JP2002214432A - Polarizing plate protection film and polarizing plate using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarizing plate protection film capable of enhancing the light transmittance of a polarizing plate used for a liquid crystal display or the like. SOLUTION: The polarizing plate protection film comprises a film having <1.0% difference between the measured light transmittance of the film and that of a laminate 3 obtained by stacking four such films 1 in the same direction and sticking these together with an adhesive 2 used in the manufacture of a polarizing plate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizing plate protective film for a polarizing plate used in a liquid crystal display device and the like, and a polarizing plate using the polarizing plate protective film.

[0002]

2. Description of the Related Art With the spread of liquid crystal displays (LCDs), the importance of polarizing plates has increased rapidly. Generally, this polarizing plate is configured by attaching a polarizing plate protective film to both sides or one side of a polarizing film having a polarizing ability via an adhesive layer. For this polarizing film, polyvinyl alcohol (hereinafter referred to as “PVA”) is mainly used, and PVA is used.
It is produced by stretching the film uniaxially and dyeing it with iodine or a dichroic dye, or by dyeing it in the opposite direction and stretching it, followed by crosslinking with a boron compound.

[0003] As a polarizing plate protective film to be bonded to a polarizing film, generally, transparency, low birefringence, moderate moisture permeability,
Cellulose acetate films are most widely used because of their appropriate rigidity. Among these properties of the cellulose acetate film, transparency (high light transmittance) is extremely important. When a polarizing plate using a polarizing plate protective film having low light transmittance is used in a liquid crystal display device, the brightness of display is reduced. In order to lower the brightness or maintain the brightness, the power consumption increases.

Accordingly, a film used as a protective film for a polarizing plate has a light transmittance as high as possible.

[0005] A cellulose acetate film used for a polarizing plate protective film having excellent light transmittance is as follows.
Conventionally, various proposals have been made. For example, Japanese Patent Application Laid-Open No. 7-1105
In Japanese Patent Application Publication No. 5 (1999) -1992, a cellulose triacetate film having excellent light transmittance and improved slipperiness is proposed by dispersing fine particles having a methyl group on the surface in cellulose triacetate. Also, Japanese Patent Application Laid-Open No. 7-2031
Japanese Patent Publication No. 7 proposes a cellulose triacetate film in which the amount of plasticizer in the vicinity of the surface of the film is specified to prevent deterioration under high temperature and high humidity (including deterioration in light transmittance due to coloring). Have been.

[0006]

However, conventionally,
The light transmittance was determined based on the light transmittance of one film, and the light transmittance when the polarizing plate was attached to the polarizing film was not considered. It did not match the light transmittance of the film.

That is, the light transmittance of one film is 9
Most of the lost light is lost due to surface reflections, although on the order of 2-93%. The reflection at the surface occurs due to the difference in the refractive index between the air and the film surface. By reducing this difference, the surface reflection can be reduced. For example, the light transmittance of a cellulose triacetate film having a thickness of 80 μm is about 92 to 93% in the air, but is more than 99% when measured in liquid paraffin having a refractive index almost the same as that of cellulose triacetate. In this case, a light transmittance of 99.9% or more can be obtained.

Incidentally, the polarizing plate protective film is usually
One sheet is attached to each side of the polarizing film. Therefore, at least one surface of the polarizing plate protective film is bonded to the polarizing film with an adhesive. Since this bonding surface is not in contact with air and is in contact with an adhesive having a higher refractive index than air, the reflection on this surface is extremely small as compared with the surface in contact with air, and is substantially zero. Close to.

[0009] As described above, even if a film having the same light transmittance as a single unit is used, the light transmittance of a polarizing plate can be compared with that of a film which maintains light transmittance by suppressing surface reflection to a low level. It will be very low. That is,
For example, if the light transmittance of two films is 92%, the reflection of one film is 7%, and the reflection of the other film is 6%, the substantial light transmittance of one film is assumed. Is 99%, and the effective light transmittance of the other film is 98%.

As described above, conventionally, the judgment was made based on the apparent light transmittance, so that the light transmittance was different from the light transmittance when actually used for the polarizing plate. Occasionally, sufficient light transmittance may not be obtained.

An object of the present invention is to solve the above problems and to provide a polarizing plate protective film having good light transmittance in a state where the polarizing plate is used.

[0012]

Means for Solving the Problems The present inventor has studied diligently to achieve the above object, and it is preferable to use a film having a high light transmittance measured without surface reflection as a protective film for a polarizing plate. The present invention has been completed by paying attention to the following.

