JP2013205500A - Method for manufacturing retardation plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing retardation plates capable of highly efficiently manufacturing a thin retardation plate uniformly having desired optical characteristics.SOLUTION: The method for manufacturing retardation plates includes: forming a film before drawing that has a resin layer (a) composed of a resin A having a positive intrinsic birefringence value and a resin layer (b) composed of a resin B having a negative intrinsic birefringence value; drawing the film in one direction at a temperature T1; and drawing it in the orthogonal direction at a temperature T2 lower than the temperature T1. The second drawing is a free uniaxial drawing, and a temperature TgH that is a higher temperature of a temperature Tg of the resin A and a temperature Tg of the resin B, and the temperature T2 satisfy the following relationship: T2<TgH-10. After the second drawing, tensile force of the film is reduced. The manufacturing method further includes a relaxation step of heat-treating the film at a temperature T4, in which a temperature TgL that is a lower temperature of the temperature Tg of the resin A and the temperature Tg of the resin B, and the temperature T4 satisfy the following relationship: TgL-20≤T4≤TgL.

Description

  The present invention relates to a method of manufacturing a retardation plate.

  A phase difference plate used for applications such as optical compensation of a liquid crystal display device is required to be able to reduce a change in color tone of the display device depending on an observation angle, and various techniques have been developed conventionally. For example, in Patent Document 1, a film before stretching having a resin layer a of resin A having a positive intrinsic birefringence value and a resin layer b of resin B having a negative intrinsic birefringence value is uniaxially stretched in a certain direction. In addition, a technique for manufacturing a retardation plate by uniaxial stretching in a direction different from the direction is described.

JP 2011-39338 A

  In the manufacturing method as described above, for example, a three-dimensional retardation plate can be manufactured. A three-dimensional retardation plate means a retardation plate having an NZ coefficient of 0 or more and 1 or less. In order to keep the NZ coefficient within the above range, generally, each resin layer containing a resin having a positive intrinsic birefringence value is made thinner than a resin layer containing a resin having a negative intrinsic birefringence value. Is required. Furthermore, when providing a resin layer having a positive intrinsic birefringence value on both sides of a resin layer having a negative intrinsic birefringence value, two intrinsic birefringence values are obtained in order to exhibit optical performance and mechanical strength. It is also required that one of the positive resin layers be thinner than the other.

  In order to manufacture such a retardation plate with a thin overall thickness and uniform optical properties, and to manufacture with high efficiency, one of the two stretching operations, preferably the second one, It is preferable to carry out by free uniaxial stretching.

  However, in the production of such a phase difference plate, when free uniaxial stretching is performed, in the film after stretching, a large residual stress is left in the film, and as a result, orientation relaxation greatly occurs after the production of the film. is there. Such orientation relaxation appears as a change with time in optical characteristics when the retardation plate is used, and thus the durability of the retardation plate is significantly reduced.

  As a measure for reducing the orientation relaxation, it can be considered that the stretching is performed at a relatively high temperature, but in this case, the occurrence of anisotropy of the retardation plate is suppressed, and as a result, it is thin and desired. It is difficult to obtain a retardation plate that obtains optical characteristics.

  Accordingly, an object of the present invention is to provide a method for producing a retardation plate that can produce a thin retardation plate having a desired optical characteristic uniformly with high efficiency.

The present inventor has studied to solve the above-mentioned problems, and in particular, has studied a means that can sufficiently relieve the residual stress of the film without causing significant orientation relaxation. As a result, it has been found that such residual stress can be relaxed by performing heat treatment under predetermined conditions in a state where the tension applied during stretching is reduced, and the present invention has been completed based on such knowledge. .
That is, according to the present invention, the following is provided.

[1] A resin layer a made of a resin A having a positive intrinsic birefringence value, and a resin layer b made of a resin B having a negative intrinsic birefringence value provided on at least one surface of the resin layer a. Forming a film comprising:
A first stretching step of stretching the film before stretching in one direction at a temperature T1 (° C.);
After the first stretching step, a second stretching step of stretching the film in the other direction substantially orthogonal to the stretching direction at a temperature T2 (° C.) lower than the temperature T1,
A method of manufacturing a retardation plate having
The second stretching step is a step of free uniaxial stretching,
Of the glass transition temperature of the resin A and the glass transition temperature of the resin B, the higher temperature TgH (° C.) and the temperature T2 satisfy the relationship of T2 <TgH-10,
After the second stretching step,
Reducing the tension of the film;
A relaxation step of heat-treating the film at a temperature T4 (° C.) in a state where tension is reduced, and the lower temperature TgL (° C.) of the glass transition temperature of the resin A and the glass transition temperature of the resin B And a relaxation step in which the temperature T4 satisfies the relationship of TgL-20 ≦ T4 ≦ TgL;
A method for producing a retardation plate further comprising:
[2] The step of reducing the tension of the film includes passing the film through a nip roll having a temperature T3 (° C.),
The heat treatment in the relaxation step includes bringing the film into contact with a roll heated to a temperature T4 (° C.),
The manufacturing method according to [1], wherein the temperature T3 and the temperature T4 satisfy a relationship of T4 ≦ T3 + 10.
[3] The manufacturing method according to [1] or [2], wherein a difference between the temperature TgH and the temperature TgL is 10 ° C. or more.
[4] The production method according to any one of [1] to [3], wherein the resin A is a resin containing a polycarbonate polymer, and the resin B is a resin containing a polystyrene polymer.
[5] The manufacturing method according to any one of [1] to [4], wherein the temperature T1 and the temperature TgH satisfy a relationship of T1> TgH.

  According to the method for producing a retardation plate of the present invention, a thin retardation plate having a desired optical characteristic can be produced with high efficiency.

Figure 1 is a high glass transition temperature Tg _A of the resin A constituting the resin layer a, when low glass transition temperature Tg _B of the resin B constituting the resin layer b, and a resin layer (a) and the resin layer b stretched 3 is a graph showing an example of the temperature dependence of retardation Δ when the film is stretched and the temperature dependence of retardation Δ when the film before stretching is stretched. FIG. 2 is a side view schematically showing a specific example of the production line for carrying out the production method of the present invention and its operation.

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

  Reference sign “A” of resin A and resin layer A, reference sign “B” of resin B and resin layer B, reference sign “a” of resin layer a, and reference sign “b” of resin layer b are all the reference signs. It is the code | symbol attached | subjected in order to distinguish the element attached | subjected from other elements, and it does not have meaning other than distinction of an element. In addition, when there are a plurality of resin layers of the same type in one film, the names may be further distinguished by attaching numbers to the names (for example, resin layer A1, resin layer A2, etc.).

  Further, a positive intrinsic birefringence value means that the refractive index in the stretching direction is larger than a refractive index in a direction perpendicular to the stretching direction, and a negative intrinsic birefringence value means that the refractive index in the stretching direction is negative. It means that the refractive index is smaller than the refractive index in the direction orthogonal thereto. The value of intrinsic birefringence can also be calculated from the dielectric constant distribution.

