JP6295617B2 - Production method of polyester film for protecting polarizing plate - Google Patents

Description

  The present invention is provided outside the polarizing plate on the viewing side of a liquid crystal display using a white light emitting diode as a backlight light source, so that the liquid crystal display can be viewed in the direction of the polarization axis even when viewed through a polarizing optical member. The present invention relates to a polarizing plate-protecting polyester film that can reduce the visibility of the resulting display image and the occurrence of light interference colors.

  In recent years, polarizing plates used for liquid crystal displays widely used as display devices for televisions, personal computers, digital cameras, mobile phones and the like are generally protective films / polarizing films / protective films or protective films / polarizing films / Consists of a retardation film. In a liquid crystal display, the display light emitted from the polarizing plate on the viewing side is linearly polarized light. For example, when a display image is viewed through an optical member having a polarizing action such as sunglasses, the polarization axis of the display light and the absorption of the optical member When the angle of the axis is not appropriate, the display image becomes dark or invisible.

  In order to solve the above problem, a method of modulating linearly polarized light into circularly polarized light by providing a λ / 4 retardation film further outside the polarizing plate on the viewing side is known (Patent Documents 1 and 2). 3) Use of a retardation film is not preferable from the viewpoint of cost. Moreover, although the method of providing a retardation plate with a large retardation outside the polarizing plate on the viewing side is known (Patent Documents 4 and 5), it is not preferable because the polarizing plate becomes thick.

JP 2000-137116 A JP 2002-22944 A JP 2008-83307 A JP-A-6-258634 JP 2004-170875 A

  The present invention has been made in view of the above circumstances, and a solution to the problem is that it is provided on the outside of a polarizing plate on the viewing side of a liquid crystal display using a white light emitting diode as a backlight light source, thereby providing an optical device having a polarizing action. An object of the present invention is to provide a polarizing plate-protecting polyester film that does not darken a display image or generate a light interference color depending on an angle even when a display image is viewed through a member.

  As a result of intensive studies in view of the above problems, the present inventors have found that the above problems can be easily solved by a specific polarizing plate protecting polyester film, and have completed the present invention.

That is, the gist of the present invention is that a sheet containing polyethylene terephthalate and polyethylene-2,6-naphthalate is stretched 1.1 to 2.8 times in the longitudinal direction to obtain a longitudinally uniaxially stretched film, and then the transverse direction. to a method of producing a film subjected to the stretching to 5.2 to 6.0 times, a content of the polyethylene-2,6-naphthalate is 10 to 80 wt%, the birefringence index of the film plane [Delta] n (below The present invention resides in a method for producing a polarizing plate-protecting polyester film , wherein (defined by formula (1)) is 0.060 or more.

Birefringence index Δn = (nx−ny) (1)
(In the above formula (1), nx represents the refractive index in the slow axis direction x where the refractive index is maximum in the in-plane direction of the film, and ny is orthogonal to the slow axis direction x in the film in-plane direction. Represents the refractive index in the direction y)

  According to the present invention, an inexpensive polyester film having excellent optical properties can be provided as a polarizing plate protective film, and the industrial value of the present invention is high.

  The polyester film referred to in the present invention is a film obtained by stretching a sheet melt-extruded from an extrusion die according to a so-called extrusion method.

  Examples of the polyethylene terephthalate constituting the film include terephthalic acid as the dicarboxylic acid, and ethylene glycol as the diol.

  In order to facilitate handling, the polyester film in the present invention may contain particles under conditions that do not impair transparency. Examples of particles used in the present invention include inorganic particles such as calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, and crosslinked polymers. Examples thereof include organic particles such as particles and calcium oxalate. Examples of the method of adding particles include a method of adding particles in a polyester as a raw material, a method of adding directly to an extruder, and the like. Two methods may be used in combination.

