. Particularly preferred are 2,2′-methylenebis ( 4- (1,1,3,3-tetramethylbutyl) -6- (2H-benzotriazol-2-yl) phenol).

The glass transition temperature Tg _B of the resin b having a negative intrinsic birefringence value is usually 80 ° C. or higher, preferably 90 ° C. or higher, more preferably 100 ° C. or higher, still more preferably 110 ° C. or higher, and particularly preferably 120 ° C. or higher. . With such a high glass transition temperature Tg _B, it is possible to reduce orientation relaxation of the resin b having a negative intrinsic birefringence value. Although not particularly limited to the upper limit of the glass transition temperature Tg _B, usually it is 200 ° C. or less.

The breaking elongation of the resin b having a negative intrinsic birefringence value at the glass transition temperature Tg _A of the resin a having a positive intrinsic birefringence value is preferably 50% or more, and more preferably 80% or more. The upper limit of the breaking elongation of the resin b having a negative intrinsic birefringence value is not particularly limited, but is usually 200% or less. If the elongation at break is within this range, the retardation film of the present invention can be stably produced by stretching. The elongation at break is determined by using a test piece type 1B test piece described in JISK7127 at a pulling speed of 100 mm / min.

The absolute value | Tg _A −Tg _B | of the difference between the glass transition temperature Tg _A of the resin a having a positive intrinsic birefringence value and the glass transition temperature Tg _B of the resin b having a negative intrinsic birefringence value is preferably 5 It is larger than ° C., more preferably 8 ° C. or higher, preferably 40 ° C. or lower, more preferably 20 ° C. or lower. If the absolute value | Tg _A −Tg _B | of the difference between the glass transition temperatures is too small, the temperature dependence of retardation development tends to be small. On the other hand, if the absolute value | Tg _A −Tg _B | of the difference between the glass transition temperatures is too large, it is difficult to stretch a resin having a high glass transition temperature, and the planarity of the multilayer film may be easily lowered. is there. The glass transition temperature Tg _A is preferably higher than the glass transition temperature Tg _B. Therefore, it is usually preferable that the resin having a positive intrinsic birefringence value and the resin b having a negative intrinsic birefringence value satisfy the relationship of Tg _A > Tg _B + 5 ° C.

  In the multilayer film of the present invention, the B layer is sandwiched between the A layers, so that even if the strength of the resin b is low, the B layer formed by the resin b is not damaged.

  When the multilayer film of the present invention is used as a material for obtaining the retardation film of the present invention, it is usually applied by, for example, stretching treatment so that the resin b having a negative intrinsic birefringence value in the layer B has a negative intrinsic birefringence value. Orient the coalesced molecules. When the molecules of the polymer are oriented, refractive index anisotropy occurs, and retardation is developed in the B layer. In the retardation film of the present invention, the retardation of the B layer expressed in this way and the retardation expressed in the A layer are synthesized to produce the desired retardation of the entire retardation film of the present invention. It is like that. Therefore, the thickness of the B layer may be set to an appropriate value according to the specific retardation to be expressed in the retardation film of the present invention.

  The multilayer film of the present invention may be provided with two or more B layers, but from the viewpoint of simplifying the control of retardation and reducing the thickness of the multilayer film of the present invention, it is provided with only one layer. Is preferred.

  The thickness of the B layer is preferably 30 μm or more, more preferably 40 μm or more, on the other hand, preferably 300 μm or less, and more preferably 250 μm or less. When there are a plurality of B layers, the total thickness of the B layers can be within the preferred range. By setting the thickness of the B layer within the above preferable range, a thin multilayer film in which the thickness of each layer can be easily controlled in-line can be created.

[1-2. C layer]
The C layer contains a resin c containing a thermoplastic acrylic resin. A thermoplastic acrylic resin is a thermoplastic resin, and is a resin containing an acrylic polymer (that is, a polymer containing a repeating unit derived from an acrylic compound).

  An acrylic compound means acrylic acid and an acrylic acid derivative. Examples of the acrylic compound include acrylic acid, acrylic ester, acrylamide, acrylonitrile, methacrylic acid, and methacrylic ester. Among these, as the acrylic compound, an acrylic acid derivative is preferable, and a (meth) acrylic acid ester is more preferable. Here, “(meth) acrylic acid” means acrylic acid and methacrylic acid.

  Examples of (meth) acrylic acid esters include alkyl esters of (meth) acrylic acid. Especially, the thing derived from (meth) acrylic acid and a C1-C15 alkanol or cycloalkanol is preferable, and the thing derived from a C1-C8 alkanol is more preferable. By reducing the number of carbon atoms as described above, the elongation at break of the multilayer film of the present invention can be reduced.

  Specific examples of the acrylate ester include methyl acrylate, ethyl acrylate, n-propyl acrylate, i-propyl acrylate, n-butyl acrylate, i-butyl acrylate, sec-butyl acrylate, and t-acrylate. -Butyl, n-hexyl acrylate, cyclohexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, n-decyl acrylate, n-dodecyl acrylate, and the like.

  Specific examples of the methacrylic acid ester include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, sec-butyl methacrylate, methacrylic acid. Examples thereof include t-butyl acid, n-hexyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, n-decyl methacrylate, and n-dodecyl methacrylate.

  Furthermore, the (meth) acrylic acid ester may have a substituent such as a hydroxyl group or a halogen atom as long as the effects of the present invention are not significantly impaired. Examples of the (meth) acrylic acid ester having such a substituent include 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-methacrylic acid 2- Examples include hydroxypropyl, 4-hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, and glycidyl methacrylate.

  In addition, an acrylic compound may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The acrylic polymer may be a polymer composed only of an acrylic compound, or may be a copolymer of an acrylic compound and a monomer copolymerizable therewith. Examples of the copolymerizable monomer include α, β-ethylenically unsaturated carboxylic acid ester monomers other than the above-mentioned (meth) acrylic acid esters, and α, β-ethylenically unsaturated carboxylic acid monomers. Examples include a monomer, an alkenyl aromatic monomer, a conjugated diene monomer, a non-conjugated diene monomer, a carboxylic acid unsaturated alcohol ester, and an olefin monomer. As the monomer copolymerizable with the acrylic compound, one type may be used alone, or two or more types may be used in combination at any ratio.

The resin c may contain components other than the acrylic polymer as long as the effects of the present invention are not significantly impaired. For example, the resin c may contain a polymer other than an acrylic polymer, a compounding agent, and the like.
When the acrylic polymer contains a unit other than the repeating unit derived from the acrylic compound, and / or when the resin c contains a component other than the acrylic polymer, the ratio of the repeating unit derived from all the acrylic compounds contained in the resin c Is preferably 50% by weight or more, more preferably 85% by weight or more, and particularly preferably 90% by weight or more, where the total of all the polymers contained in the resin c is 100% by weight. The upper limit can be 100% by weight.

  Of these acrylic polymers, polymethacrylate is preferred, and polymethyl methacrylate is more preferred.

The glass transition temperature Tg _C of the resin c is preferably lower than the glass transition temperature Tg _A of the resin a forming the A layer and the glass transition temperature Tg _B of the resin b forming the B layer. Furthermore, the glass transition temperatures Tg _A and Tg _C are more preferably Tg _A > Tg _C + 20 ° C., and particularly preferably Tg _A > Tg _C + 35 ° C. The glass transition temperatures Tg _B and Tg _C are more preferably Tg _B > Tg _C + 15 ° C., and particularly preferably Tg _B > Tg _C + 18 ° C. When the layer made of the resin a, the layer made of the resin b, and the layer made of the resin c containing the acrylic polymer are co-stretched, stretching at a temperature in the vicinity of Tg _A or Tg _B results in a combination of the A layer and the B layer. Refractive characteristics can be expressed sufficiently and uniformly. At this time, since the layer made of the resin c containing the acrylic polymer is stretched at a temperature higher than the glass transition temperature Tg _C by, for example, 20 ° C. or more, the acrylic polymer is hardly oriented and non-oriented. It becomes the state of. Therefore, by making the glass transition temperature Tg _C lower than the glass transition temperature Tg _A and the glass transition temperature Tg _B , the C layer can be easily non-oriented, and the resin is oriented in the A layer and the B layer. Appropriate retardation can be developed. Further, the lower limit of Tg _C is preferably 20 ° C, more preferably 25 ° C, and further preferably 30 ° C.

  The resin c may contain rubber particles. By including the rubber particles, the flexibility of the resin c can be increased, and the impact resistance of the multilayer film of the present invention can be improved.

  Examples of the rubber forming the rubber particles include acrylate polymer rubber, polymer rubber mainly composed of butadiene, and ethylene-vinyl acetate copolymer rubber. Examples of the acrylate polymer rubber include those having butyl acrylate, 2-ethylhexyl acrylate or the like as a main component of a monomer unit. Among these, acrylic acid ester polymer rubber mainly composed of butyl acrylate and polymer rubber mainly composed of butadiene are preferable.

  The rubber particles may contain two or more types of rubber. Further, these rubbers may be mixed uniformly, but may be layered. Examples of rubber particles in which rubber is layered include a core composed of a rubber elastic component obtained by grafting an alkyl acrylate such as butyl acrylate and styrene, and one or both of polymethyl methacrylate and methyl methacrylate and an alkyl acrylate. Examples thereof include particles in which a hard resin layer (shell) made of a polymer forms a layer with a core-shell structure.

  The rubber particles preferably have a number average particle diameter of 0.05 μm or more, more preferably 0.1 μm or more, and preferably 0.3 μm or less, and 0.25 μm or less. Is more preferable. By setting the number average particle diameter to be equal to or more than the lower limit of the above range, the flexibility of the film is increased, and even if a load is applied during stretching, the film can be hardly broken. Moreover, by making a number average particle diameter below the upper limit of the said range, the haze of the multilayer film of this invention can be restrained low and a light transmittance can be made high.