  The in-plane retardation (in-plane retardation) of the film or layer is a value represented by (nx−ny) × d unless otherwise specified. Further, the retardation in the thickness direction (thickness direction retardation) of the film or layer is a value represented by {| nx + ny | / 2−nz} × d unless otherwise specified. Here, nx is a direction (in-plane direction) perpendicular to the thickness direction of the film or layer and represents the refractive index in the stretching direction of the film or layer, and ny is the in-plane direction and orthogonal to the nx direction. Nz represents the refractive index in the thickness direction, and d represents the thickness of the film or layer. Moreover, when extending | stretching in multiple times, let the extending | stretching direction in the 1st extending | stretching be the nx direction, and let the direction be the nx direction also in the extending | stretching after the 2nd time. Retardation can be measured using a commercially available phase difference measuring device (for example, an automatic birefringence meter (“KOBRA-21ADH” manufactured by Oji Scientific Instruments)) or the Senarmont method. These measurement wavelengths are 550 nm unless otherwise specified.

  Further, the NZ coefficient of the film or layer is a value represented by (Nx−Nz) / (Nx−Ny) unless otherwise specified. Here, Nx represents the refractive index in the direction (in-plane direction) perpendicular to the thickness direction of the film or layer and gives the maximum refractive index, and Ny is the in-plane direction and orthogonal to the Nx direction. The refractive index Ny in the direction to be measured, Nz represents the refractive index in the thickness direction, and d represents the thickness of the film or layer. These measurement wavelengths are 550 nm unless otherwise specified.

  Further, the “long” means one having a length of at least 5 times the width, preferably 10 times or more, and specifically wound in a roll shape. It has a length enough to be stored or transported.

Further, “substantially orthogonal” means that the angle formed is usually 85 ° or more, preferably 89 ° or more, usually 95 ° or less, preferably 91 ° or less.
Further, “substantially parallel” means that the formed angle is within a range of ± 5 °, preferably within a range of ± 1 °, in addition to the case where the formed angle is 0 ° (when truly parallel). .

  Further, the “polarizing plate” and the “retardation plate” include not only a rigid member but also a flexible member such as a resin film.

  In this application, the temperature related variables are in degrees Celsius (° C.).

[1. Overview]
The method for producing a retardation plate of the present invention includes a resin layer a made of a resin A having a positive intrinsic birefringence value, and a resin B having a negative intrinsic birefringence value provided on one surface of the resin layer a. A step of forming a pre-stretching film (pre-stretching film forming step) comprising a resin layer b comprising: a first stretching of stretching the film before stretching in one direction at a temperature T1 (° C.) And a second stretching step of stretching the film in the other direction substantially orthogonal to the stretching direction at a temperature T2 (° C.) lower than the temperature T1 after the step and the first stretching step, and then the tension of the film And a relaxation step of heat-treating the film at a temperature T4 (° C.) in a state where the tension is reduced.

[2. Film forming step before stretching]
[2.1. Resin A]
Resin A is a resin having a positive intrinsic birefringence value. Usually, a thermoplastic resin having a positive intrinsic birefringence value is used as the resin A.
Since the intrinsic birefringence value of the resin A is positive, the resin A usually contains a polymer having a positive intrinsic birefringence value. Examples of this polymer include olefin polymers such as polyethylene and polypropylene; polyester polymers such as polyethylene terephthalate and polybutylene terephthalate; polyarylene sulfide polymers such as polyphenylene sulfide; polyvinyl alcohol polymers, polycarbonate polymers, poly Examples include arylate polymers, cellulose ester polymers, polyethersulfone polymers, polysulfone polymers, polyallylsulfone polymers, polyvinyl chloride polymers, norbornene polymers, and rod-like liquid crystal polymers. These polymers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. The polymer may be a homopolymer or a copolymer. Among these, a polycarbonate polymer is preferable from the viewpoints of expression of retardation, stretchability at low temperature, and adhesiveness between the resin layer a and a layer other than the resin layer a.

  As the polycarbonate polymer, any polymer may be used as long as it is a polymer having a repeating unit due to a carbonate bond (—O—C (═O) —O—). Examples of the polycarbonate polymer include bisphenol A polycarbonate, branched bisphenol A polycarbonate, o, o, o ', o'-tetramethylbisphenol A polycarbonate, and the like.

  Resin A may contain a compounding agent. Examples of compounding agents include layered crystal compounds; fine particles; antioxidants, heat stabilizers, light stabilizers, weathering stabilizers, ultraviolet absorbers, near infrared absorbers and other stabilizers; plasticizers; dyes and pigments, etc. Colorants; antistatic agents; and the like. Among these, an ultraviolet absorber is preferable because it can improve weather resistance. Moreover, a compounding agent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  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 suitable is 2,2'-methylenebis (4- (1,1,3,3-tetramethylbutyl) -6- (2H-benzotriazol-2-yl) phenol).

  The amount of the compounding agent can be appropriately determined as long as the effects of the present invention are not significantly impaired. For example, the total light transmittance in a 1 mm thickness of the pre-stretched film can be in a range where 80% or more can be maintained.

  It is preferable to adjust the weight average molecular weight of the polymer in the resin A to a range in which the melt extrusion method or the solution casting method can be performed using the resin A.

The glass transition temperature Tg _A of the resin A, usually 80 ° C. or higher, preferably 90 ° C. or higher, more preferably 100 ° C. or higher, more preferably 110 ° C. or higher, particularly preferably 120 ° C. or higher. With such a high glass transition temperature Tg _A , orientation relaxation of the resin A can be reduced. Particular upper limit to the glass transition temperature Tg _A is not, but usually is 200 ° C. or less.

The breaking elongation of the resin A at the glass transition temperature Tg _B of the resin B described later is preferably 50% or more, and more preferably 80% or more. When the elongation at break is within this range, the retardation plate 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. Moreover, although there is no restriction | limiting in particular in the upper limit of the breaking elongation of resin A, Usually, it is 200% or less.

[2.2. Resin B]
Resin B is a resin having a negative intrinsic birefringence value. Usually, a thermoplastic resin having a negative intrinsic birefringence value is used as the resin B.
Since the intrinsic birefringence value of the resin B is negative, the resin B usually contains a polymer having a negative intrinsic birefringence value. Examples of this polymer include polystyrene polymers including homopolymers of styrene or styrene derivatives or copolymers with other monomers; polyacrylonitrile polymers, polymethylmethacrylate polymers, or multicomponent copolymers thereof. Examples thereof include polymers. Examples of other monomers contained in the polystyrene polymer include acrylonitrile, maleic anhydride, methyl methacrylate, and butadiene. These polymers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. Among these, a polystyrene polymer is preferable from the viewpoint of high retardation development, and a copolymer of styrene or a styrene derivative and maleic anhydride is particularly preferable from the viewpoint of high heat resistance. In this case, the amount of the structural unit (maleic anhydride unit) formed by polymerizing maleic anhydride with respect to 100 parts by weight of the styrene polymer is preferably 5 parts by weight or more, more preferably 10 parts by weight or more. The amount is particularly preferably 15 parts by weight or more, preferably 30 parts by weight or less, more preferably 28 parts by weight or less, and particularly preferably 26 parts by weight or less.