  The particle size of the particles used is usually 0.05 to 5.0 μm, preferably 0.1 to 4.0 μm. When the average particle size is larger than 5.0 μm, the haze of the film increases and the transparency of the film may be lowered. If the average particle size is smaller than 0.1 μm, the surface roughness becomes too small, and the film may be difficult to handle. The particle content is usually 0.001 to 30.0 wt%, preferably 0.01 to 10.0 wt%, based on the polyester. If the particle content is large, the haze increases and the transparency of the film may be lowered. If the particle content is small, the film may be difficult to handle.

  The method of adding particles to the polyester is not particularly limited, and a conventionally known method can be adopted. For example, it can be added at any stage for producing the polyester, but the polycondensation reaction may proceed preferably after the esterification stage or after the transesterification reaction. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a method of blending dried particles and a polyester raw material using a kneading extruder. It is performed by the method of blending.

  In the polarizing plate using the polyester film of the present invention, the in-plane birefringence Δn is 0.060 or more, preferably 0.100 or more, more preferably 0.120 or more, particularly preferably 0.150 or more. It is. When Δn of the stretched film is smaller than 0.060, the interference color of light becomes strong and the original color of the image cannot be obtained in the liquid crystal display.

  The polyester film of the present invention contains 10 to 80% by weight of a resin composition having a refractive index higher than that of polyethylene terephthalate by 0.02 or more. Examples of the resin composition having a refractive index higher by 0.02 or more than polyethylene terephthalate include polyethylene-2,6-naphthalate, polymethacrylic acid resin, polystyrene, fluororesin (PTFE), vinylidene fluoride, and silicon resin. From the standpoint of compatibility with polyethylene terephthalate, polyethylene-2,6-naphthalate is preferred.

  The content of the resin composition is preferably 10 to 50% by weight, more preferably 10 to 20% by weight. When the content of the resin composition is small, the interference color of light becomes strong when the polyester film of the present invention is used for a liquid crystal display, and the original color of the image cannot be obtained on the liquid crystal display. On the other hand, if the content is large, a good color can be obtained in a liquid crystal display, but this is not preferable because of increased cost.

  It is preferable that the thickness of the polyester film of this invention is 4-100 micrometers, More preferably, it is 23-75 micrometers, More preferably, it is 38-50 micrometers. When the thickness of the film is less than 4 μm, it is difficult to form the film and it is difficult to handle the film. When the thickness of the film is greater than 100 μm, the polarizing plate becomes thick for mobile use, which may not be preferable.

  In the present invention, other additives may be added as necessary. Examples of such additives include stabilizers, lubricants, cross-linking agents, anti-blocking agents, antioxidants, dyes, pigments, and ultraviolet absorbers.

  In the present invention, a single-layer polyester film using a single polyester melt extruder can be used, or two or more polyester films can be used to form a laminated film of at least three layers by a so-called coextrusion method. . As the structure of the layer, an A structure using only the A raw material, an A / B / A structure using the A raw material and the B raw material, an A / B / C structure using the C raw material, or other three or more layers. It can be set as the film of a structure.

  In the present invention, a polyester chip dried by a known method is supplied to a melt extrusion apparatus and heated to a temperature equal to or higher than the melting point of each polymer to be melted. Next, the molten polymer is extruded from a die and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition point to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed.

  In the present invention, the sheet thus obtained is preferably stretched in the biaxial direction to form a film. Specifically describing the stretching conditions, the unstretched sheet is preferably stretched 1.1 to 6 times at 80 to 150 ° C. in the longitudinal direction to form a longitudinal uniaxially stretched film, and then 80 to 160 ° C. in the lateral direction. The film is preferably stretched 1.1 to 6 times and heat-treated at 150 to 240 ° C. for 1 to 600 seconds. Further, at this time, a method of relaxing 0.1 to 20% in the longitudinal direction and / or the transverse direction in the maximum temperature zone of the heat treatment and / or the cooling zone at the heat treatment outlet is preferable.

  As the polarizing plate, it is preferable to provide a coating layer on at least one surface in order to make it adhere to the polarizing plate protective film or to improve the adhesion to the hard coat.