The rubber particles have a refractive index n _p (λ) at a wavelength of 380 nm to 780 nm between the refractive index n _m (λ) at a wavelength of 380 nm to 780 nm of the acrylic polymer serving as a matrix, and | n _p (λ) −n It is preferable to satisfy the relationship of _m (λ) | ≦ 0.05. In particular, it is more preferable that | n _p (λ) −n _m (λ) | ≦ 0.045. Note that n _p (λ) and n _m (λ) are average values of the main refractive indexes at the wavelength λ. By keeping the value of | n _p (λ) −n _m (λ) | in the above range, the reflection at the interface between the acrylic polymer and the rubber particles is suppressed, and the transparency of the multilayer film of the present invention is reduced. Can be high.

  The amount of the rubber particles is preferably 5 parts by weight or more and preferably 50 parts by weight or less with respect to 100 parts by weight of the acrylic polymer. By setting the amount of the rubber particles to be equal to or more than the lower limit of the above range, the impact resistance of the retardation film of the present invention can be enhanced and the handling property can be improved. Moreover, the transparency of the retardation film of this invention can be made high by making the quantity of a rubber particle below the upper limit of the said range.

  In the multilayer film of the present invention, each thickness of the C layer is 5 μm or more. By having such a thickness, it is possible to satisfactorily measure the film thicknesses of the A layer to the C layer with the interference film thickness meter. The thickness of the C layer is preferably 7 μm or more, more preferably 10 μm or more, on the other hand, preferably 30 μm or less, more preferably 25 μm or less, and further preferably 20 μm or less. More preferred.

  The tensile elastic modulus of the C layer is preferably 2.0 GPa or more, more preferably 3.0 GPa or more. When the tensile elastic modulus of the C layer is within this range, when the multilayer film of the present invention is stretched to obtain a retardation film, the shift between the layers can be suppressed, and the orientation angle can be easily controlled.

[1-3. A layer]
The A layer is made of a resin a having a positive intrinsic birefringence value. The resin a having a positive intrinsic birefringence value includes at least polycarbonate. Polycarbonate is a polymer excellent in retardation development, low temperature stretchability, and adhesion to other layers.

  Any polycarbonate can be used as long as it is a polymer having a repeating unit due to a carbonate bond (—O—C (═O) —O—). Moreover, what consists of one type of repeating unit may be used for a polycarbonate, and what combined two or more types of repeating units in arbitrary ratios may be used.

Examples of the polycarbonate include bisphenol A polycarbonate, branched bisphenol A polycarbonate, o, o, o ′, o′-tetramethylbisphenol A polycarbonate, and the like.
As the polycarbonate, one type may be used alone, or two or more types may be used in combination at any ratio.

  Further, the resin a having a positive intrinsic birefringence value may contain components other than polycarbonate as long as the effects of the present invention are not significantly impaired. For example, the resin a having a positive intrinsic birefringence value may contain a polymer other than a polycarbonate polymer, a compounding agent, and the like.

  Examples of polymers other than the polycarbonate and acrylic polymer that the resin a having a positive intrinsic birefringence value may contain include olefin polymers such as polyethylene and polypropylene; polyesters such as polyethylene terephthalate and polybutylene terephthalate; polyphenylene Polyarylene sulfide such as sulfide; polyvinyl alcohol; cellulose ester; polyethersulfone; polysulfone; polyallylsulfone; polyvinyl chloride; norbornene polymer; Moreover, the structural component of these polymers may be contained as a repeating unit in a part of polycarbonate. Furthermore, these may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. However, from the viewpoint of remarkably exhibiting the advantages of the present invention, the amount of the polymer other than the polycarbonate polymer in the resin a is preferably small, for example, 10 parts by weight or less is preferable with respect to 100 parts by weight of the polycarbonate. More preferably, it is more preferably 3 parts by weight or less. Among these, it is particularly preferable that no polymer other than polycarbonate is contained.

Resin a may also contain an acrylic polymer as a polymer other than polycarbonate. Thereby, the difference of the average refractive index of resin a and resin c can be adjusted, the affinity of C layer and A layer can be improved, and adhesive strength can also be improved. Furthermore, since the acrylic polymer has high strength and is hard, the strength of the A layer can be increased, and consequently the strength of the multilayer film of the present invention can be increased.
Specific examples of the acrylic polymer can be the same as the specific examples of the material of the resin c.

When the resin a contains an acrylic polymer as a component of a polymer other than polycarbonate, the amount of the repeating unit derived from the acrylic compound in the resin a is 100% by weight as the total of all the polymers contained in the resin a. It is preferably 45% by weight or less, more preferably 40% by weight or less, and particularly preferably 30% by weight or less. The lower limit may be 0% by weight.
By setting the amount of the repeating unit derived from the acrylic compound to be equal to or less than the upper limit of the above range, the average refractive index difference between the C layer and the B layer can be sufficiently expressed.

  Examples of the compounding agent that may include the resin a having a positive intrinsic birefringence value include the same examples as the compounding agent that may include the resin b having a negative intrinsic birefringence value. In addition, a compounding agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. The amount of the compounding agent can be appropriately determined within a range that does not significantly impair the effects of the present invention. For example, the total light transmittance in terms of 1 mm thickness of the multilayer film may be maintained within 80% or more. .

The glass transition temperature Tg _A of the resin a having a positive intrinsic birefringence value is usually 80 ° C. or higher, preferably 90 ° C. or higher, more preferably 100 ° C. or higher, still more preferably 110 ° C. or higher, and particularly preferably 120 ° C. or higher. . With such a high glass transition temperature Tg _{A, the} relaxation of the orientation of the resin a having a positive intrinsic birefringence value can be reduced. Although not particularly limited to the upper limit of the glass transition temperature Tg _A, usually at 200 ° C. or less.

The breaking elongation of the resin a having a positive intrinsic birefringence value at the glass transition temperature Tg _B of the resin b having a negative intrinsic birefringence value is preferably 50% or more, and more preferably 80% or more. . When the elongation at break is within this range, a retardation film can be stably produced by stretching.

  Usually, the A layer is provided exposed on the main surface of the multilayer film of the present invention. That is, the A layer is usually the outermost layer of the multilayer film of the present invention. Even if the A layer is exposed as described above, the strength of the A layer is usually strong, so that it is difficult to break during handling and the handling property is not deteriorated.

  When the multilayer film of the present invention is used as a material for obtaining the retardation film of the present invention, usually, for example, by applying a stretching treatment, the weight of the resin a having a positive intrinsic birefringence value in the layer A is included. Orient the coalesced molecules. When the molecules of the polymer are oriented, refractive index anisotropy occurs, and retardation is developed in the A layer. In the retardation film of the present invention, the retardation of the A layer expressed in this way and the retardation expressed in the B layer are synthesized to produce the desired retardation of the entire retardation film of the present invention. . Therefore, the thickness of the A layer may be set to an appropriate value according to the specific retardation to be expressed in the retardation film of the present invention.

  The multilayer film of the present invention may be provided with three or more A layers, but only two layers are provided from the viewpoint of simplifying the control of retardation and reducing the thickness of the multilayer film of the present invention. Is preferred.

  The thickness of the A layer is preferably 1 μm or more, more preferably 3 μm or more, and on the other hand, preferably 20 μm or less, more preferably 15 μm or less. When there are a plurality of A layers, the total thickness of the A layers can be within the preferred range. By setting the thickness of the A layer within the above preferable range, a retardation film having a low photoelastic coefficient and a film thickness of each layer that can be controlled in-line can be obtained.

[1-4. Measurement of difference in average refractive index of resin a to resin c and film thickness measurement]
In the multilayer film of the present invention, the difference in average refractive index between the resin a and the resin c and the difference in average refractive index between the resin b and the resin c are both usually 0.01 or more and 0.02 The above is preferable. Thereby, the film thickness of each layer can be measured by the interference type film thickness meter. Among these, from the viewpoint of measuring the thickness of each layer stably and accurately, the difference in average refractive index between the resin b and the resin a is preferably 0.02 or more. The refractive index difference between the resin a and the resin b is usually small, and it is difficult to measure with an interference film thickness meter, but the resin c layer intervenes between them to interfere with the film thickness of each layer. It is possible to accurately measure in-line with a film thickness meter.

FIG. 1 is a cross-sectional view schematically showing a cross section of an example of the multilayer film of the present invention taken along a plane perpendicular to the film main surface. As shown in FIG. 1, the multilayer film 100 includes a B layer 111 made of a resin b, C layers 131 and 132 made of a resin c formed on both surfaces of the B layer 111, and a C layer B layer. A layers 121 and 122 made of resin a formed on the surface far from the surface. Since the A layers 121 and 122 are the outermost layers, the surfaces 125 and 126 on the side far from the C layer are the front and back main surfaces of the multilayer film 100 as a whole.
An adhesive layer or the like is provided between the A layer 121 and the C layer 131, between the C layer 131 and the B layer 111, between the B layer 111 and the C layer 132, and between the C layer 132 and the A layer 122. The other layers are not interposed, and the layers of these sets are in direct contact with each other. Therefore, the interface 135 between the resin a and the resin c, the resin c and the resin b, , An interface 116 between the resin b and the resin c, and an interface 136 between the resin c and the resin a.
In the multilayer film 100, the difference in average refractive index between the resin a and the resin c is large, and the difference in average refractive index between the resin b and the resin c is also large. Therefore, the interfaces 135, 115, 116, and 136 reflect light. To do. Therefore, the light L ₁₀₀ irradiated to the multilayer film 100 is indicated by arrows L ₁₂₅ , L on the one main surface 125, the interface 135, the interface 115, the interface 116, the interface 136, and the other main surface 126 of the multilayer film 100, respectively. Reflected as indicated by L ₁₃₅ , L ₁₁₅ , L ₁₁₆ , L ₁₃₆ , L ₁₂₆ . In the interference film thickness meter, the reflected light L ₁₂₅ , L ₁₃₅ , L ₁₁₅ , L ₁₁₆ , L ₁₃₆ , L ₁₂₆ is detected, and the A layers 121 and 122, the B layer 111, and the C layer 131 and The film thickness of 132 is measured.

