JP2007237436A - Biaxially stretched multilayered laminated film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially stretched multilayered laminated film excellent in humidity resistance and thermal dimensional stability and having optical isotropy. <P>SOLUTION: The is constituted by alternately laminating a first layer with a thickness of 1-50 nm comprising a crystalline thermoplastic resin having positive intrinsic double refractivity and a second layer with a thickness of 1-50 nm comprising a crystalline thermoplastic resin so as to form at least 101 layers and, when the direction vertical to the surfaces of the layers is set to a z-direction and the directions crossing to z-direction at a right angle and crossing to each other at a right angle are set to an x-direction and a y-direction, the difference between the x-direction and the y-direction is 0.02 or below and at least either one of the difference between the x-direction and the z-direction and the difference between the y-direction and the z-direction is 0.02 or below with respect to the refractivity of light with a wavelength of 633 nm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optically isotropic biaxially stretched multilayer laminated film used for a protective film for a polarizing film for liquid crystal display.

  As a polarizing film for liquid crystal display, a uniaxially oriented polyvinyl alcohol film containing iodine or a dichroic dye is used as a polarizing element, and a non-axially oriented cellulose triacetate film obtained by a casting method is used as a polarizing element. It is known that it is bonded to both sides of the.

  Recently, the use of liquid crystal display devices has been expanded, and it is going to be used for automobile instruments, meters for external equipment, etc., but the non-axially oriented cellulose triacetate film on the support substrate has a high moisture permeability, so it can be used under high temperature and high humidity. If the film is exposed for a long time, the film hydrolyzes to generate minute bubbles in the support substrate, and the image clarity as a polarizing film for liquid crystal display is lowered, or the polarizing element is deteriorated. There are practical problems.

  As a solution to this problem, as disclosed in Japanese Patent Application Laid-Open No. 51-143344, “a non-axial or non-axial or anti-axial retardation film is bonded to the surface of a polarizing plate made of polyvinyl alcohol. And a “broken uniaxially oriented polyester film” useful for the production of polarizing elements as disclosed in JP-A No. 58-132523 is known.

Japanese Patent Laid-Open No. 51-143344 JP 58-132523 A Japanese National Patent Publication No. 9-506837 Japanese National Patent Publication No. 11-511322

  However, since a uniaxially oriented film causes a refractive index shift due to a slight axial shift from the polarizing element due to refractive index anisotropy, an optically isotropic non-oriented film is generally used. On the other hand, a biaxially stretched film of a crystalline thermoplastic resin typified by a polyethylene terephthalate (hereinafter sometimes referred to as “PET”) film exhibits optical anisotropy, and thus serves as a support for a polarizing element or a protective film. Not suitable for use. However, if optical isotropy can be imparted, it can be used for these applications.

  An object of the present invention is to provide a biaxially stretched thermoplastic resin film having excellent moisture resistance, excellent thermal dimensional stability, and optical isotropy.

  That is, the present invention provides a first layer having a thickness of 1 to 50 nm made of a crystalline thermoplastic resin having positive intrinsic birefringence and a thickness of 1 to 50 nm made of a crystalline thermoplastic resin having negative intrinsic birefringence. When the direction perpendicular to the plane of the layer is the z direction and the directions perpendicular to the z direction and perpendicular to each other are the x direction and the y direction, the wavelength is Regarding the refractive index of 633 nm light, the difference between the x direction and the y direction is 0.02 or less, and at least one of the difference between the x direction and the z direction and the difference between the y direction and the z direction is 0.02 or less. It is a biaxially stretched multilayer laminated film characterized by being.

  According to the present invention, a biaxially stretched thermoplastic resin film having excellent moisture resistance, excellent thermal dimensional stability, and optical isotropy can be provided.

  The first layer and the second layer of the present invention each have a thickness of 1 to 50 nm, preferably 1 to 30 nm. When the thickness exceeds 50 nm, color development occurs due to light interference due to a difference in refractive index between layers, and transparency cannot be maintained. Moreover, the thickness of 1 nm or more is required from a viewpoint of maintaining crystallinity.

[Crystalline thermoplastic resin having positive intrinsic birefringence]
The crystalline thermoplastic resin having positive intrinsic birefringence in the present invention is a crystalline thermoplastic resin having a positive intrinsic birefringence value calculated from the dielectric constant distribution with the molecular chain direction of the polymer as an axis. is there. As the polymer satisfying the above, it is preferable to use crystalline polyester. For example, polyethylene terephthalate, polybutylene terephthalate, polyethylene 2,6-naphthalenedicarboxylate or a copolymer thereof can be used.