  Further, the coating layer may contain an antistatic agent, an antifoaming agent, a coating property improving agent, a thickener, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment, and the like.

  As a coating method of the coating agent, a reverse roll coater, a gravure coater, a rod coater, an air doctor coater, or a coating apparatus other than these can be used.

  In addition, in order to improve the applicability | paintability and adhesiveness to the film of a coating agent, you may give a chemical process and an electrical discharge process to a film before application | coating. Further, in order to further improve the surface characteristics, a discharge treatment may be performed after the coating layer is formed.

  The thickness of the coating layer is usually in the range of 0.02 to 0.5 μm, preferably 0.03 to 0.3 μm, as the final dry thickness. When the thickness of the coating layer is less than 0.02 μm, the effect of the present invention may not be sufficiently exhibited. When the thickness of the coating layer exceeds 0.5 μm, the films tend to stick to each other, and particularly when the coated film is re-stretched to increase the strength of the film, it adheres to the roll in the process. There is a tendency to become easy. The above problem of sticking appears particularly when the same coating layer is formed on both sides of the film.

  If necessary, the film may be coated by a method called offline coating which is coated after the production of the film. The coating material may be either water-based and / or solvent-based in the case of off-line coating.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. Various physical properties and characteristics are measured or defined as follows. In the examples, “%” means “% by weight”.

(1) Measurement of refractive index The refractive index of each resin was measured using a precision refractometer KPR-2000 manufactured by Shimadzu Corporation.

(2) Definition of in-plane birefringence index Δn The birefringence index Δn is defined by the following equation.
Birefringence index Δn = (nx−ny) (1)
(In the above formula (1), nx represents the refractive index in the slow axis direction x where the refractive index is maximum in the in-plane direction of the film, and ny is the slow axis direction x in the in-plane direction of the film. Represents the refractive index in the orthogonal direction y)

(3) Measurement of nx and ny Using an Abbe refractometer manufactured by Atago Co., Ltd., the refractive index nx in the slow axis direction x where the refractive index in the film in-plane direction is maximized, and orthogonal to the slow axis direction x The refractive index ny in the direction y was measured, and the birefringence Δn was calculated from the above equation (1). In addition, the measurement was performed at 23 degreeC using the sodium D line | wire.

(4) Inspection of visibility Using a polyvinyl alcohol (PVA) film (manufactured by Kuraray Co., Ltd., polymerization degree 2400), after stretching 3 times in the first bath (iodine, KI aqueous solution-30 ° C), the second bath ( The total draw ratio was stretched up to 6 times in boric acid and KI aqueous solution (55 ° C.) to obtain a polarizer. Then, using a PVA-based adhesive, a TAC film having a thickness of 40 μm is bonded to both surfaces, and the angle between the polyester film and the absorption axis of the polarizing plate is 45 ° via an adhesive on one side TAC film. In this manner, a polarizing plate was prepared. The polarizing plate was mounted on a liquid crystal panel using a phosphor-type white light emitting diode as a backlight light source so that the polyester film was on the outside of the viewing side, and the visibility was confirmed.
A: There is no light interference color and it is good. ○: There is a little light interference color but no problem. X: The light interference color is strong and cannot be used as a polarizing plate.

(5) Productivity and handleability ◎: Good film productivity and good handleability during processing ○: No problem in both film productivity and processability ×: Film productivity and processing Either or both of the handling times are poor and not suitable for production or processing

(6) Comprehensive judgment Visibility, productivity, and handleability are comprehensively evaluated. The most excellent film for polarizing plate is ◎, the best is ◯, the acceptable is △, and is insufficient. Was marked with x. △ More than pass.
◎: No light interference color, good processing suitability, no problem as polarizing plate ○: Either light interference color or processing suitability is slightly inferior, but can be used as a polarizing plate △: Light interference color, processing proper Although it is inferior, it is an acceptable level as a polarizing plate. X: Either the light interference color or the processing suitability is inferior and cannot be used as a polarizing plate.