  In general, in view of the fact that an optical film has been developed to reduce the difference in refractive index of each layer included in the optical film, the resin c that forms the C layer and the resin that forms the other layers as in the present invention ( Providing a difference in the average refractive index positively between the resin a and the resin b) has significant significance. Normally, there is no circumstance that the measurement accuracy improves as the difference in the average refractive index increases, and if the difference in average refractive index is within the above range, the film thicknesses of the A layer and the B layer are sufficiently accurate. It can be easily measured. Therefore, although there is no particular limitation on the upper limit of the difference in average refractive index between the resin a and the resin c and the difference in average refractive index between the resin b and the resin c, both are usually 0.15 or less, preferably 0.10. It is as follows.

  The average refractive index means an average value of refractive indexes in all measurement directions of a resin to be measured. Therefore, when the molecules contained in the resin are not oriented, for example, when the multilayer film is not stretched, the refractive index of the resin is usually constant regardless of the measurement direction. Even if the refractive index is measured in the direction, the refractive index value itself may be adopted as the average refractive index. In addition, when the molecules contained in the resin are oriented, for example, when a multilayer film is stretched, the refractive index of the resin may vary depending on the measurement direction. The rate is measured, and the average of the measured values is obtained as the average refractive index. However, the average refractive index of the resin oriented in the multilayer film is usually obtained as an average value of the refractive index at the fast axis, the refractive index at the slow axis, and the refractive index in the thickness direction in the plane of the multilayer film. May be.

  In addition, the measurement wavelength of the average refractive index of the resin a to the resin c is usually 532 nm.

In order to keep the difference in the average refractive index between the resin a and the resin c and the difference in the average refractive index between the resin b and the resin c within the above ranges, for example, the refractive index of each resin may be adjusted. . Although there is no restriction | limiting in the method of adjusting the refractive index of each resin, For example, it is refracting with the essential component (resin a is a polycarbonate, resin b is a styrene polymer, resin c is a thermoplastic acrylic resin) of resin a-resin c. Examples thereof include a method in which polymers or monomer units having different rates are included, or a compounding agent is included.
As a specific example, when the average refractive index of the resin a or b is too far from the average refractive index of the resin c, the resin a or b containing an acrylic polymer can be used. For example, in the case of the resin a, the resin a is a mixture of a polycarbonate polymer and an acrylic polymer, and the ratio thereof is adjusted, or a copolymer containing a polymerization unit of polycarbonate and a polymerization unit of acrylic compound is used. By adjusting the ratio, the refractive index of the resin a can be changed, and the difference in average refractive index between the resin a and the resin c can be within the above range. For example, in the case of the resin b, the resin b is a mixture of a styrene polymer and an acrylic polymer, the ratio of these is adjusted, or a copolymer containing a polymer unit of a styrene polymer and a polymer unit of an acrylic compound By adjusting the ratio of these units, the refractive index of the resin b can be changed, and the difference in average refractive index between the resin b and the resin c can be kept within the above range.

[1-5. Other layers]
The multilayer film of the present invention may be provided with other layers in addition to the A layer to the C layer as long as the effects of the present invention are not significantly impaired.
For example, the multilayer film of the present invention has, on its surface, a mat layer that improves the slipperiness of the film, a hard coat layer that prevents scratches on the film surface, and an antireflection layer that suppresses reflection of light on the film surface. Further, an antifouling layer or the like for preventing the adhesion of dirt may be provided. For example, when the multilayer film of the present invention is stretched to obtain the retardation film of the present invention, another layer may be additionally provided on the stretched multilayer film to form a retardation film. . Examples of the layer that can be additionally provided include the same layers as those listed above.

[1-6. Retardation film)
The retardation film of the present invention is formed by stretching the multilayer film of the present invention.
The retardation film of the present invention preferably has a total light transmittance of 85% or more from the viewpoint of stably exhibiting the function as an optical member. The light transmittance can be measured using a spectrophotometer (manufactured by JASCO Corporation, ultraviolet-visible near-infrared spectrophotometer “V-570”) in accordance with JIS K0115.

  The haze of the retardation film of the present invention is preferably 5% or less, more preferably 3% or less, and particularly preferably 1% or less. By setting the haze to a low value, the sharpness of the display image of the display device incorporating the retardation film of the present invention can be enhanced. Here, the haze is an average value obtained by measuring five points using a “turbidimeter NDH-300A” manufactured by Nippon Denshoku Industries Co., Ltd. according to JIS K7361-1997.

  In the retardation film of the present invention, ΔYI is preferably 5 or less, and more preferably 3 or less. When this ΔYI is in the above range, there is no coloring and the visibility is good. ΔYI is measured using a “spectral color difference meter SE2000” manufactured by Nippon Denshoku Industries Co., Ltd. according to ASTM E313. The same measurement is performed five times, and the arithmetic average value is obtained.

The retardation film of the present invention preferably has a desired retardation depending on the use as a retardation film. For example, when used as a retardation film for a liquid crystal display device, the multilayer film of the present invention has a retardation Re at an incident angle of 0 ° and a retardation R ₄₀ at an incident angle of 40 ° of 0.92 ≦ R ₄₀ /. It is preferable to satisfy the relationship of Re ≦ 1.08. Among these, R ₄₀ / Re is preferably 0.95 or more, and preferably 1.05 or less. Since Re and R ₄₀ have such a relationship, when the retardation film of the present invention is applied to a display device such as a liquid crystal display device, the angle dependency of the display color tone of the device is particularly preferably reduced. Can do.

Here, the incident angle of 0 ° is the normal direction of the main surface of the retardation film, and the incident angle of 40 ° is an angle inclined by 40 ° from the normal direction of the main surface of the retardation film. In measuring R _40, the direction in which the observation angle is tilted is not particularly limited, and the value of R ₄₀ when tilted in any one direction may satisfy the requirement.
The measurement wavelength of retardation Re and R ₄₀ may be any wavelength within the visible light region, but is preferably 590 nm.

The retardations Re and R ₄₀ at the incident angles of 0 ° and 40 ° can be measured by a parallel Nicol rotation method using KOBRA-WR manufactured by Oji Scientific Instruments. When Re and R ₄₀ satisfy the above relationship, the refractive indices nx and ny in the principal axis direction and the refractive index nz in the thickness direction in the plane of the retardation film generally satisfy nx>nz> ny. Here, the refractive indexes nx, nz and ny are weighted averages n _ave of the refractive indexes in the respective directions of the respective layers included in the retardation film of the present invention, and the refractive index of the i-layer resin is ni and the i-layer film. The thickness is determined by the following equation, where Li is:
n _ave = Σ (ni × Li) / ΣLi

  The retardation Re at an incident angle of 0 ° of the retardation film of the present invention is preferably 50 nm or more, more preferably 100 nm or more, and preferably 400 nm or less, preferably 350 nm or less. More preferred.

  The outer surface of the retardation film of the present invention is in the MD direction (machine direction; the flow direction of the film in the production line, and usually coincides with the long direction of the long film, also referred to as the vertical direction). It is preferably flat without substantially having irregularly extending linear concave portions or linear convex portions (so-called die lines). Here, “the surface is substantially free of irregularly formed linear recesses and linear protrusions and is flat” means that the depth is less than 50 nm even if linear recesses and linear protrusions are formed. Or it is a linear recessed part with a width larger than 500 nm, and a linear convex part with a height less than 50 nm or a width larger than 500 nm. More preferably, it is a linear concave part having a depth of less than 30 nm or a width of more than 700 nm, and a linear convex part having a height of less than 30 nm or a width of more than 700 nm. By adopting such a configuration, it is possible to prevent the occurrence of light interference and light leakage based on the light refraction at the linear concave portions or the linear convex portions, and the optical performance can be improved. In addition, irregularly occurring means that it is formed with an unintended size, shape, or the like at an unintended position.

The depth of the linear concave portion described above, the height of the linear convex portion, and the width thereof can be obtained by the following method. Light is applied to the retardation film, the transmitted light is projected onto the screen, and the lighted or dark stripes of light appearing on the screen (this part has the depth of the linear recesses and the height of the linear protrusions). Cut out at 30 mm square. The surface of the cut film piece is observed using a three-dimensional surface structure analysis microscope (field region 5 mm × 7 mm), converted into a three-dimensional image, and a cross-sectional profile is obtained from the three-dimensional image. The cross-sectional profile is obtained at 1 mm intervals in the visual field region.
In this cross-sectional profile, an average line is drawn, the length from the average line to the bottom of the linear concave portion is the depth of the linear concave portion, and the length from the average line to the top of the linear convex portion is the height of the linear convex portion. It becomes. The distance between the intersection of the average line and the profile is the width. The maximum values are obtained from the measured values of the linear concave portion depth and the linear convex portion height, respectively, and the width of the linear concave portion or the linear convex portion showing the maximum value is obtained. The maximum value of the linear recess depth and the height of the linear convex portion obtained from the above, the width of the linear concave portion and the width of the linear convex portion showing the maximum value, the depth of the linear concave portion of the film Let the height of the linear protrusions and their widths.

  The retardation film of the present invention is subjected to heat treatment at 60 ° C., 90% RH, 100 hours, in the MD direction and the TD direction (traverse direction; a direction parallel to the film surface and perpendicular to the MD direction, usually in the width direction). May be contracted in the horizontal direction), but the contraction rate is preferably 0.5% or less, more preferably 0.3% or less. With such a small shrinkage rate, it is possible to prevent the retardation film of the present invention from being deformed by the shrinkage stress and being peeled off from the display device even in a high temperature and high humidity environment.

  The retardation film of this invention is good also considering the dimension of the TD direction as 1000 mm-2000 mm, for example. Moreover, although the retardation film of this invention does not have a restriction | limiting in the dimension of the MD direction, It is preferable that it is a long film. Here, the “long” film means a film having a length of at least 5 times the width of the film, preferably a length of 10 times or more, specifically a roll. It has a length enough to be wound up into a shape and stored or transported.