Resin B may contain a compounding agent. Examples thereof include the same compounding agents that may be contained in the resin A. Moreover, 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 as long as the effects of the present invention are not significantly impaired. For example, the total light transmittance in a 1 mm thickness of the pre-stretched film can be in a range where 80% or more can be maintained.

  It is preferable to adjust the weight average molecular weight of the polymer contained in the resin B to a range in which the resin B can be used for the melt extrusion method or the solution casting method.

The glass transition temperature Tg _B of the resin B is usually 80 ° C. or higher, preferably 90 ° C. or higher, more preferably 100 ° C. or higher, more preferably 110 ° C. or higher, particularly preferably 120 ° C. or higher. With such a high glass transition temperature Tg _{B, the} relaxation of orientation of the resin B can be reduced. Particular upper limit to the glass transition temperature Tg _B is not, but usually is 200 ° C. or less.

The breaking elongation of the resin B at the glass transition temperature Tg _A of the resin _A is preferably 50% or more, and more preferably 80% or more. When the elongation at break is within this range, the retardation plate of the present invention can be stably produced by stretching. Although there is no restriction | limiting in particular in the upper limit of the breaking elongation of resin B, Usually, it is 200% or less.

The difference between the higher temperature TgH and the lower temperature TgL of the glass transition temperature Tg _A of the resin _A and the glass transition temperature Tg _B of the resin B (that is, the absolute value of the difference between Tg _A and Tg _B ) is: Preferably it is 10 degreeC or more, More preferably, it is 12 degreeC or more, Preferably it is 40 degrees C or less, More preferably, it is 20 degrees C or less. By making the absolute value of the difference between the glass transition temperatures larger than the lower limit of the range, the temperature dependence of the expression of retardation can be increased. On the other hand, by making the absolute value of the difference between the glass transition temperatures below the upper limit of the above range, the resin having a high glass transition temperature can be easily stretched, and the flatness of the retardation plate can be enhanced. The glass transition temperature Tg _A is preferably higher than the glass transition temperature Tg _B. Therefore, it is preferable that the resin A and the resin B normally satisfy the relationship of Tg _A ≧ Tg _B + 10 ° C.

[2.3. Method for forming film before stretching]
Examples of the method for forming the film before stretching include a co-extrusion method and a co-casting method, and the co-extrusion method is preferable. The co-extrusion method is a method of extruding and molding a plurality of resins in a molten state. The coextrusion method is excellent in terms of production efficiency and in that volatile components such as a solvent do not remain in the film before stretching.

  Examples of the coextrusion method include a coextrusion T-die method, a coextrusion inflation method, and a coextrusion lamination method. Among these, the coextrusion T-die method is preferable. The coextrusion T-die method includes a feed block method and a multi-manifold method. Among them, the multi-manifold method is particularly preferable in that variation in the thickness of the resin layer a can be reduced.

  When the co-extrusion T-die method is adopted, the melting temperature of the resin in the extruder having the T-die is preferably 80 ° C. or higher, and 100 ° C. higher than the glass transition temperature (Tg) of each resin. 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. By setting the melting temperature of the resin in the extruder to the lower limit value or more of the above range, the fluidity of the resin can be sufficiently increased, and by setting the melting temperature or less to the upper limit value or less, deterioration of the resin can be prevented.

Usually, the sheet-like molten resin extruded from the opening of the die is brought into close contact with the cooling drum. The method for bringing the molten resin into close contact with the cooling drum 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 drums is not particularly limited, but is usually two or more. Examples of the arrangement method of the cooling drum include, but are not 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 drum is not particularly limited.

  The degree of adhesion of the extruded sheet-like resin to the cooling drum varies depending on the temperature of the cooling drum. When the temperature of the cooling drum is raised, the adhesion is improved. However, when the temperature is raised too much, the sheet-like resin may not be peeled off from the cooling drum and may be wound around the drum. Therefore, the cooling drum temperature is preferably (Tg + 30) ° C. or less, more preferably (Tg−5) ° C., where Tg is the glass transition temperature of the resin in the layer contacting the drum among the resin A and resin B extruded from the die. ˜ (Tg−45) ° C. By doing so, problems such as slipping and scratches can be prevented.

  Here, it is preferable to reduce the content of the residual solvent in the film before stretching. Examples of the means include (1) reducing the residual solvent of the resin as a raw material; (2) pre-drying the resin before forming the pre-stretch film. For example, the preliminary drying is performed by a hot air dryer or the like in the form of pellets or the like. The drying temperature is preferably 100 ° C. or more, and the drying time is preferably 2 hours or more. By performing preliminary drying, the residual solvent in the film before stretching can be reduced, and foaming of the extruded sheet-like resin can be prevented.

[2.4. Film before stretching)
The film before stretching includes a resin layer a containing the resin A and a resin layer b containing the resin B. Usually, the resin layer a and the resin layer b are in direct contact.

  The thicknesses of the resin layer a and the resin layer b can be set according to the retardation of the retardation plate to be manufactured.

  The number of layers of the resin layer a and the resin layer b in the pre-stretched film is not particularly limited and can be one or more, respectively, but one resin layer a, resin layer b, and another layer Having a three-layer structure including the resin layer a in this order is preferable because a retardation plate having excellent retardation and strength can be obtained.

  Furthermore, when a three-dimensional retardation plate is manufactured as the retardation plate, the thickness of the resin layer b is usually larger than the thickness of the resin layer a. For example, when a three-dimensional retardation plate is manufactured using a resin containing a polycarbonate polymer as the resin A and a resin containing a polystyrene polymer as the resin B, the total thickness of the resin layer a and the resin layer b The ratio to the total thickness (total thickness of resin layer a / total thickness of resin layer b) is usually 1/20 or more, preferably 1/15 or more, and usually 1/4 or less. Thereby, the temperature dependence of the expression of retardation can be increased.

  The total thickness of the film before stretching 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 300 μm or less, and particularly preferably 250 μm or less. By making the pre-stretch film above the lower limit of the range, sufficient retardation can be developed and the mechanical strength can be increased, and by making the film below the upper limit of the range, the retardation plate has sufficient flexibility. Thus, handling can be improved.

  The film before stretching is stretched in different directions substantially orthogonal to each other at different temperatures T1 and T2, so that retardation occurs in each of the resin layer a and the resin layer b according to each temperature T1 and T2 and the stretching direction. It has the property. When a three-dimensional retardation plate is manufactured by the method for manufacturing a retardation plate of the present invention, the retardation plate is manufactured using this property. In this case, by synthesizing the retardation generated in the resin layer a and the retardation generated in the resin layer b, the NZ coefficient of the retardation plate can be set to 0 or more and 1 or less.

  The size of retardation generated by stretching in the resin layer a and the resin layer b is determined according to the thickness of the film before stretching, the stretching temperature, the stretching ratio, and the like. Therefore, the specific configuration of the pre-stretch film is preferably determined according to an optical function such as a polarizing plate compensation function to be developed.