The raw materials used in the following examples and comparative examples were prepared as follows.
(Method for producing polyester A)
Take 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, and 0.07 part of calcium acetate monohydrate in a reactor, heat up and evaporate methanol to conduct transesterification, and take about 4 and a half hours after starting the reaction. The temperature was raised to 230 ° C. to substantially complete the transesterification reaction.

  Next, 0.04 part of phosphoric acid and 0.035 part of antimony trioxide were added and polymerized in accordance with a conventional method. That is, the reaction temperature was gradually raised to finally 280 ° C., while the pressure was gradually reduced to finally 0.05 mmHg. After 4 hours, the reaction was completed, and a chip was obtained according to a conventional method to obtain polyester A. The obtained polyethylene terephthalate had a refractive index of 1.58.

(Method for producing polyester B)
When the polyester A was produced, 6000 ppm of amorphous silica having an average particle size of 3.2 μm was added to prepare polyester B.
(Polyester C manufacturing method)
Polyester C which is polyethylene-2,6-naphthalate was obtained by changing the dicarboxylic acid raw material in the production method of polyester A. The obtained polyethylene-2,6-naphthalate had a refractive index of 1.64.

Example 1:
Raw materials in which the polyesters A, B, and C were mixed at a ratio of 85%, 5%, and 10%, respectively, were melt extruded by a melt extruder to obtain an amorphous sheet. Subsequently, the sheet was coextruded on a cooled casting drum and solidified by cooling to obtain a non-oriented sheet. Next, the film was stretched 2.5 times in the longitudinal direction at 90 ° C., and further subjected to a preheating step in the tenter, stretched 5.3 times in the lateral direction at 120 ° C., and heat-treated at 180 ° C. for 10 seconds, The roll was wound up to obtain a polyester film having a thickness of 50 μm. The evaluation results are shown in Table 1 below.

Example 2:
In Example 1, the film was stretched 1.5 times in the longitudinal direction at 90 ° C., and further subjected to a preheating step in the tenter and then stretched 5.2 times in the lateral direction at 120 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 3:
In Example 1, a polyester film was obtained in the same manner as in Example 1 except that a raw material in which polyesters A, B, and C were mixed at a ratio of 75%, 5%, and 20%, respectively, was used. The evaluation results are shown in Table 1.

Example 4:
In Example 3, the film was stretched 1.8 times in the longitudinal direction at 90 ° C., and further subjected to a preheating step in the tenter and then stretched 5.3 times in the transverse direction at 120 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 5:
In Example 3, the film was stretched 1.5 times in the machine direction at 90 ° C., and further subjected to a preheating step in the tenter and then stretched 5.2 times in the transverse direction at 120 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 6:
In Example 3, a polyester film was obtained in the same manner as in Example 3 except that the thickness of the film was 75 μm. The evaluation results are shown in Table 1.

Example 7:
In Example 1, using raw materials in which polyesters A, B, and C were mixed in proportions of 45%, 5%, and 50%, respectively, the film was stretched 2.8 times in the longitudinal direction at 120 ° C., and further in a tenter. A polyester film was obtained in the same manner as in Example 1 except that a preheating step was performed and the film was stretched 5.3 times in the transverse direction at 120 ° C. and heat-treated at 150 ° C. for 10 seconds to obtain a film. The evaluation results are shown in Table 1.

Example 8:
In Example 7, the film was stretched 2.0 times in the longitudinal direction at 120 ° C., and further subjected to a preheating step in the tenter and then stretched 5.4 times in the lateral direction at 120 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 9:
In Example 7, the film was stretched 1.5 times in the longitudinal direction at 120 ° C., and further subjected to a preheating step in the tenter and then stretched 5.2 times in the transverse direction at 120 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 10:
In Example 1, using raw materials in which polyesters A, B, and C were mixed at a ratio of 15%, 5%, and 80%, respectively, the film was stretched 2.8 times in the longitudinal direction at 130 ° C., and further in the tenter. A polyester film was obtained in the same manner as in Example 1 except that a preheat process was performed and the film was stretched 5.2 times in the transverse direction at 130 ° C. and heat-treated at 150 ° C. for 10 seconds to obtain a film. The evaluation results are shown in Table 1.