  The specific thickness of the retardation film of the present invention may be set according to the film strength required according to the application and the size of retardation to be expressed, but is preferably 10 μm or more, more preferably 30 μm or more. Moreover, 200 micrometers or less are preferable and 150 micrometers or less are more preferable.

[2. (Method for producing multilayer film)
[2-1. (Co-extrusion method)
The production method of the multilayer film of the present invention is not limited. For example, a coextrusion method such as coextrusion T-die method, coextrusion inflation method, coextrusion lamination method, etc .; film lamination molding method such as dry lamination; A coating molding method such as coating a resin solution on the surface of the resin film; Among these, the co-extrusion method is preferable from the viewpoint of manufacturing efficiency and preventing a volatile component such as a solvent from remaining in the film.

  When the coextrusion method is employed, the multilayer film is obtained, for example, by coextruding a resin a having a positive intrinsic birefringence value, a resin b having a negative intrinsic birefringence value, and a resin c. Examples of the co-extrusion method include a co-extrusion T-die method, a co-extrusion inflation method, and a co-extrusion lamination method. Among them, the co-extrusion T-die method is preferable. Further, the coextrusion T-die method includes a feed block method and a multi-manifold method, but the multi-manifold method is particularly preferable in that variation in thickness can be reduced.

  When the co-extrusion T-die method is employed, the melting temperature of the resin in the extruder having the T-die is preferably 80 ° C. higher than the glass transition temperature of the resin a to the resin c, and is 100 ° C. higher. It is more preferable to set it above, and it is preferable that the temperature be 180 ° C. or higher, and it is more preferable that the temperature be 150 ° C. or higher. If the melting temperature in the extruder is excessively low, the fluidity of the resin may be insufficient. Conversely, if the melting temperature is excessively high, the resin may be deteriorated.

In the coextrusion method, the film-like molten resin extruded from the opening of the die is usually brought into close contact with a cooling roll (also referred to as a cooling drum). The method for bringing the molten resin into close contact with the cooling roll is not particularly limited, and examples thereof include an air knife method, a vacuum box method, and an electrostatic contact method.
The number of cooling rolls is not particularly limited, but is usually 2 or more. In addition, examples of the arrangement method of the cooling roll include, but are not particularly limited to, a linear type, a Z type, and an L type. Further, the way of passing the molten resin extruded from the opening of the die through the cooling roll is not particularly limited.

  The degree of adhesion of the extruded film-like resin to the cooling roll varies depending on the temperature of the cooling roll. When the temperature of the cooling roll is raised, the adhesion is improved. However, if the temperature is raised too much, the film-like resin may not be peeled off from the cooling roll, and there is a possibility that a problem of winding around the drum may occur. Therefore, the temperature of the cooling roll is preferably (Tg + 30) ° C. or less, more preferably (Tg-5) ° C. to (Tg−), where Tg is the glass transition temperature of the resin of the layer that is extruded from the die and contacts the drum. 45) Set to a range of ° C. By doing so, problems such as slipping and scratches can be prevented.

  Moreover, it is preferable to reduce content of the residual solvent in a multilayer film. Means for that purpose include (1) reducing the residual solvent contained in resin a to resin c as raw materials; (2) pre-drying resin a to resin c before forming a multilayer film; Is mentioned. For example, the preliminary drying is performed with a hot air dryer or the like in the form of pellets of the resins a to c. The drying temperature is preferably 100 ° C. or more, and the drying time is preferably 2 hours or more. By performing preliminary drying, it is possible to reduce the residual solvent in the multilayer film, and to prevent foaming of the extruded film-like resin.

[2-2. Method for producing retardation film]
When producing a retardation film in which Re and R ₄₀ satisfy the relationship of 0.92 ≦ R ₄₀ /Re≦1.08 as the retardation film of the present invention, resin b, resin c, and resin a are usually added. A coextrusion step of co-extrusion to obtain a predetermined pre-stretching film, a first stretching step in which the pre-stretching film is uniaxially stretched in a single direction at a predetermined temperature, and a uniaxial stretching process in the first stretching step. A retardation film is produced by a production method having a second stretching step in which a uniaxial stretching process is performed at a predetermined temperature different from the first stretching step in a direction orthogonal to the first direction. Hereinafter, this manufacturing method will be described in detail.

-Coextrusion process In a coextrusion process, resin b, resin c, and resin a are coextruded, and the multilayer film of this invention is manufactured as a film before extending | stretching.

The unstretched film is stretched to different angles substantially orthogonal to each other at different angles of temperatures T1 and T2, thereby causing retardation in each of the A layer and the B layer according to the temperatures T1 and T2 and the stretching direction. In this way, the retardation produced in the A layer and the retardation produced in the B layer are synthesized, and the desired retardation is exhibited as a whole retardation film. In addition, “substantially orthogonal” means that they intersect at an angle of usually 85 ° or more, preferably 89 ° or more, and usually 95 ° or less, preferably 91 ° or less.
The retardation based on the C layer may be manifested by stretching at temperatures T1 and T2, but by appropriately selecting the material of the resin c, the retardation is not manifested by stretching at temperatures T1 and T2, and is obtained. The C layer in the phase difference film can be isotropic.

  The size of the retardation developed in the A layer and the B layer by stretching depends on the thickness of the film before stretching, the stretching temperature, the stretching ratio, and the like. Therefore, it is preferable that the configuration of the pre-stretched film is appropriately determined according to the retardation to be expressed.

Although the specific composition of the film before stretching can be variously set, among them, the film before stretching is oriented in the film plane with respect to the X-axis and the uniaxial stretching direction in the direction of stretching in a certain direction (that is, uniaxial stretching direction). Linearly polarized light (hereinafter referred to as “XZ-polarized light” as appropriate) is incident perpendicularly to the film surface and the vibration plane of the electric vector is in the XZ plane when the orthogonal direction is the Y-axis and the film thickness direction is the Z-axis. The phase with respect to linearly polarized light (hereinafter referred to as “YZ polarized light” where appropriate) that is perpendicularly incident on the film surface and whose electric vector vibration surface is on the YZ plane is
Delayed when uniaxially stretched in the X-axis direction at temperature T1,
It is preferable to satisfy the requirement (hereinafter referred to as “requirement P” as appropriate) that the process proceeds when uniaxially stretching in the X-axis direction at a temperature T2 different from the temperature T1.

  The requirement P may be satisfied when at least one of the various directions in the plane of the unstretched film is taken as the X axis. Usually, since the film before stretching is an isotropic raw film, the above-mentioned requirement P is satisfied when one direction in the plane is the X axis, and any other direction is the X axis. The requirement P can be satisfied.

  In a film in which a slow axis appears in the X axis by uniaxial stretching, the phase of XZ polarized light is usually delayed from that of YZ polarized light. Conversely, in a film in which a fast axis appears on the X-axis by uniaxial stretching, the phase of XZ polarized light is usually advanced relative to that of YZ polarized light. The pre-stretch film according to the present invention is a multilayer film utilizing these properties, and is a film in which the appearance of the slow axis or the fast axis depends on the stretching temperature. The temperature dependence of the expression of such retardation can be adjusted, for example, by adjusting the relationship between the photoelastic coefficient of the resin a and the resin b and the film thickness ratio of each layer.

The in-plane retardation of a certain layer is the difference between the refractive index nx in the X-axis direction that is the stretching direction and the refractive index ny in the Y-axis direction that is perpendicular to the stretching direction (= | nx−ny |). It is a value obtained by multiplying the film thickness d. The retardation of the multilayer film comprising the A layer and the B layer is synthesized from the retardation of the A layer and the retardation of the B layer. Therefore, for example, in order to reverse the sign of retardation developed in the entire film by stretching at a high temperature T _H and a low temperature T _L , (i) the glass transition temperature of the stretching at a low temperature T _L the absolute value of the retardation high resin is expressed is smaller than the absolute value of the retardation expressing low resin glass transition temperature, a draw in (ii) a higher temperature T _H, low glass transition temperature resin is expressed It is preferable to adjust the film thicknesses of the A layer and the B layer so that the absolute value of retardation is smaller than the absolute value of retardation at which a resin having a high glass transition temperature is developed.

Thus, as the resin constituting the A layer and the B layer, the refractive index in the X-axis direction and the refractive index in the Y-axis direction are respectively applied to the A layer and the B layer by stretching in one direction (that is, uniaxial stretching). The combination of the resin a and the resin b that can cause a difference is selected, and the above requirement P is satisfied by adjusting the total thickness of the A layer and the total thickness of the B layer in consideration of the stretching conditions. A film before stretching can be obtained.
The temperature T1 is one of T _{H and} T _L , and the temperature T2 is either T _H or T _L that is different from T1.

The expression of retardation when a pre-stretch film satisfying the above requirement P is stretched will be specifically described with reference to the drawings. FIG. 2 shows that when the glass transition temperature Tg _A of the resin a forming the A layer is high and the glass transition temperature Tg _B of the resin b forming the B layer is low, the A layer and the B layer of the film before stretching. The temperature dependence of the retardation of each of the A layer and the B layer when the film is stretched, and the temperature dependence of the retardation Δ of the film before stretching when the film before stretching (here, A layer + B layer) is stretched. An example is shown. The pre-stretch film as shown in FIG. 2, since the larger the negative retardation expressed in B layer compared with the retardation of the positive expressed in A layer in stretching at a temperature T _b, minus the A layer + B layer Retardation Δ is developed. On the other hand, since a stretching at the temperature T _a is smaller for negative retardation expressed in B layer compared with the retardation of the positive expressed in A layer, it will express a positive retardation Δ in A layer + B layer . Therefore, by combining the drawing of such different temperatures T _a and T _b, by combining the retardation caused by stretching at each temperature can be produced a retardation film having a desired retardation.