Among them, the pre-stretch film has a stretch direction in one direction (that is, a uniaxial stretch direction) as an X axis, a direction perpendicular to the uniaxial stretch direction in the film plane as a Y axis, and a film thickness direction as a Z axis. The linearly polarized light (hereinafter referred to as “XZ-polarized light” where appropriate) that is incident perpendicularly to the film surface and whose electric vector vibration surface is in the XZ plane is incident perpendicularly to the film surface and the electric vector vibration surface. Is a phase with respect to linearly polarized light (hereinafter referred to as “YZ polarized light” where appropriate) in the YZ plane,
Delayed when uniaxially stretched in the X-axis direction at temperature Ta,
When the uniaxial stretching is performed in the X-axis direction at a temperature Tb different from the temperature Ta,
(Hereinafter referred to as “requirement P” as appropriate). In the production method of the present application, when the temperature T1 of the first stretching step and the temperature T2 of the second stretching step are set to Ta and Tb described above, it is performed using a film before stretching that satisfies the requirement P, This is particularly preferable from the viewpoint of developing a good phase difference.

  Of the various directions in the plane of the film before stretching, those satisfying the requirement P when at least one direction is taken as the X axis can be preferably used in the present invention as a film satisfying the requirement P. Usually, the pre-stretch film is an isotropic (that is, has no anisotropy) raw film, so if one of the in-plane directions is taken as the X axis and the requirement P is satisfied, any other direction The requirement P can also be satisfied when the X axis is used.

In a film in which a slow axis appears in the X axis by uniaxial stretching, the phase of XZ polarized light is 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 advances with respect to YZ polarized light.
Thus, when the requirement P is satisfied, the film before stretching becomes a film in which the slow axis or the fast axis appears 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 thickness ratio of each layer.

  The in-plane retardation is obtained by multiplying the thickness d by the difference (= nx−ny) 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. Value. The retardation of the film before stretching when the resin layer a and the resin layer b are laminated is synthesized from the retardation of the resin layer a and the retardation of the resin layer b. Therefore, in order to reverse the sign of the retardation of the pre-stretching film including the resin layer a and the resin layer b by stretching at the high temperature Ta and the low temperature Tb, (i) In stretching at the low temperature Tb The absolute value of retardation developed by a resin having a high glass transition temperature is smaller than the absolute value of retardation developed by a resin having a low glass transition temperature. (Ii) The glass transition temperature is low by stretching at a high temperature Ta. It is preferable to adjust the thicknesses of the resin layer a and the resin layer b so that the absolute value of the retardation expressed by the resin is smaller than the absolute value of the retardation expressed by the resin having a high glass transition temperature.

  As described above, the difference between the refractive index nx in the X-axis direction and the refractive index ny in the Y-axis direction that develops in each of the resin layer a and the resin layer b by stretching in one direction (that is, uniaxial stretching); By adjusting the total thickness of the resin layer b and the total thickness of the resin layer b, the requirement P (that is, when the phase of the XZ polarized light with respect to the YZ polarized light is uniaxially stretched in the X axis direction at the temperature Ta is delayed, It is possible to obtain a pre-stretched film that satisfies the requirement that it proceeds when uniaxially stretched in the X-axis direction.

The expression of retardation when a pre-stretch film satisfying the requirement P is stretched will be specifically described with reference to the drawings. Figure 1 is a high glass transition temperature Tg _A of the resin A constituting the resin layer a, when low glass transition temperature Tg _B of the resin B constituting the resin layer b, and a resin layer (a) and the resin layer b stretched 3 is a graph showing an example of the temperature dependence of retardation Δ when the film is stretched and the temperature dependence of retardation Δ when the film before stretching is stretched. In the film before stretching as shown in FIG. 1, in the stretching at the temperature Tb, the negative retardation expressed in the resin layer b is larger than the positive retardation expressed in the resin layer a. A negative retardation Δ is developed. On the other hand, in the stretching at the temperature Ta, since the negative retardation expressed in the resin layer b is smaller than the positive retardation expressed in the resin layer a, the retardation plate as a whole expresses a positive retardation Δ. become. Therefore, by combining such stretching at different temperatures Ta and Tb, the retardation produced by stretching at each temperature is synthesized to have a desired retardation and thus exhibit a desired optical function. A board can be realized stably.

  Moreover, in the pre-stretch film, it is preferable that variations in the thicknesses of the resin layer a and the resin layer b are 1 μm or less over the entire surface. Thereby, when a phase difference plate is provided in a display device, variation in color tone of the display device can be reduced. In addition, the color change after long-term use can be made uniform.

As described above, in order to set the variation in the film thickness of the resin layer a and the resin layer b to 1 μm or less over the entire surface, for example, the following (1) to (6) can be performed.
(1) A polymer filter having an opening of 20 μm or less is provided in the extruder.
(2) The gear pump is rotated at 5 rpm or more.
(3) An enclosure means is arranged around the die.
(4) The air gap is 200 mm or less.
(5) Edge pinning is performed when the film is cast on a cooling roll.
(6) A twin-screw extruder or a single-flight extruder with a double flight type screw is used as the extruder.

The thickness of each resin layer is measured using a commercially available contact-type thickness meter, then the total thickness of the film is measured, then the thickness measurement part is cut and the cross section is observed with an optical microscope, and the thickness ratio of each layer is determined. It can be calculated from the ratio. Further, this operation may be performed at regular intervals in the MD direction (film flow direction) and TD direction (film width direction) of the film, and the average value and variation of the thickness may be obtained.
The variation in thickness is calculated from the following equation using the arithmetic average value T _ave of the measured values measured above as a reference, with the maximum value of the measured thickness T being T _max and the minimum value being T _min .
Thickness variation (μm) = the larger of “T _ave −T _min ” and “T _max −T _ave ”.

  The film before stretching may have any other layer in addition to the resin layer a and the resin layer b as long as the effects of the present invention are not significantly impaired. Examples of the optional layer include an adhesive layer that bonds the resin layers, a mat layer that improves the slipperiness of the film, a hard coat layer such as an impact-resistant polymethacrylate resin layer, an antireflection layer, and an antifouling layer. It is done. An arbitrary layer may be provided later on the pre-stretched film obtained by coextrusion. However, when the resin A and the resin B are coextruded, the formation material of the arbitrary layer is the same as that of the resin A and the resin B You may make it extrude.

  The pre-stretch film preferably has a total light transmittance of 85% or more. This is to make the obtained retardation plate suitable as an optical member. The upper limit is ideally 100%. 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 film before stretching 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 manufactured retardation plate can be improved. The lower limit is ideally zero. 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 pre-stretch film, 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. The lower limit is ideally zero. 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 outer surface of the pre-stretch film is preferably flat without substantially having irregularly formed linear recesses or linear projections (so-called die lines) extending in the MD direction. 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 recess having a depth of less than 30 nm or a width of more than 700 nm, and a linear protrusion 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. Moreover, irregularly occurring means that it is formed at an unintended position with an unintended size, shape, or the like.

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. Irradiate the film before stretching, project the transmitted light on the screen, and display the light bright or dark stripes 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.
An average line is drawn on this cross-sectional profile. The length from the average line to the bottom of the linear concave portion is the linear concave portion depth, and the length from the average line to the top of the linear convex portion is the linear convex portion height. 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 pre-stretch film may have a dimension in the TD direction of, for example, 500 mm to 2000 mm. Further, the film before stretching is not limited in the dimension in the MD direction, but is preferably a long film.