  From the viewpoint of uniformly maintaining the laminated state, the melt viscosity ratio (melt viscosity of thermoplastic resin 1 / melt viscosity of thermoplastic resin 2) at a shear rate of about 1000 (1 / second) of the thermoplastic resin to be laminated is: Preferably it is 0.6-1.4, More preferably, it is 0.8-1.2 or less. If the ratio of melt viscosity is less than 0.6 or exceeds 1.4, the layer structure at the time of lamination is disturbed, causing streaky defects and the like, which is not preferable.

[Crystalline thermoplastic resin having negative intrinsic birefringence]
The crystalline thermoplastic resin having negative intrinsic birefringence in the present invention is a crystalline thermoplastic resin having a negative intrinsic birefringence value calculated from a dielectric constant distribution with the molecular chain direction of the polymer as an axis. is there. Examples of the polymer satisfying the above include syndiotactic polystyrene, isotactic polystyrene, polyethylene-1,4-naphthalenedicarboxylate and copolymers thereof.

It is known that the birefringence developed after stretching is represented by the following formula.
Δn = f · Δn (0)
Δn: orientation birefringence (birefringence after stretching)
Δn (0): Intrinsic birefringence f: Degree of orientation

[Refractive index difference]
The biaxially stretched multilayer laminated film of the present invention has a refractive index of light having a wavelength of 633 nm when the direction perpendicular to the layer surface is the z direction, and the directions perpendicular to the z direction and perpendicular to each other are the x direction and the y direction. The difference between the x direction and the y direction is 0.02 or less, and at least one of the difference between the x direction and the z direction and the difference between the y direction and the z direction is 0.02 or less. is required. That is, the difference in refractive index between two directions perpendicular to the in-plane direction of the layer is 0.02 or less, and the difference in refractive index between at least one direction in the in-plane direction and the thickness direction is 0.02 or less. It is. If the difference between the x direction and the y direction exceeds 0.02, or the difference between the x direction and the z direction and the difference between the y direction and the z direction both exceed 0.02, birefringence is exhibited. It cannot be handled as an isotropic film.
The biaxially stretched multilayer laminated film of the present invention preferably has a total light transmittance of 85% or more. If it is less than 85%, it is not preferable because sufficiently high transparency cannot be obtained.

[Production method]
The biaxially stretched multilayer laminated film of the present invention includes a first layer crystalline thermoplastic resin supplied from a first extruder and a second layer crystalline thermoplastic supplied from a second extruder. A state in which at least 101 layers or more are alternately superposed in a molten state is formed into a multilayer unstretched film (a step of forming a sheet-like material). The multilayer unstretched film thus obtained is stretched in the biaxial direction (the direction along the film surface) in the film forming direction and the width direction perpendicular thereto. The stretching temperature is preferably in the range of the glass transition temperature (Tg) to Tg + 50 ° C. of the crystalline thermoplastic resin of the first layer. The area magnification at this time is preferably 5 to 50 times. The larger the draw ratio is, the smaller the variation in the plane direction in the individual layers of the first layer and the second layer is due to the thinning by stretching, that is, the refractive index of the multilayer stretched film becomes uniform in the plane direction. Therefore, it is preferable. The stretching method for stretching in two directions may be sequential biaxial stretching or simultaneous biaxial stretching.

Hereinafter, the present invention will be further described based on examples.
The physical property values and characteristic values were measured as follows.

(1) Melting point and glass transition point (Tg) of crystalline thermoplastic resin
10 mg of the crystalline thermoplastic resin was sampled, and the melting point was measured using DSC (trade name: DSC2920, manufactured by TA Instruments) at a heating rate of 20 ° C./min.

(2) Thickness of each layer A sample is cut into triangles, fixed to an embedded capsule, and then embedded in an epoxy resin. And the embedded sample was cut | disconnected along the film forming direction and thickness direction with the microtome (ULTRACUT-S, manufacturer: Reichert), and it was set as the thin film slice | slice of thickness 50nm. The obtained thin film slices were observed and photographed at an accelerating voltage of 100 kV using a transmission electron microscope (manufacturer: JEOL Ltd., trade name: JEM2010), and the thickness of each layer was measured from the photograph.

(3) Refractive index
The refractive index in each direction at a wavelength of 633 nm was measured in the film forming direction (nMD), the width direction (nTD), and the thickness direction using a prism coupler manufactured by Metricon.

(4) Total light transmittance According to JISK6714-1958, total light transmittance _Tt (%) was measured using the haze measuring machine (Nippon Denshoku Industries Co., Ltd. product, NDH-20).