  When the example of the structure of the film before extending | stretching is given, when resin b is resin containing a styrene-maleic anhydride copolymer, for example, with the sum total of the film thickness of A layer, and the sum total of the film thickness of B layer The ratio (total thickness of layer A / total thickness of layer B) is usually 1/15 or more, preferably 1/10 or more, and usually 1/4 or less. Even if the A layer becomes too thick or the B layer becomes too thick, the temperature dependency of the expression of retardation tends to decrease.

  The total thickness of the film before stretching, that is, the multilayer film of the present invention is preferably 10 μm or more, more preferably 20 μm or more, particularly preferably 30 μm or more, preferably 500 μm or less, more preferably 400 μm or less, particularly preferably 300 μm or less. It is. If the pre-stretch film is thinner than the lower limit of the range, sufficient retardation tends to be difficult and mechanical strength tends to be weakened. If the film is thicker than the upper limit of the range, the flexibility is deteriorated and handling may be hindered. There is sex.

  When the number of A layers included in the pre-stretch film is 2, the ratio of the thickness of one A layer to the other A layer (thickness of the thicker A layer / thickness of the thinner A layer) Is preferably 1.5 / 1 or more from the viewpoint of ensuring light leakage of the polarizing plate when combined with the polarizing plate in a liquid crystal display device. Further, from the viewpoint of maintaining the accuracy of the film thickness of the thinner A layer, the ratio of the film thickness between one A layer and the other A layer is preferably 10/1 or less.

  In the pre-stretch film, the variation in the film thicknesses of the A layer and the B layer is preferably 1 μm or less over the entire surface. Thereby, also in the A layer and the B layer of the retardation film, the variation in the film thickness is 1 μm or less over the entire surface, and the variation in the color tone of the display device including the retardation film can be reduced. In addition, the color tone change after long-term use of the retardation film can be made uniform.

  In order to make the variation in the film thicknesses of the A layer and the B layer 1 μm or less as described above, for example, (1) a polymer filter having an opening of 20 μm or less is provided in the extruder; (2) the gear pump is 5 rpm or more (3) Arrange the surrounding means around the die; (4) Set the air gap to 200 mm or less; (5) Perform edge pinning when casting the film on a chill roll; and (6) Extrusion As the machine, a twin screw extruder or a double-flight type single screw extruder may be used. Further, for example, the thickness of the A layer and the B layer can be reduced by measuring the film thickness of the A layer and the B layer during the manufacturing process and performing feedback control based on the film thickness as in the embodiment described later. can do.

The film thickness variations of the A layer and the B layer are measured at regular intervals in the MD direction and TD direction of the film, and the measured film thickness T is measured based on the arithmetic average value T _ave of the measured values. The maximum value is T _max , and the minimum value is T _min . Note that the variation in film thickness (μm) is the larger of T _ave −T _min and T _max −T _ave .

The above-mentioned film before stretching is usually produced by a coextrusion method. The coextrusion method is as described above.
Moreover, although an isotropic raw film is usually used as the pre-stretch film, a film once stretched may be used as a pre-stretch film, which may be further stretched.

-1st extending | stretching process In a 1st extending | stretching process, the uniaxial stretching process is performed to one direction at the temperature of either the temperature T1 or T2. When the film is stretched at the temperature T1, the phase of the XZ polarized light with respect to the YZ polarized light is delayed in the pre-stretched film that satisfies the requirement P. On the other hand, when the uniaxial stretching is performed at the temperature T2, the phase of the XZ polarized light with respect to the YZ polarized light advances.

When the relationship between the glass transition temperatures is Tg _A > Tg _B , the temperature T1 is preferably higher than Tg _B, more preferably higher than Tg _B + 5 ° C., and preferably lower than Tg _A + 40 ° C. More preferably, it is lower than Tg _A + 20 ° C. By making the temperature T1 higher than the lower limit of the above range, the desired retardation can be stably expressed in the B layer, and by making the temperature T1 lower than the upper limit, the desired retardation can be stably expressed in the A layer. it can.
Furthermore, when the relationship between the glass transition temperatures is Tg _A > Tg _B , the temperature T2 is preferably higher than Tg _B −20 ° C., more preferably higher than Tg _B −10 ° C., and Tg _B + 5 ° C. Preferably, it is lower than Tg _B. By making the temperature T2 higher than the lower limit of the above range, it is possible to prevent the pre-stretched film from being broken or clouded during stretching, and by making the temperature T2 lower than the upper limit, the desired retardation is stably expressed in the resin B. be able to.
In this case the glass transition temperature relation is Tg _A> Tg _B, it is preferably carried out at a temperature T1 in the first stretching step.

When the relationship of glass transition temperature is Tg _B > Tg _A , the temperature T1 is preferably higher than Tg _A, more preferably higher than Tg _A + 5 ° C., and preferably lower than Tg _B + 40 ° C. More preferably, it is lower than Tg _B + 20 ° C. By making the temperature T1 higher than the lower limit of the above range, the desired retardation can be stably expressed in the A layer, and by making the temperature T1 lower than the upper limit, the desired retardation can be stably expressed in the B layer. it can.
Furthermore, when the relationship between the glass transition temperatures is Tg _B > Tg _A , the temperature T2 is preferably higher than Tg _A −20 ° C., more preferably higher than Tg _A −10 ° C., and Tg _A + 5 ° C. Preferably, it is lower than Tg _A. By making the temperature T2 higher than the lower limit of the above range, it is possible to prevent the pre-stretching film from being broken or clouded during stretching, and by making the temperature T2 lower than the upper limit, the desired retardation is stably expressed in the resin A. be able to.
In this case the glass transition temperature relation is Tg _B> Tg _A, is preferably performed at a temperature T2 in the first stretching step.

  The uniaxial stretching process can be performed by a conventionally known method. For example, a method of uniaxially stretching in the MD direction using a difference in peripheral speed between rolls, a method of uniaxially stretching in the TD direction using a tenter, and the like can be mentioned. Examples of the uniaxial stretching method in the MD direction include an IR heating method between rolls, a float method, and the like. Of these, the float method is preferred because a retardation film having high optical uniformity can be obtained. On the other hand, as a method of uniaxially stretching in the TD direction, a tenter method can be mentioned.

  In the uniaxial stretching treatment, in order to reduce stretching unevenness and thickness unevenness, a temperature difference may be created in the TD direction in the stretching zone. In order to create a temperature difference in the TD direction in the stretching zone, for example, a known method such as adjusting the opening degree of the hot air nozzle in the TD direction or controlling the heating by arranging the IR heaters in the TD direction may be used. it can.

-2nd extending process A 2nd extending process is performed after performing a 1st extending process. In the second stretching step, the film stretched in one direction in the first stretching step is uniaxially stretched in a direction orthogonal to the direction in which the uniaxial stretching process was performed in the first stretching step.
In the second stretching step, uniaxial stretching is performed at a temperature T2 or T1 different from that in the first stretching step. In the second stretching step, when the relationship of glass transition temperature is Tg _A > Tg _B , it is preferable to perform uniaxial stretching at temperature T2, and when Tg _B > Tg _A , uniaxial stretching is performed at temperature T1. preferable.

  The difference between the temperature T1 and the temperature T2 is usually 5 ° C. or higher, preferably 10 ° C. or higher. By increasing the difference between the temperature T1 and the temperature T2 as described above, a desired retardation can be stably expressed in the retardation film. In addition, although there is no restriction | limiting in the upper limit of the difference of temperature T1 and temperature T2, it is 100 degrees C or less from a viewpoint of industrial productivity.

  For the uniaxial stretching treatment in the second stretching step, a method similar to the method that can be adopted in the uniaxial stretching treatment in the first stretching step can be applied. However, the uniaxial stretching process in the second stretching process is preferably performed at a smaller stretching ratio than the uniaxial stretching process in the first stretching process. Specifically, the first draw ratio is preferably 2 to 4 times, and the second draw ratio is preferably 1.1 to 2 times.

  The combinations of the stretching directions in the first stretching process and the second stretching process are, for example, stretching in the MD direction in the first stretching process and stretching in the TD direction in the second stretching process, or stretching in the TD direction in the first stretching process. What is necessary is just to extend | stretch to MD direction at a 2nd extending | stretching process, or to extend | stretch to the diagonal direction substantially orthogonal to it by extending | stretching to an oblique direction at a 1st extending | stretching process. Especially, it is preferable to extend | stretch in a TD direction at a 1st extending | stretching process, and to extend | stretch to MD direction at a 2nd extending | stretching process. This is because the variation in the direction of the optical axis can be reduced over the entire width of the obtained retardation film by performing stretching in the MD direction in the second stretching step having a small stretching ratio.

  As described above, by performing the first stretching step and the second stretching step on the pre-stretching film, the stretching temperature, stretching direction and stretching are performed on the A layer and the B layer in each of the first stretching step and the second stretching step. Retardation according to the magnification and the like occurs. For this reason, in the retardation film obtained through the first stretching step and the second stretching step, the retardation expressed in the A layer and the B layer in each of the first stretching step and the second stretching step is synthesized. The desired retardation will occur.

  Also, by co-stretching the pre-stretched film comprising the A layer and the B layer, the manufacturing process is shortened compared to the case of manufacturing the retardation film by laminating the separately stretched A layer and B layer. Cost can be reduced. In addition, the B layer made of the resin b having a negative intrinsic birefringence value is difficult to be stretched by itself and may cause stretching unevenness or breakage. However, it is possible to stably co-stretch by laminating with the A layer. And the thickness unevenness of the B layer can be reduced.

-Other process In the manufacturing method of the retardation film mentioned above, you may make it perform other processes other than a coextrusion process, a 1st extending process, and a 2nd extending process.
For example, you may provide the process (preheating process) of heating a film before extending | stretching previously before extending | stretching the film before extending | stretching. Examples of the means for heating the pre-stretched film include an oven-type heating device, a radiation heating device, or immersion in a liquid. Of these, an oven-type heating device is preferable. The heating temperature in the preheating step is usually a stretching temperature of −40 ° C. or higher, preferably a stretching temperature of −30 ° C. or higher, and is usually a stretching temperature of + 20 ° C. or lower, preferably a stretching temperature of + 15 ° C. or lower. The stretching temperature means the set temperature of the heating device.