[3. First stretching step]
In the first stretching step, the film before stretching is stretched in one direction at a temperature T1. That is, the film before stretching is uniaxially stretched at a temperature T1. When stretched at the temperature T1, retardation occurs in each of the resin layer a and the resin layer b depending on the composition of the film before stretching, the stretching temperature T1, the stretching ratio, and the like, and the resin layer a and the resin layer b as a whole are also lettered. Cause a foundation. At this time, for example, when the film before stretching satisfies the requirement P, the phase of the XZ polarized light with respect to the YZ polarized light is delayed.

The temperature T1 is set to an appropriate temperature so that a desired retardation is obtained. For example, in relation to the higher temperature TgH of the glass transition temperature Tg _A of the resin _A and the glass transition temperature Tg _B of the resin B, T1> TgH can be satisfied. On the other hand, the upper limit of the temperature T1 is not particularly limited, but is preferably TgH + 40 ° C. or less, and more preferably TgH + 20 ° C. or less. By setting the temperature T1 within the above temperature range, a retardation plate having a desired retardation can be produced.

  In the first stretching step, the film can be stretched at a high stretching ratio of usually 3 times or more, preferably 3.3 times or more, more preferably 3.5 times or more. Thus, by extending | stretching by a high draw ratio, it is possible to make the thickness of the phase difference plate obtained thin. From the viewpoint of stably producing the retardation film, the upper limit is usually 6 times or less, preferably 5 times or less, more preferably 4 times or less.

  Uniaxial stretching can be performed by a conventionally known method. For example, in the case of stretching a long film before stretching, for example, a method of uniaxially stretching in the longitudinal direction (usually coincides with the MD direction) using a difference in peripheral speed between rolls; using a tenter And uniaxial stretching in the transverse direction (usually coincides with the TD direction). In the production method of the present invention, since the second stretching step is preferably performed by stretching in the longitudinal direction, the first stretching step is preferably performed by stretching in the transverse direction.

  In stretching, in order to reduce stretching unevenness and thickness unevenness, a temperature difference may be created in the width direction of the film in the stretching zone. In order to create a temperature difference in the width direction of the film in the stretching zone, for example, a known technique such as adjusting the opening degree of the hot air nozzle in the width direction or controlling the heating by arranging IR heaters in the width direction is used. sell.

[4. Second stretching step]
After the first stretching step, a second stretching step is performed. In the second stretching step, the film stretched in one direction in the first stretching step is stretched in another direction substantially orthogonal to the stretching direction in the first stretching step.

  In the second stretching step, the film is stretched at a temperature T2 lower than the temperature T1. That is, the film is uniaxially stretched at a relatively low temperature T2. When stretched at the temperature T2, retardation occurs in each of the resin layer a and the resin layer b depending on the film configuration, the stretching temperature T2, the stretch ratio, and the like, and the entire resin layer a and the resin layer b are also retardationd. Arise. At this time, for example, if the film satisfies the requirement P, the phase of the XZ polarized light with respect to the YZ polarized light advances by stretching in the second stretching step.

  In the second stretching step, stretching is performed by free uniaxial stretching. Free uniaxial stretching of a film is uniaxial stretching that does not restrict deformation of the film other than in the stretching direction. For example, in the uniaxial stretching in the longitudinal direction, the film tends to shrink in the width direction, but in the free uniaxial stretching in the longitudinal direction, stretching is performed without restricting the shrinkage in the width direction.

  Free uniaxial stretching can be performed by, for example, a method of uniaxially stretching in the longitudinal direction using a difference in peripheral speed between rolls, for example, when stretching a long pre-stretched film. More specifically, free uniaxial stretching can be performed by providing a pair of nip rolls at two locations upstream and downstream of the production line, and rotating the peripheral speed of the nip roll downstream of the upstream nip roll faster. .

  The uniaxial stretching in the second stretching step is preferably performed at a smaller stretching ratio than the uniaxial stretching in the first stretching step. Specifically, the draw ratio in the second drawing step is preferably 1.1 to 2 times, and more preferably 1.1 to 1.5 times.

Temperature T2 of the second stretching step is in relation to the temperature TgH the higher ones of the glass transition temperature Tg _B of the glass transition temperature Tg _A and the resin B of the resin A, satisfy the relationship T2 <TgH-10. When free uniaxial stretching is performed at such a low temperature, the phase difference can be expressed higher than when free uniaxial stretching is performed at a high temperature, but the orientation relaxation after stretching becomes large. Therefore, in the present invention, a specific relaxation step is performed after the second stretching step. Thereby, a residual stress can be relieved in the aspect by which orientation relaxation was suppressed, As a result, favorable expression of a phase difference and suppression of orientation relaxation can be made compatible.
The lower limit of the temperature T2 is not particularly limited, but from the viewpoint of stably expressing the phase difference, the relationship with the lower temperature TgL of the glass transition temperature Tg _A of the resin _A and the glass transition temperature Tg _B of the resin B. In this case, the temperature is preferably TgL-20 ° C or higher, and more preferably TgL-10 ° C or higher.

  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, the polarizing plate compensation function can be stably exhibited in the retardation plate. 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.

  As described above, by performing the first stretching step and the second stretching step on the film before stretching, the stretching temperature and the stretching direction are applied to the resin layer a and the resin layer b in each of the first stretching step and the second stretching step. And retardation according to a draw ratio etc. arises. For this reason, in the phase difference plate obtained through the first stretching step and the second stretching step, the retardation generated in the resin layer a and the resin layer b in each of the first stretching step and the second stretching step is synthesized. As a result, a desired retardation is produced. Therefore, a phase difference plate having a desired retardation can be obtained by the second stretching step.

  In addition, by co-stretching the pre-stretched film including the resin layer a and the resin layer b, the production process is compared with the case where the resin layer a and the resin layer b that are stretched separately are bonded to produce a retardation plate. This shortens the manufacturing cost. In addition, the resin layer b containing the resin B having a negative intrinsic birefringence value is difficult to be stretched by itself and may cause uneven stretching or breakage. Co-stretching can be performed, and uneven thickness of the resin layer b can be reduced.

[5. Process for reducing tension]
After the second stretching step, a step of reducing the tension of the film is performed.
When the second stretching process is performed by two sets of nip rolls provided at two locations upstream and downstream of the production line, the film unloaded from the downstream pair of nip rolls is simply transported without applying tension. Thus, a reduction in the tension can be achieved. However, the method for reducing the tension is not limited to this. For example, another nip roll is provided further downstream than the pair of nip rolls downstream of the second stretching step, and the film unloaded from the nip roll is conveyed without applying tension. Also, a reduction in tension can be achieved.

  The extent to which the tension of the film is reduced by this process is not particularly limited as long as the tension is less than the tension applied to the film for stretching in the second stretching process, but it should be 100 N / m or less. Is preferred.

[6. Mitigation process]
In the production method of the present invention, a relaxation step of heat-treating the film is performed in a state where the tension is reduced by the step of reducing the tension of the film. Here, the temperature T4 of the heat treatment satisfies the relationship of TgL-20 ≦ T4 ≦ TgL in the relationship between the glass transition temperature Tg _A of the resin _A and the lower temperature TgL of the glass transition temperature Tg _B of the resin B. . It is more preferable that TgL and T4 satisfy the relationship of TgL-15 ≦ T4 ≦ TgL-5.