Example 11:
In Example 10, the film was stretched 2.0 times in the longitudinal direction at 130 ° C., and further subjected to a preheating step in the tenter and then stretched 5.4 times in the lateral direction at 130 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 12:
In Example 10, the film was stretched 1.5 times in the machine direction at 130 ° C., then further subjected to a preheating step in the tenter and 5.3 times in the transverse direction at 130 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 13:
In Example 10, the film was stretched 1.5 times in the longitudinal direction at 130 ° C., and further subjected to a preheating step in the tenter and then 6.0 times in the transverse direction at 130 ° C. to obtain a film. A polyester film was obtained in the same manner. The evaluation results are shown in Table 1.

Example 14:
In Example 10, a polyester film was obtained in the same manner as in Example 10 except that the thickness of the film was 23 μm. The evaluation results are shown in Table 1.

Comparative Example 1:
In Example 1, using raw materials in which polyesters A and B were mixed at a ratio of 95% and 5%, respectively, the film was stretched 1.1 times in the longitudinal direction at 90 ° C., and further subjected to a preheating step in a tenter. A polyester film was obtained in the same manner as in Example 1 except that a film was obtained by stretching 5.0 times in the transverse direction at ° C. The evaluation results are shown in Table 1.

Comparative Example 2:
In Example 1, using raw materials in which polyesters A, B, and C were mixed in proportions of 5%, 5%, and 90%, respectively, the film was stretched 2.5 times in the longitudinal direction at 130 ° C. A polyester film was obtained in the same manner as in Example 1 except that a film was obtained by performing a preheating step and then stretching the film 5.2 times in the transverse direction at 130 ° C. and performing heat treatment at 150 ° C. for 10 seconds. The evaluation results are shown in Table 1.

Comparative Example 3:
In Example 1, after using a raw material in which polyesters A, B, and C were mixed at a ratio of 75%, 5%, and 20%, respectively, the film was stretched 3.0 times in the longitudinal direction at 90 ° C., and further in a tenter. A polyester film was obtained in the same manner as in Example 1 except that a film was obtained by stretching the film by 5.2 times in the transverse direction at 120 ° C. through a preheating step. The evaluation results are shown in Table 1.

Comparative Example 4:
In Example 1, using raw materials in which polyesters A, B, and C were mixed at a ratio of 15%, 5%, and 80%, respectively, the film was stretched 3.0 times in the longitudinal direction at 130 ° C. A polyester film was obtained in the same manner as in Example 1 except that a preheat process was performed and the film was stretched 4.5 times in the transverse direction at 130 ° C. and heat-treated at 150 ° C. for 10 seconds to obtain a film. The evaluation results are shown in Table 1.

  The obtained results are summarized in Table 1 below.

  The film of the present invention can be suitably used, for example, as a polarizing plate protective film.

Claims (1)

A sheet containing polyethylene terephthalate and polyethylene-2,6-naphthalate is stretched 1.1 to 2.8 times in the longitudinal direction to form a longitudinally uniaxially stretched film, and then 5.2 to 6.0 in the lateral direction. This is a method for producing a film that is stretched twice. The polyethylene-2,6-naphthalate content is 10 to 80% by weight, and the in-plane birefringence index Δn (defined by the following formula (1)) ) Is 0.060 or more, A method for producing a polarizing plate-protecting polyester film.
Birefringence index Δn = (nx−ny) (1)
(In the above formula (1), nx represents the refractive index in the slow axis direction x where the refractive index is maximum in the in-plane direction of the film, and ny is orthogonal to the slow axis direction x in the film in-plane direction. Represents the refractive index in the direction y)