  For example, the stretched film may be fixed after one or both of the first stretching step and the second stretching step. The temperature in the fixing treatment is usually room temperature or higher, preferably stretching temperature −40 ° C. or higher, and usually stretching temperature + 30 ° C. or lower, preferably stretching temperature + 20 ° C. or lower.

  Furthermore, for example, a step of providing, for example, a mat layer, a hard coat layer, an antireflection layer, an antifouling layer, or the like on the surface of the obtained retardation film may be performed.

[2-3. (Feedback control based on film thickness)
The multilayer film of the present invention can measure the thickness of the A layer to the C layer even in the course of in-line production by using an interference film thickness meter. Therefore, the film thicknesses of the A layer to the C layer can be measured during the production, and the feedback control of the production conditions can be performed based on the measured film thickness. By performing such feedback control, it becomes possible to precisely control the film thicknesses of the A-layer to C-layer of the multilayer film, so that a retardation film having a desired retardation can be stably produced.

  FIG. 3 is a diagram schematically showing an outline of a retardation film manufacturing apparatus according to an embodiment of a method for manufacturing a multilayer film and a retardation film of the present invention. In the manufacturing apparatus 200 shown in FIG. 3, the film 300 before extending | stretching is manufactured as a multilayer film of this invention, The film 300 before extending | stretching is extended | stretched, and the phase difference film 400 is manufactured.

  As shown in FIG. 3, the manufacturing apparatus 200 includes a hopper 210, an extruder 220, a die 230, a cooling roll 240, a first stretching machine 250, a second stretching machine 260, an interference film thickness meter 270 and a control device 280. Is provided.

  The hopper 210 is a device that can supply the resin a to the resin c to the extruder 220. The extruder 220 is a device that can send molten resins a to c to the die 230 by, for example, a screw (not shown). Resins a to c supplied from the hopper 210 to the extruder 220 are sent from the extruder 220 to the die 230 by screws.

  The die 230 has a slit 231 as an opening through which molten resin a to resin c can be coextruded. The shape of the slit 231 is set according to the width of the unstretched film 300 that is a multilayer film and the film thicknesses of the A layer to the C layer. Specifically, the slit length is set according to the width of the film 300 before stretching, and the slit width is set according to the film thickness of each of the A layer to the C layer. A flow path (not shown) through which resin a to resin c circulate is formed inside the die 230, and the resin a to resin c sent from the extruder 220 passes through the flow path of the die 230. A layer of resin a, a layer of resin c, a layer of resin b, a layer of resin c, and a layer of resin a are coextruded from a slit in the form of a three-kind five-layer film.

  In addition, adjustment bolts 232 are provided at a plurality of positions in the longitudinal direction of the slit 231 in the slit 231 of the die 230. The adjusting bolt 232 is an adjusting unit that can adjust the size (specifically, the slit width) of the slit 231 and is loosened or tightened by an appropriate mechanism (not shown). Therefore, the slit width of the slit 231 can be adjusted by the adjustment bolt 232, and increases when the adjustment bolt 232 is loosened, and decreases when tightened.

  Further, the die 230 is provided with a heater 233 for independently heating the resins a to c flowing through the flow path. The temperature of the heater 233 can be adjusted. By adjusting the temperature of the heater 233, the temperatures of the resin a and the resin b flowing through the flow path can be controlled. Therefore, the heater 233 functions as an extrusion rate adjusting unit, adjusts the viscosity of the resin a to the resin c by controlling the temperature, and the extrusion rate of one or both of the resin a to the resin c extruded from the slit 231 (i.e., The speed at which the resin is extruded) can be adjusted.

  The cooling roll 240 is a roll that can cool the resin a to the resin c coextruded in a film shape from the slit 231 of the die 230. The resin a to the resin c which have been melted by being cooled by the cooling roll 240 are cured, and a pre-stretching film 300 as a multilayer film of the present invention is obtained. The obtained pre-stretching film 300 is sent to the first stretching machine 250 and then sent to the second stretching machine 260.

  The first stretching machine 250 is a device that can perform a uniaxial stretching process in one direction on the pre-stretching film 300 at either the temperature T1 or T2. Further, the second stretching machine 260 performs the first stretching in the direction orthogonal to the direction in which the uniaxial stretching process is performed on the pre-stretching film 300 stretched by the first stretching machine 250. This is an apparatus that can perform uniaxial stretching at a temperature T2 or T1 different from the uniaxial stretching in the machine 250. Therefore, when the uniaxial stretching process is performed by the first stretching machine 250 and the second stretching machine 260, a desired retardation is developed in the unstretched film 300, and the retardation film 400 is obtained. ing.

  The interference-type film thickness meter 270 is a measuring instrument that can measure the film thicknesses of the A layer made of the resin a, the B layer made of the resin b, and the C layer made of the resin c of the unstretched film 300. The interference-type film thickness meter 270 irradiates light to the film 300 before being stretched, detects the reflected light, and measures the film thickness of each of the A layer to the C layer. The measured value is sent to the control device 280 as indicated by an arrow A1.

  The control device 280 includes an opening control unit 281 that can adjust the slit width of the slit 231 of the die 230. The opening controller 281 can adjust the slit width of the slit 231 to a desired size at a desired position by performing control to loosen or tighten the adjustment bolt 232 as indicated by an arrow A2. . At this time, the opening controller 281 reduces the variation in the film thickness in the TD direction of the unstretched film 300 according to the film thickness of each of the A layer to C layer sent from the interference film thickness meter 270. In addition, the slit width of the slit 231 is adjusted so that the variation in the total film thickness in the MD direction of the unstretched film 300 is reduced.

  In addition, the control device 280 includes an extrusion speed control unit 282 that controls the heating temperature of the heater 233 to adjust one or more extrusion speeds of the resins a to c. The extrusion speed control unit 282 adjusts the viscosity of one or both of the resin a to the resin c by controlling the temperature of the heater 233 provided in the die 230, as indicated by an arrow A3, and the slit 231 of the die 230. The extrusion speed of one or both of resin a to resin c extruded from can be adjusted. At this time, the extrusion speed control unit 282 has a small variation in film thickness in the MD direction of each of the A layer to C layer according to the film thickness of each of the A layer to C layer sent from the interference type film thickness meter 270. Thus, one or more extrusion speeds of the resin a to the resin c are adjusted.

  The hardware configuration of the control device 280 is not limited, but is usually configured by a computer including a processor such as a CPU, a memory such as a RAM and a ROM, and an interface such as an input / output terminal. Then, control is performed according to the control content recorded in advance in a memory or the like.

  Since the manufacturing apparatus 200 of the present embodiment is configured as described above, when the retardation film 400 is manufactured, the resin a and the resin b are supplied to the hopper 210 as indicated by an arrow A4. The supplied resin a and resin b are sent to the die 230 by the extruder 220. Resin a to resin c sent to the die 230 are coextruded as a film-like molten resin from the slit 231 and cooled by the cooling roll 240 to become the pre-stretching film 300 (coextrusion step).

  The unstretched film 300 is sent to the first stretching machine 250 and subjected to a uniaxial stretching process (first stretching step). Thereafter, the unstretched film 300 that has been subjected to the uniaxial stretching process by the first stretching machine 250 is sent to the second stretching machine 260, and the uniaxial stretching process is performed at a temperature and direction different from those of the first stretching machine 250. (Second stretching process). Thereby, since a desired retardation is developed in the pre-stretched film 300, the retardation film 400 is obtained. The obtained retardation film 400 is wound up in the MD direction and collected as a roll 410.

  Furthermore, in this embodiment, before performing the uniaxial stretching process with the first stretching machine 250, the film thicknesses of the A layer to the C layer of the unstretched film 300 are measured by the interference film thickness meter 270 (measuring step). ). The measured film thickness data is sent to the controller 280.

  In the control device 280, the slit width of the slit 231 is adjusted by controlling the opening control unit 281 to loosen or tighten the adjustment bolt 232 (opening adjustment process). At this time, according to the film thickness measured by the interference film thickness meter 270, the opening control unit 281 performs slits at a position where the total film thickness of the unstretched film 300 is thicker than the target value in the TD direction of the unstretched film 300. Control is performed such that the slit width of the slit 231 is reduced and the slit width of the slit 231 is increased at a position where the total film thickness is thinner than the target value. Thereby, since the dispersion | variation in the total film thickness in the TD direction of the film 300 before extending | stretching can be made small, it becomes possible to control the thickness of the film 300 before extending | stretching and the retardation film 400 in the TD direction precisely, and also the phase difference film 400 by extension. It is possible to precisely control the retardation of the film.

  Further, the opening control unit 281 determines the slit 231 when the total film thickness of the unstretched film 300 becomes larger than the target value in the MD direction of the unstretched film 300 in accordance with the film thickness measured by the interference film thickness meter 270. When the slit width is reduced and the total film thickness becomes thinner than the target value, control is performed to increase the slit width of the slit 231. Thereby, since the dispersion | variation in the total film thickness in MD direction of the film 300 before extending | stretching can be made small, it becomes possible to control the thickness of MD direction in the film 300 before extending | stretching and the phase difference film 400 precisely, and by extension, phase difference film 400 It is possible to precisely control the retardation of the film.

  Further, in the control device 280, the extrusion speed control unit 282 controls the temperature of the heater 233 to adjust one or more viscosities of the resin a to the resin c, whereby one or more of the resin a to the resin c from the slit 231 is adjusted. The extrusion speed is adjusted (speed adjustment process). At this time, the extrusion speed control unit 282 reduces the extrusion speed when the film thickness of each layer becomes larger than the target value in the MD direction of the pre-stretch film 300 in accordance with the film thickness measured by the interference film thickness meter 270. Control is performed such that the film thickness is adjusted to be thin, and when the film thickness of each layer becomes thinner than the target value, the extrusion speed is increased to adjust the film thickness to be thick. Thereby, since the dispersion | variation in the film thickness in the MD direction of each layer of the film 300 before extending | stretching can be made small, it becomes possible to control the thickness of the MD direction in each layer of the film 300 before extending | stretching and the phase difference film 400 precisely, and by extension. The retardation of the retardation film 400 can be precisely controlled. For temperature adjustment of the resin a to the resin c by the heater 233, for example, JP-A-2006-188018 may be referred to.