  According to the finding of the present inventor, as described above, by performing a relaxation process in which heat treatment is performed within a predetermined temperature range in a state in which the tension of the film is reduced, the film remains without greatly generating orientation relaxation. Stress can be relaxed sufficiently. Therefore, by combining the second stretching step for generating a high residual stress in the film and the present relaxation step, it is possible to achieve both good retardation and suppression of orientation relaxation.

  The heat treatment of the film can be a process such as a process of bringing the film into contact with a heated roll or a process of passing the film through an oven. In particular, the treatment of bringing the film into contact with a heated roll is preferable because the cost of the manufacturing process can be reduced by downsizing the equipment.

  The heat treatment time can be arbitrarily determined as long as the residual stress of the film can be relaxed. For example, when the film is brought into contact with a heated roll, it is preferable that the film temperature reaches T4. It is 1 to 20 seconds, more preferably 5 to 15 seconds. When the film is passed through an oven, it can be preferably 30 to 90 seconds, more preferably 45 to 75 seconds after the film temperature reaches T4.

  When heat treatment is performed to bring a long film into contact with a heated roll, the film expands in the width direction due to heating, while the film in contact with the roll cannot easily extend in the width direction,皺 may occur. In order to prevent the occurrence of such wrinkles, in a preferred embodiment, the step of reducing the tension of the film includes passing the film through a nip roll at a temperature T3, and the heat treatment comprises bringing the film into contact with a heated roll. In addition, the temperature T3 and the temperature T4 may satisfy the relationship of T4 ≦ T3 + 10. Here, the nip roll at the temperature T3 can also serve as a nip roll downstream of the second stretching step. In that case, since the second stretching process is completed when the nip roll is reached, the temperature T3 can be set in the downstream nip roll independently of the stretching temperature T2 in the second stretching process.

[7. Other processes]
In the method for producing a retardation plate of the present invention, as long as a desired retardation plate is obtained, the above-described pre-stretching film forming step, first stretching step, second stretching step, step of reducing film tension, and relaxation Any process other than the process can be performed.
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.

  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 plate may be performed.

[8. Embodiment]
Next, specific embodiments of the production method of the present invention will be described with reference to the drawings.
FIG. 2 is a side view schematically showing a specific example of the production line for carrying out the production method of the present invention and its operation. The production line shown in FIG. 2 has a first extending part 100, a second extending part 200, and a relaxing part 300 in order from the upstream.

  The first stretching unit 100 is provided with a tenter stretching machine 101 including a gripping tool 102 that grips a film.

  The second stretching unit 200 is provided with a pair of nip rolls 201, a preheating roll 211, an oven 220, and a pair of nip rolls 202 in order from the upstream. The oven 220 includes a housing 221 and a blower 222.

  The relaxing part 300 is provided with a heating roll 311.

  In operation, the unstretched film 10 prepared by an apparatus for forming a pre-stretch film (not shown) is supplied to the first stretching section 100 along the arrow A1, and is fed to the gripping tool 102 of the tenter stretching machine 101. The left and right end portions (that is, portions at both ends in the width direction of the long film) are gripped. The tenter stretching machine 101 is configured such that the width direction expands as the film 10 advances along the arrow A1, and as a result, the film 10 is stretched in the width direction.

  The film 10 stretched in the width direction is then supplied to the second stretching section 200, nipped by the nip roll 201, preliminarily heated in contact with the roll 211, and in the oven 220 from the blower 222 in the direction of arrow A4. It is heated by the hot air that is jetted out and is nipped by the nip roll 202. The nip rolls 201 and 202 are driven to rotate in the directions of arrows A2 and A3, respectively, and the nip roll 202 is driven at a faster peripheral speed. Thus, the film 10 is stretched in the longitudinal direction (that is, the longitudinal direction of the long film) in a heated state.

  The stretched film 10 is conveyed further downstream from the nip roll 202. Here, the tension of the film can be reduced by cutting the tension with the nip roll 202 and transporting the film downstream without applying the tension.

  The film 10 transported in a state where the tension is reduced is further supplied to the relaxation unit 300, where it is heat-treated by the heating roll 311. The heat-treated film can be used as a retardation film as a product as it is or after being subjected to any operation (applying an optional additional layer, cutting, winding, etc.) as it is.

[9. Embodiment: Modification]
The manufacturing method of the present invention is not limited to the above-described embodiment, and can also be performed by various modifications to the above-described embodiment.
For example, in the example shown in FIG. 2, a heating roll is used as the heating means in the relaxation process, but the heating means is not limited to this, and heating by an oven can also be performed.
In the example shown in FIG. 2, the pre-stretching film forming process, the first stretching process, the second stretching process, the tension reducing process, and the relaxation process are performed on a continuous line, but these are not always continuous. There is no need to do this. For example, the film carried out from the nip roll 202 can be wound up once and supplied to another line for the relaxation process to perform the relaxation process.

[10. Phase difference plate]
A phase difference plate is obtained by the manufacturing method described above. The retardation plate obtained by the method including the first stretching step and the second stretching step described above is (1) a resin layer a after stretching (hereinafter referred to as “resin layer A”) depending on the stretching conditions of each stretching step. And the slow axis of the stretched resin layer b (hereinafter referred to as “resin layer B”) are substantially parallel to each other; (2) the slow axis of the resin layer A is the resin layer Two embodiments are possible: an embodiment substantially perpendicular to the slow axis of B. Among these, usually, (1) the slow axis of the resin layer A and the slow axis of the resin layer B are substantially parallel to each other.

  Moreover, it is preferable that the phase difference plate manufactured by the manufacturing method mentioned above is a three-dimensional phase difference plate. That is, the NZ coefficient of the produced retardation plate is preferably 0 or more, more preferably 0.3 or more, particularly preferably 0.5 or more, preferably 1 or less, more preferably 0.9 or less, particularly Preferably it is 0.8 or less. Such a three-dimensional retardation plate can exhibit an excellent optical compensation function when mounted on a display device.

  The retardation plate may have a retardation that does not change greatly even when it is exposed to a high temperature for a long period of time. Specifically, the Re change rate (== resistivity index) for the in-plane retardation Re2 before heat treatment at 80 ° C. for 500 hours in the atmosphere and the in-plane retardation Re3 after the treatment (= (Re3 / Re2) × 100 (%)) is preferably 95% or more, more preferably 98% or more.

  The thickness of the phase difference plate is generally 10 μm or more, preferably 30 μm or more, and usually 200 μm or less, preferably 150 μm or less, as the total thickness of the resin layer A and the resin layer B. Further, it is preferable that the thickness variation of the resin layer A and the resin layer B is 1 μm or less over the entire surface. Thereby, variation in color tone in the display device can be reduced. In addition, the color change after long-term use can be made uniform. In order to realize this, the thickness variation of the resin layer a and the resin layer b in the pre-stretch film may be 1 μm or less over the entire surface.

  Regarding the retardation plate, the total light transmittance, haze, ΔYI, JIS pencil hardness, and the outer surface is substantially flat and substantially free from linear concave portions or linear convex portions. Same as film.