(5) Transmittance variation Using Hitachi's fractured double beam spectrophotometer 228A, the absorption axes of the two polarizing plates are orthogonal to each other, and the orthogonal axes are inclined at 45 ° and inserted, and the light transmittance for wavelengths from 385 nm to 785 nm. When a strong rainbow pattern is measured, it is known that this light transmittance varies depending on the wavelength. Therefore, when the light transmittance exceeds 1% due to this variation, x indicates that the light transmittance exceeds 1%. evaluated.

(6) Hue From the transmission spectrum of the test film with respect to the standard light C, L ^* , a ^* and b ^* in the L ^* a ^* b ^* color system were determined according to JIS standard Z8729. The ab chroma (C ^* ab) obtained from the following formula was used to evaluate the deviation in saturation from the achromatic color.
C ^* ab = (a ^* ^ 2 + b ^* ^ 2) ^ (1/2)
◎: C ^* ab is less than 5 ○: C ^* ab is 5 or more and less than 10
×: C ^* ab is 10 or more

(7) Hue spots Ten A4-sized sample films were prepared, and the streaks and spots visible in the sample film were visually evaluated under the following evaluation criteria under illumination of 30 lux.
○: There are no visible streaks or spots in the sample.
X: Hue spots that clearly appear as spots or streaks can be confirmed.

[Example 1]
Copolymer polyethylene terephthalate (PET) having an intrinsic viscosity (orthochlorophenol, 35 ° C.) of 0.62 is used as the first layer thermoplastic resin, and the second layer thermoplastic resin is made by Idemitsu Petrochemical Co., Ltd. A tick polystyrene resin (XAREC 300ZC) was prepared. Then, the polyester for the first layer and the polyester for the second layer are dried at 170 ° C. and 100 ° C. for 3 hours, respectively, supplied to the extruder, heated to 290 ° C. to be in a molten state, and used for the first layer. Branching the polyester into 801 layers and the second layer polyester into 800 layers, and then using a multi-layer feed block device in which the first and second layers are stacked alternately, the layered state is maintained. As it was, it was led to a die and cast on a casting drum to produce a total of 1601 unstretched multilayer laminated films in which the first layer and the second layer were alternately laminated. At this time, the extrusion amount of the first layer and the second layer was adjusted to be 20:80, and the both end layers were laminated so as to be the first layer. This multilayer unstretched film was stretched 3.2 times in the film forming direction at a temperature of 110 ° C., further stretched 3.4 times in the width direction at a temperature of 120 ° C., and heat-set at 230 ° C. for 3 seconds. . Table 2 shows the physical properties of the obtained biaxially stretched multilayer laminated film.

[Example 2 and Comparative Examples 1 to 3]
The same procedure as in Example 1 was performed except that the first layer thermoplastic resin, the second layer thermoplastic resin, and the production conditions were changed as shown in Table 1. The physical properties of the obtained multilayer stretched film are summarized in Table 2.

The polymers shown in Table 1 are as follows.
PET:
Polyethylene terephthalate PEN having an intrinsic viscosity (orthochlorophenol, 35 ° C.) of 0.62:
Polyethylene 2,6-naphthalenedicarboxylate 300ZC having an intrinsic viscosity (orthochlorophenol, 100 ° C.) of 0.50:
Idemitsu Petrochemical Co., Ltd. Syndiotactic Polystyrene Resin XAREC 300ZC
130ZC:
Idemitsu Petrochemical Co., Ltd. Syndiotactic Polystyrene Resin XAREC 130ZC

  The biaxially stretched multilayer laminated film of the present invention is a film having optical isotropy, and can be suitably used, for example, as a support for a polarizing element of a liquid crystal display device or a protective film.

Claims (3)

  A first layer having a thickness of 1 to 50 nm made of a crystalline thermoplastic resin having a positive intrinsic birefringence, and a second layer having a thickness of 1 to 50 nm made of a crystalline thermoplastic resin having a negative intrinsic birefringence; The refractive index of light having a wavelength of 633 nm is obtained by alternately laminating 101 layers or more, and the direction perpendicular to the plane of the layers is the z direction, and the directions perpendicular to the z direction and perpendicular to each other are the x direction and the y direction. The difference between the x direction and the y direction is 0.02 or less, and at least one of the difference between the x direction and the z direction and the difference between the y direction and the z direction is 0.02 or less. A biaxially stretched multilayer laminated film.   The biaxially stretched multilayer laminated film according to claim 1, wherein the thermoplastic resin of the first layer is a crystalline polyester.   The biaxially stretched multilayer laminated film according to claim 1, wherein the total light transmittance is 85% or more.