As mentioned above, although one Embodiment of the manufacturing method of the retardation film using the feedback control based on a film thickness was described, the said embodiment may be changed further and may be implemented.
For example, the interference type film thickness meter 270 is provided between the first stretching machine 250 and the second stretching machine 260, and after the uniaxial stretching process by the first stretching machine 250 and by the second stretching machine 260. You may make it measure the film thickness of the film 300 before extending | stretching before uniaxial stretching. Further, for example, the interference type film thickness meter 270 is also provided downstream of the second stretching machine 260 and measures the film thickness of the retardation film 400 after uniaxial stretching by the second stretching machine 260. Also good. Here, from the viewpoint that the film thickness can be measured with high accuracy, it is preferable that the film thickness of each layer when measuring the film thickness is large. Specifically, each film thickness of the A layer to the C layer is preferably 3 μm or more, and more preferably 5 μm or more. The upper limit of the film thickness is preferably 300 μm or less, and more preferably 250 μm or less. Furthermore, it is preferable that the thickness of each layer is different. Specifically, the difference in film thickness of each layer is preferably 3 μm or more, more preferably 5 μm or more, and preferably 300 μm or less, more preferably 250 μm or less.

In addition, for example, only one of the control by the opening control unit 281 and the control by the extrusion speed control unit 282 may be performed.
Further, instead of automatically controlling by the control unit 280, the user may control based on the film thicknesses of the A layer to the C layer measured by the interference film thickness meter 270. Good.
Furthermore, the manufacturing apparatus 200 for the retardation film 400 may be provided with components other than those described above.

[3. Liquid crystal display device)
By obtaining the retardation film of the present invention by using the multilayer film of the present invention, a retardation film with precisely controlled retardation can be realized, so that high birefringence compensation is possible. For this reason, for example, the retardation film of the present invention alone or in combination with other members is used for a liquid crystal display device, an organic electroluminescence display device, a plasma display device, an FED (field emission) display device, an SED (surface electric field). ) It may be applied to a display device such as a display device.

  A liquid crystal display device generally includes a pair of polarizers (light incident side polarizing plate and light emitting side polarizing plate) whose absorption axes are substantially orthogonal to each other, and a liquid crystal cell provided between the pair of polarizers. . When providing the retardation film of this invention in a liquid crystal display device, you may provide the retardation film of this invention between said pair of polarizers, for example. At this time, the retardation film of the present invention may be provided on the light incident side of the liquid crystal cell, or may be provided on the light emission side of the liquid crystal cell.

  Usually, the pair of polarizers, the retardation film of the present invention, and the liquid crystal cell are combined into a single member called a liquid crystal panel, and this liquid crystal panel is irradiated with light from a light source to the light output side of the liquid crystal panel. An image is displayed on an existing display surface. At this time, the retardation film of the present invention has an excellent polarizing plate compensation function because the retardation is precisely controlled, and can reduce light leakage when the display surface of the liquid crystal display device is viewed obliquely. Is possible. Moreover, since the retardation film of the present invention usually has an excellent optical function in addition to the polarizing plate compensation function, it is possible to further improve the visibility of the liquid crystal display device.

  Liquid crystal cell driving methods include, for example, in-plane switching (IPS) method, vertical alignment (VA) method, multi-domain vertical alignment (MVA) method, continuous spin wheel alignment (CPA) method, hybrid alignment nematic (HAN) Examples thereof include a twisted nematic (TN) method, a super twisted nematic (STN) method, and an optically compensated bend (OCB) method. Of these, the in-plane switching method and the vertical alignment method are preferable, and the in-plane switching method is particularly preferable. Although the in-plane switching type liquid crystal cell has a wide viewing angle, the viewing angle can be further expanded by applying the retardation film of the present invention.

The retardation film of the present invention may be bonded to a liquid crystal cell or a polarizing plate. For example, the retardation film may be bonded to both sides of the polarizing plate, or may be bonded only to one side. A known adhesive can be used for bonding.
In addition, the retardation film of the present invention may be used alone or in combination of two or more.
Furthermore, when providing the retardation film of this invention in a display apparatus, you may use it in combination with another retardation film. For example, when the retardation film of the present invention is provided as a retardation film in a liquid crystal display device having a vertical alignment type liquid crystal cell, the viewing angle characteristics are improved between the pair of polarizers in addition to the retardation film of the present invention. Another retardation film may be provided.

[4. Other matters]
The retardation film of the present invention may be used as a quarter wavelength plate, for example. In this case, the retardation film of the present invention is made a quarter wave plate by setting the in-plane retardation of the retardation film of the present invention to 120 nm to 160 nm, and this quarter wave plate can be combined with a linear polarizer. Thus, a circularly polarizing plate can be obtained. At this time, the angle formed by the slow axis of the quarter-wave plate and the absorption axis of the linear polarizer is preferably 45 ° ± 2 °.

  Further, the multilayer film of the present invention may be used as a protective film in the polarizing plate as it is or after being subjected to a stretching treatment. The polarizing plate usually includes a polarizer and protective films bonded to both sides thereof. In this case, instead of the protective film, the multilayer film of the present invention may be bonded to a polarizer, and the multilayer film may be used as a protective film. In this case, since the protective film is omitted, the liquid crystal display device can be reduced in thickness, weight, and cost.

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following examples, and may be arbitrarily changed without departing from the gist of the present invention and its equivalent scope. May be implemented. In the following description, “%” and “parts” representing amounts are based on weight unless otherwise specified.

[Evaluation method]
[Average refractive index]
The resin to be measured was pressed for 1 minute with an electric heating press adjusted to 200 ° C., and a sample having a film thickness of about 100 μm was molded. The molded sample was measured at a measurement wavelength of 532 nm using a refractive index film thickness measuring device (“Prism Coupler” manufactured by Metricon).

[Measurement of film thickness by interference type film thickness meter]
Using an interference film thickness meter (manufactured by Otsuka Electronics Co., Ltd.), the film thicknesses of the resin layer A1, the resin layer C1, the resin layer B1, the resin layer C2, the resin layer A2, and the entire film of the film before stretching were measured.

[Thickness measurement by microscope]
After embedding the film before stretching in an epoxy resin, it is sliced using a microtome (“RUB-2100” manufactured by Yamato Kogyo Co., Ltd.), and the cross section is observed using a scanning electron microscope. The film thickness of was measured.

[Measurement of retardation]
About the stretched film, the retardation Re at an incident angle of 0 ° and the retardation R ₄₀ at an incident angle of 40 ° were measured using an automatic birefringence meter (“KOBRA-21ADH” manufactured by Oji Scientific Instruments) at a measurement wavelength of 590 nm. It was measured.

[Tensile modulus of resin c]
Acrylic resin is molded in a single layer, a 1 cm × 25 cm test piece is cut out, and the tensile speed is 25 mm / min using a tensile tester (manufactured by Toyo Baldwin, Tensilon UTM-10T-PL) based on ASTM882. Measured with The same measurement was performed 5 times, and the arithmetic average value was taken as the representative value of the tensile modulus.

[Measurement of glass transition temperature (Tg)]
The resin to be measured was measured by differential scanning calorimetry (DSC) at a heating rate of 10 ° C./min using a differential scanning calorimeter (“DSC-6200” manufactured by Seiko Instruments Inc.) according to JIS K7121.

[Delamination strength]
From the pre-stretched films obtained in each Example and Comparative Example, a 10 mm wide film piece was cut into a strip shape as a test piece, and a 180 ° peel test was performed at a tensile rate of 100 mm / min. The evaluation was “A” when cohesive failure occurred at the interface, “B” when partial cohesive failure occurred, and “C” when peeled at the interface.

[Example 1]
[Preparation of film before stretching]
A film forming apparatus for coextrusion molding of three types and five layers (type of forming a film consisting of five layers with three types of resins) was prepared.
Pellets of polycarbonate resin (“Wanderlite PC-115” manufactured by Asahi Kasei Co., Ltd., glass transition temperature 145 ° C.) were put into a single screw extruder equipped with a double flight type screw and melted. This polycarbonate resin corresponds to a resin a having a positive intrinsic birefringence value.

  Also, another uniaxial shaft provided with a double-flight type screw of pellets of styrene-maleic anhydride copolymer resin (“Dylark D332” manufactured by Nova Chemicals, maleic anhydride unit content 17% by weight, glass transition temperature 129 ° C.) It was put into an extruder and melted. This styrene-maleic anhydride copolymer resin corresponds to the resin b having a negative intrinsic birefringence value.

  Furthermore, pellets of polymethyl methacrylate resin ("Delpet 80NH" manufactured by Asahi Kasei Co., Ltd., glass transition temperature 110 ° C., tensile elastic modulus 3.3 GPa) are put into yet another single-screw extruder equipped with a double flight screw. And melted. This polymethyl methacrylate resin corresponds to the thermoplastic acrylic resin c.

  The melted polycarbonate resin at 260 ° C. was supplied to one manifold of a multi-manifold die (die slip surface roughness Ra = 0.1 μm) through a leaf disk-shaped polymer filter having an opening of 10 μm. Further, the molten 260 ° C. styrene-maleic anhydride copolymer resin was supplied to another manifold through a leaf disk-shaped polymer filter having an opening of 10 μm. Furthermore, the melted polymethylmethacrylate resin at 260 ° C. was supplied to another manifold through a leaf disk-shaped polymer filter having an opening of 10 μm.