The retardation plate may have an arbitrary layer other than the resin layer A and the resin layer B. Examples of the optional layer include the same layers as described in the section of the film before stretching.
Moreover, the phase difference plate is good also considering the dimension of the width direction as 1000 mm-2000 mm.

[11. Liquid crystal display device]
The retardation plate produced by the production method of the present invention usually has an excellent polarizing plate compensation function. Therefore, this retardation plate is used alone or in combination with other members, such as 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) display device, etc. It may be applied to the display device.

  The liquid crystal display device generally includes a pair of polarizing plates (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 polarizing plates. In the case where a retardation plate is provided in the liquid crystal display device, a retardation plate may be provided between the pair of polarizing plates. In this case, the phase difference plate may be provided on the light incident side of the liquid crystal cell, may be provided on the light emission side of the liquid crystal cell, or provided on both the light incident side and the light emission side of the liquid crystal cell. Also good. Usually, the pair of polarizing plates, the retardation plate and the liquid crystal cell are integrally provided as a liquid crystal panel, and the liquid crystal panel is irradiated with light from a light source, and an image is displayed on the display surface existing on the light emitting side of the liquid crystal panel. It has become so. At this time, since the retardation plate exhibits an excellent polarizing plate compensation function, it is possible to reduce light leakage when the display surface of the liquid crystal display device is viewed obliquely. In addition, since the retardation plate usually has an excellent optical function in addition to the polarizing plate compensation function, the visibility of the liquid crystal display device can be further improved.

  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, it is possible to further widen the viewing angle by applying a retardation plate.

The retardation plate may be bonded to a liquid crystal cell or a polarizing plate. A known adhesive can be used for bonding.
One retardation plate may be used alone, or two or more retardation plates may be used.
Furthermore, when providing a phase difference plate in a liquid crystal display device, the phase difference plate manufactured by the manufacturing method of the present invention may be used in combination with another phase difference plate. For example, when the retardation plate manufactured by the manufacturing method of the present invention is provided in a liquid crystal display device having a vertical alignment type liquid crystal cell, in addition to the retardation plate manufactured by the manufacturing method of the present invention between a pair of polarizing plates. Thus, another retardation plate for improving the viewing angle characteristics may be provided.

[12. Other matters]
The retardation plate manufactured by the manufacturing method of the present invention can be used for applications other than those described above. For example, by setting the in-plane retardation Re of the retardation plate to 120 nm to 160 nm, the retardation plate can be a quarter wavelength plate, and this quarter wavelength plate can be combined with a linear polarizer to form a circularly polarizing plate. be able to. 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 °.

  Moreover, a phase difference plate can also be used as a protective film of a polarizing plate. The polarizing plate usually includes a polarizer and protective films bonded to both sides thereof. If the retardation plate is bonded to the polarizer, the retardation plate can be used as a protective film. In this case, since the protective film is omitted, the liquid crystal display device can be thinned.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the examples described below, and does not depart from the scope of the claims of the present invention and the equivalents thereof. It can be implemented with any change.
In the following description, “%” and “part” representing amounts are based on weight unless otherwise specified. In addition, the operations described below were performed under normal temperature and normal pressure conditions unless otherwise specified.

  In the following examples and comparative examples, various properties were evaluated as follows.

〔Glass-transition temperature〕
The glass transition temperature of the resin was measured by raising the temperature at 20 ° C./min using a differential scanning calorimeter (EXSTAR 6220 manufactured by Seiko Instruments Inc.).

[Measurement of retardation]
The in-plane retardation Re and NZ coefficient of the film were measured at a measurement wavelength of 550 nm using an automatic birefringence meter (“KOBRA-21ADH” manufactured by Oji Scientific Instruments).

[Film wrinkles after relaxation process]
The presence or absence of wrinkles on the film after the relaxation process was determined visually, and the presence or absence of wrinkles generated by the relaxation process was examined.

[Production Example 1]
[P1-1. Preparation of film before stretching)
A film forming apparatus for coextrusion 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 charged into one uniaxial extruder equipped with a double flight type screw and melted. This polycarbonate resin corresponds to the resin A having a positive intrinsic birefringence value.

  In addition, a pellet of styrene-maleic anhydride copolymer resin (“Dylark D332” manufactured by Nova Chemicals, maleic anhydride unit content 17% by weight, glass transition temperature 130 ° C.) is formed on the other uniaxial shaft provided with a double flight type screw. 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 resin A at 260 ° C. was supplied to one manifold of a multi-manifold die (surface roughness Ra = 0.1 μm) through a leaf disk-shaped polymer filter having an opening of 10 μm. Further, the melted resin B at 260 ° C. was supplied to the other manifold through a leaf disk-shaped polymer filter having an opening of 10 μm.

  Resin A and Resin B are simultaneously extruded from the multi-manifold die at 260 ° C., and a film-like molten resin having a three-layer structure including a resin A layer, a resin B layer, and a resin A layer in this order is obtained. Prepared continuously. The film-like molten resin was 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 resin layer a1 (thickness 7.1 μm). ), A resin layer b (thickness 169.9 μm), and a resin layer a2 (thickness 19.5 μm) were prepared in this order to prepare a film before stretching having a three-layer structure.

[P1-2. (Phase development of film before stretching)
The pre-stretch film prepared in the above step [P1-1] was stretched freely uniaxially at a stretch temperature of 150 ° C. and a stretch ratio of 1.25 times. The stretched film is incident perpendicular to the film surface 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 in-plane retardation Re of the linearly polarized light having the vibration plane of the electric vector in the XZ plane and perpendicularly incident on the film plane and having the vibration plane of the electric vector in the YZ plane was measured to be 74.4 nm. Yes, it was found that the phase was delayed.
Moreover, about the film before extending | stretching prepared by another said process [P1-1], the same free uniaxial stretching was performed except having made extending | stretching temperature 130 degreeC. For a stretched film, a plane for linearly polarized light that is incident perpendicularly to the film surface and whose electric vector vibration plane is in the XZ plane, and for a linearly polarized light that is incident perpendicularly to the film plane and whose electric vector vibration plane is in the YZ plane When the internal retardation Re was measured, it was found to be -160.4 nm, indicating that the phase was advanced.

[Example 1]
The unstretched film prepared in Production Example 1 was supplied to a tenter transverse uniaxial stretching machine and stretched in the transverse direction at a stretching temperature of 152 ° C. and a stretching ratio of 3.5 times (first stretching step). Subsequently, the stretched film is supplied to a longitudinal uniaxial stretching machine that uniaxially stretches using a difference in peripheral speed between the upstream pair of nip rolls and the downstream pair of nip rolls, and the stretching ratio is 1.25 times. And then stretched in the longitudinal direction (second stretching step). In the oven between the two sets of nip rolls, the stretching temperature was set to 130 ° C. The temperature of the downstream nip roll was set to 115 ° C., and in the region downstream of the downstream nip roll, the film was conveyed at a low tension, thereby reducing the film tension to 80 N / m.
Next, the film in a state where the tension was reduced was heat-treated for 7 seconds using a ceramic roll having a temperature of 120 ° C. (relaxation step). Thereby, a retardation film having a three-layer structure including the resin layer A1, the resin layer B, and the resin layer A2 in this order was manufactured.
In the obtained retardation plate, 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, and the NZ coefficient was 0.67. .
The in-plane retardation Re1 before the relaxation step after the second stretching step and the in-plane retardation Re2 after the relaxation step are measured, and the Re retention rate (= (Re2 / Re1) × 100 (%)) in the relaxation step is obtained. It was.
Further, the obtained retardation plate was left in the atmosphere at 80 ° C. for 500 hours, and then the in-plane retardation Re3 was further measured, and the Re change rate (= (Re3 / Re2) × 100 (%)).
Furthermore, about the obtained retardation plate, the presence or absence of a wrinkle of the film after a relaxation process was evaluated. The results are shown in Table 1.