A polycarbonate resin, a styrene-maleic anhydride copolymer resin, and a polymethyl methacrylate resin are simultaneously extruded from the multi-manifold die at 260 ° C. to obtain a polycarbonate resin layer / polymethyl methacrylate resin layer / styrene-maleic anhydride copolymer. A five-layer film consisting of polymer resin / polymethyl methacrylate resin layer / polycarbonate resin layer was formed. The molten resin coextruded into a film in this way is cast on a cooling roll adjusted to a surface temperature of 115 ° C., and then passed between two cooling rolls adjusted to a surface temperature of 120 ° C. to obtain a polycarbonate resin layer (Resin layer A1), polymethyl methacrylate resin layer (resin layer C1), styrene-maleic anhydride copolymer resin layer (resin layer B1), polymethyl methacrylate resin layer (resin layer C2), and polycarbonate resin A film before stretching having a five-layer structure including the layers (resin layer A2) in this order was obtained (coextrusion step). The width of the obtained film before stretching was 600 mm. About the obtained film before extending | stretching, the film thickness measurement with an interference type film thickness meter and a microscope was performed. The results are shown in Table 1.
Further, as described above, the delamination strength was also measured. The results are shown in Table 1.

[Production of stretched film]
The film before stretching was supplied to a tenter transverse uniaxial stretching machine and stretched in the transverse direction at a stretching temperature of 150 ° C. and a stretching ratio of 3.0 (first stretching step). Subsequently, the stretched film was supplied to a longitudinal uniaxial stretching machine and stretched in the longitudinal direction at a stretching temperature of 126 ° C. and a stretching ratio of 1.2 to obtain a retardation film (second stretching step). The stretching temperature in the second stretching step is the glass transition temperature of the mixed resin that forms the B layer. In the obtained retardation film, the slow axis of the resin layer A1, the slow axis of the resin layer B, and the slow axis of the resin layer A2 were substantially parallel to each other. The retardation of the obtained stretched film was measured. The results are shown in Table 1.

[Example 2]
Example 1 except that the polymethyl methacrylate resin was changed from a pellet of “Delpet 80NH” to a pellet of “HT 55Z” (manufactured by Sumitomo Chemical Co., Ltd., rubber particle-containing polymethyl methacrylate, tensile elastic modulus 2.5 GPa). In the same manner as above, a pre-stretch film and a stretched film were prepared and evaluated. The results are shown in Table 1.

[Example 3]
Stretching in the same manner as in Example 1 except that the polymethyl methacrylate resin was changed from “Delpet 80NH” pellets to “TX-100S” (Denka Co., Ltd., methyl methacrylate-styrene copolymer). A pre-film and a stretched film were prepared and evaluated. The results are shown in Table 1.

[Comparative Example 1]
[Preparation of film before stretching]
A film forming apparatus for co-extrusion molding of two types and three layers (type of forming a film composed of three layers with two types of resins) was prepared.
Pellets of polycarbonate resin (“Wanderlite PC-115” manufactured by Asahi Kasei Co., Ltd., glass transition temperature 145 ° C.) were put into a single screw extruder equipped with a double flight type screw and melted. This polycarbonate resin corresponds to a resin a having a positive intrinsic birefringence value.

  Also, another uniaxial shaft provided with a double-flight type screw of pellets of styrene-maleic anhydride copolymer resin (“Dylark D332” manufactured by Nova Chemicals, maleic anhydride unit content 17% by weight, glass transition temperature 129 ° C.) It was put into an extruder and melted. This styrene-maleic anhydride copolymer resin corresponds to the resin b having a negative intrinsic birefringence value.

  The melted polycarbonate resin at 260 ° C. was supplied to one manifold of a multi-manifold die (die slip surface roughness Ra = 0.1 μm) through a leaf disk-shaped polymer filter having an opening of 10 μm. Further, the molten 260 ° C. styrene-maleic anhydride copolymer resin was supplied to another manifold through a leaf disk-shaped polymer filter having an opening of 10 μm.

A polycarbonate resin and a styrene-maleic anhydride copolymer resin are simultaneously extruded from the multi-manifold die at 260 ° C. to form a three-layer structure comprising a polycarbonate resin layer / styrene-maleic anhydride copolymer resin / polycarbonate resin layer. It was made into the film form. The molten resin coextruded into a film in this way is cast on a cooling roll adjusted to a surface temperature of 115 ° C., and then passed between two cooling rolls adjusted to a surface temperature of 120 ° C. to obtain a polycarbonate resin layer A pre-stretched film having a three-layer structure comprising (resin layer A1), a styrene-maleic anhydride copolymer resin layer (resin layer B1), and a polycarbonate resin layer (resin layer A2) in this order was obtained. Extrusion process). The width of the obtained film before stretching was 600 mm. About the obtained film before extending | stretching, the film thickness measurement with an interference type film thickness meter and a microscope was performed. The results are shown in Table 1. However, the interference type film thickness meter cannot measure the film thickness because the difference in refractive index between layers is small.
Further, as described above, the delamination strength was also measured. The results are shown in Table 1.

[Production of stretched film]
The film before stretching was supplied to a tenter transverse uniaxial stretching machine and stretched in the transverse direction at a stretching temperature of 150 ° C. and a stretching ratio of 3.0 (first stretching step). Subsequently, the stretched film was supplied to a longitudinal uniaxial stretching machine and stretched in the longitudinal direction at a stretching temperature of 126 ° C. and a stretching ratio of 1.2 to obtain a retardation film (second stretching step). The stretching temperature in the second stretching step is the glass transition temperature of the mixed resin that forms the B layer. In the obtained retardation film, the slow axis of the resin layer A1, the slow axis of the resin layer B, and the slow axis of the resin layer A2 were substantially parallel to each other. The retardation of the obtained stretched film was measured. The results are shown in Table 1.

〔Consideration〕
As can be seen from Table 1, by providing resin layers C1 and C2 having a large refractive index difference between the resin layers A1 and A2 and the resin layer B1, the film thicknesses of the resin layers A1 and A2 and the resin layer B1. Can be measured with an interference type film thickness meter, and the measurement result is sufficiently close to the film thickness measurement by microscopic observation. From this, it turns out that the multilayer film which has the structure of this invention can measure the film thickness of each layer in-line with an interference type film thickness meter. Moreover, it turns out that the delamination strength is also improving in Examples 1-3 compared with the comparative example 1 which does not have C layer.

100 multilayer film 111 B layer 115,116 Interface between resin b and resin c 121,122 A layer 125,126 Surface of A layer (main surface of multilayer film 100)
131, 132 C layer 135, 136 Interface between resin a and resin c 200 Retardation film production apparatus 210 Hopper 220 Extruder 230 Die 231 Slit (opening)
232 Adjustment bolt 233 Heater 240 Cooling roll 250 First stretching machine 260 Second stretching machine 270 Interference film thickness meter 280 Controller 281 Opening control unit 282 Extrusion speed control unit 300 Film before stretching 400 Phase difference film 410 Phase difference film Roll of

Claims (8)

A B layer made of a resin b having a negative intrinsic birefringence value;
C layer made of resin c formed on both surfaces of the B layer;
A multilayer film comprising the C layer and an A layer formed of a resin a having a positive intrinsic birefringence value formed on a surface far from the B layer;
The resin b includes a styrene polymer,
The resin c includes a thermoplastic acrylic resin;
The resin a includes polycarbonate;
The multilayer film whose thickness of each said C layer is 5 micrometers or more.   The multilayer film according to claim 1, wherein the styrenic polymer is a copolymer containing a repeating unit derived from maleic anhydride.   A retardation film formed by stretching the multilayer film according to claim 1. A retardation Re at an incident angle of 0 °, the retardation _{R 40} at an incident angle of 40 ° satisfies the relation of 0.92 ≦ _R 40 /Re≦1.08, retardation film according to claim 3. A method for producing a multilayer film according to claim 1 or 2,
A resin b containing a styrenic polymer and having a negative intrinsic birefringence value;
A resin c containing a thermoplastic acrylic resin;
A resin a containing polycarbonate and having a positive intrinsic birefringence value;
The manufacturing method of a multilayer film including co-extrusion. A resin b containing a styrenic polymer and having a negative intrinsic birefringence value;
A resin c containing a thermoplastic acrylic resin;
A resin a containing polycarbonate and having a positive intrinsic birefringence value;
And extrude
When the uniaxial stretching direction is the X axis, the direction perpendicular to the uniaxial stretching direction in the film plane is the Y axis, and the film thickness direction is the Z axis, the plane of incidence is perpendicular to the film plane and the vibration plane of the electric vector is The phase of linearly polarized light in the XZ plane that is perpendicularly incident on the film surface and the plane of vibration of the electric vector in the YZ plane is delayed when uniaxially stretched in the X-axis direction at the temperature T1 and is different from the temperature T1. A coextrusion step for obtaining a pre-stretched film, which proceeds when uniaxially stretched in the X-axis direction at a temperature T2,
A first stretching step for subjecting the pre-stretched film to a uniaxial stretching treatment in one direction at a temperature T1 or T2.
A second stretching step in which a uniaxial stretching treatment is performed at a temperature T2 or T1 different from the above in a direction orthogonal to the direction in which the uniaxial stretching treatment is performed in the first stretching step;
The film before stretching is
A B layer made of the resin b;
C layer composed of the resin c formed on both surfaces of the B layer, each having a thickness of 5 μm or more;
The manufacturing method of retardation film which is a multilayer film provided with the A layer which consists of the said resin a formed in the surface on the side far from the said B layer of the said C layer. The co-extrusion step includes extruding one or more of the resin a, the resin b, and the resin c from the opening of a die having an opening whose size can be adjusted;
The manufacturing method further includes:
A measuring step of measuring the film thickness of each of the layer consisting of the resin a, the layer consisting of the resin b, and the layer consisting of the resin c of the pre-stretch film with an interference film thickness meter;
The manufacturing method according to claim 6, further comprising: an opening adjusting step of adjusting the size of the opening of the die according to the measured film thickness of each layer.   The manufacturing method of Claim 7 which further has a speed adjustment process of adjusting one or more extrusion speeds of the said resin a, the said resin b, and the said resin c according to the film thickness of each measured layer.

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