[Example 2]
A retardation plate was produced and evaluated in the same manner as in Example 1 except that the longitudinal stretching temperature was changed to 128 ° C. and the downstream nip roll set temperature was changed to 114 ° C. The results are shown in Table 1.

[Example 3]
A retardation plate was produced and evaluated in the same manner as in Example 1 except that the longitudinal stretching temperature was changed to 124 ° C. and the setting temperature of the downstream nip roll was changed to 116 ° C. The results are shown in Table 1.

[Comparative Example 1]
The unstretched film prepared in Production Example 1 was supplied to a tenter transverse uniaxial stretching machine and stretched in the transverse direction at a stretching temperature of 152 ° C. and a stretching ratio of 3.5 times (first stretching step). Subsequently, the stretched film is supplied to a longitudinal uniaxial stretching machine that uniaxially stretches using a difference in peripheral speed between the upstream pair of nip rolls and the downstream pair of nip rolls, at a stretching temperature of 130 ° C. The film was stretched in the machine direction at a magnification of 1.25 (second stretching process). The temperature of the downstream nip roll is set to 25 ° C., and in the region downstream of the downstream nip roll, the film is conveyed with a low tension, and this is passed through the ceramic roll at a temperature of 25 ° C. without passing through the relaxation step, A phase difference plate having a three-layer structure including the resin layer A1, the resin layer B, and the resin layer A2 in this order was obtained.
In the obtained retardation plate, 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.
After measuring the in-plane retardation Re1 of the obtained retardation plate, the retardation plate was allowed to stand in the atmosphere at 80 ° C. for 500 hours, and then the in-plane retardation Re3 was further measured. The Re change rate (= (Re3 / Re1) × 100 (%)) was obtained. The results are shown in Table 1.

[Comparative Example 2]
A retardation plate was produced and evaluated in the same manner as in Comparative Example 1 except that the longitudinal stretching temperature was changed to 128 ° C. The results are shown in Table 1.

[Comparative Example 3]
A retardation plate was produced and evaluated in the same manner as in Comparative Example 1 except that the longitudinal stretching temperature was changed to 124 ° C. The results are shown in Table 2.

[Comparative Example 4]
A retardation plate was produced and evaluated in the same manner as in Comparative Example 1 except that the longitudinal stretching temperature was changed to 140 ° C. The results are shown in Table 2.

[Comparative Example 5]
A retardation film was produced and evaluated in the same manner as in Comparative Example 1 except that the longitudinal stretching temperature was changed to 150 ° C. The results are shown in Table 2.

[Comparative Example 6]
In the same manner as in Example 1, except that the longitudinal stretching temperature was changed to 128 ° C., the setting temperature of the downstream nip roll was changed to 105 ° C., and the temperature of the ceramic roll in the relaxation process was changed to 100 ° C. Plates were manufactured and evaluated. The results are shown in Table 2.

[Comparative Example 7]
In the same manner as in Example 1, except that the longitudinal stretching temperature was changed to 128 ° C., the setting temperature of the downstream nip roll was changed to 114 ° C., and the temperature of the ceramic roll in the relaxation process was changed to 140 ° C. Plates were manufactured and evaluated. The results are shown in Table 2.

From the results of Tables 1 and 2, the following can be understood.
The retardation plates obtained in Examples 1 to 3 in which the production method satisfying the requirements of the present invention was carried out had a high value of in-plane retardation and high durability. On the other hand, in Comparative Examples 1 to 3 in which the relaxation process was not performed, orientation relaxation due to a high temperature load was larger than in the corresponding Examples (Examples 1 to 3, respectively). Further, in Comparative Examples 4 to 5 in which stretching was performed at a high stretching temperature and the relaxation process was not performed, the reduction in in-plane retardation after applying a load due to high temperature could be reduced, but the retardation itself immediately after the stretching process was It was small. In Comparative Example 6 in which the temperature of the heat treatment in the relaxation process is too low, orientation relaxation due to a high temperature load was large. In Comparative Example 7 in which the temperature of the heat treatment in the relaxation step was too high, the orientation was also relaxed by the heat treatment. Moreover, since the amount of temperature increase in the heat treatment was large, wrinkles occurred in the film.

DESCRIPTION OF SYMBOLS 10: Film 100: 1st extending | stretching part 101: Tenter stretching machine 102: Gripping tool 200: 2nd extending | stretching part 201: Nip roll 202: Nip roll 211: Roll for preheating 220: Oven 221: Housing 222: Blower 300: Relaxation Part 311: Heating roll

Claims (5)

A film comprising a resin layer a made of a resin A having a positive intrinsic birefringence value, and a resin layer b made of a resin B having a negative intrinsic birefringence value provided on at least one surface of the resin layer a. Forming, and
A first stretching step of stretching the film before stretching in one direction at a temperature T1 (° C.);
After the first stretching step, a second stretching step of stretching the film in the other direction substantially orthogonal to the stretching direction at a temperature T2 (° C.) lower than the temperature T1,
A method of manufacturing a retardation plate having
The second stretching step is a step of free uniaxial stretching,
Of the glass transition temperature of the resin A and the glass transition temperature of the resin B, the higher temperature TgH (° C.) and the temperature T2 satisfy the relationship of T2 <TgH-10,
After the second stretching step,
Reducing the tension of the film;
A relaxation step of heat-treating the film at a temperature T4 (° C.) in a state where tension is reduced, and the lower temperature TgL (° C.) of the glass transition temperature of the resin A and the glass transition temperature of the resin B And a relaxation step in which the temperature T4 satisfies the relationship of TgL-20 ≦ T4 ≦ TgL;
A method for producing a retardation plate further comprising: Reducing the tension of the film includes passing the film through a nip roll having a temperature of T3 (° C.);
The heat treatment in the relaxation step includes bringing the film into contact with a roll heated to a temperature T4 (° C.),
The manufacturing method according to claim 1, wherein the temperature T3 and the temperature T4 satisfy a relationship of T4 ≦ T3 + 10.   The manufacturing method of Claim 1 or 2 whose difference of the said temperature TgH and the said temperature TgL is 10 degreeC or more.   The manufacturing method according to any one of claims 1 to 3, wherein the resin A is a resin containing a polycarbonate polymer, and the resin B is a resin containing a polystyrene polymer.   The manufacturing method of any one of Claims 1-4 with which the said temperature T1 and the said temperature TgH satisfy | fill the relationship of T1> TgH.

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