[0013] The polarizing plate protective film according to the present invention is laminated with four films so that the light transmittance measured on one film and the film are oriented in the same direction, and bonded using an adhesive used when manufacturing a polarizing plate. The film is characterized by comprising a film having a difference from the light transmittance measured for the combined laminate within 1.0%.

In the polarizing plate protective film of the present invention,
The light transmittance of the case of one film and the case of the laminate of four films are measured. In the case of one film,
The reflection on the surface of the film and the attenuation of light on the film itself are related to the light transmittance. In the case of a laminate, the reflection on the surface of the film disposed on the surface and the attenuation of light on each of the four films themselves Is related to the light transmittance. Therefore, the difference between these light transmittances is substantially equal to the attenuation of light in the three films, and the smaller the difference in light transmittance, the higher the light transmittance of the film. When the difference in light transmittance is within 1.0%, it is extremely suitable when used for a polarizing plate.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the polarizing plate protective film of the present invention, the light transmittance measured on one film and the four films are stacked so that they are oriented in the same direction, and the adhesive used for producing a polarizing plate. The difference from the light transmittance measured for the laminate bonded with the agent is used as a criterion for determination.

Here, the film is stacked in the same direction when a film is formed into a film-forming support (casting support).
Means that the surfaces in contact with each other overlap in the same direction. Therefore, the bonding is such that the surface in contact with the film-forming support during film formation and the surface not in contact with the film-forming support during film formation are bonded face to face via an adhesive. become.

In the present invention, the four films are bonded together with an adhesive to form a laminate, and the light transmittance of the laminate is measured. When a polarizing plate is used as a part of a liquid crystal display device, two polarizing plates are often used. Therefore, four polarizing plate protective films are used. This is to increase the accuracy.

The difference in the light transmittance is required to be within 1.0%, preferably within 0.6%.
More preferably, it is within 3%. If the difference in light transmittance exceeds 1.0%, it is difficult to obtain practically sufficient light transmittance when used for a polarizing plate.

The adhesive used for laminating the films is not particularly limited as long as it is used for producing a polarizing plate. For example, various adhesives such as PVA, acrylic, styrene and vinyl acetate can be used. Agents can be used.

The thickness of the adhesive is preferably thin so that the effect of the adhesive is as small as possible if a sufficient adhesive strength can be obtained. For example, the thickness after drying is preferably within 20 μm, more preferably within 10 μm, and most preferably within 1 μm.

The light transmittance of the film is not particularly limited, and various means that can be used in principle can be used. For example, the following means can be used in combination. Use highly transparent raw materials (for example, high quality linter cotton for cellulose triacetate). Bleach the ingredients. Minimize the amount of matting agent added for imparting slipperiness and adhesion resistance. Thin the film.

The polarizing plate protective film according to the present invention preferably has a thickness of 30 μm or more and 150 μm or less,
30 μm or more and 100 μm or less, more preferably 40 μm
It is most preferably not less than 80 μm. It is clear that the thinner the film, the higher the light transmittance will be. However, if the film is too thin, the strength of the film will be insufficient, and the function as a polarizing plate protective film will be reduced. Therefore, it is preferable that the thickness of the polarizing plate protective film is at least 30 μm at its thinnest.

As the material for the polarizing plate protective film, cellulose ester, polycarbonate and the like are included, but cellulose ester is preferred because of good transparency and low birefringence. As the cellulose ester, lower fatty acid esters of cellulose (eg, cellulose acetate,
Cellulose acetate butyrate and cellulose acetate propionate) are representative. A lower fatty acid means a fatty acid having 6 or less carbon atoms. Among cellulose esters, cellulose acetate is preferable, and the cellulose acetate includes cellulose triacetate (TAC) and cellulose diacetate (DAC).

The degree of acetylation of the cellulose acetate is 5
It is preferably 5.0% or more and 62.5% or less.
8.0% or more is more preferable, and 60% or more is most preferable. The degree of acetylation of cellulose acetate is 6 according to the definition.
Naturally, it is not more than 2.5%, but the lower the degree of acetylation, the greater the dependence of the size on humidity, and the lower the function as a polarizing plate protective film. Therefore, in this sense, the degree of acetylation is preferably at least 55.0% or more.

The polarizing plate protective film preferably contains a phosphate or phthalate as a plasticizer, and the content of the plasticizer is preferably 4 to 20% by weight, more preferably 10 to 15% by weight. If the content of the plasticizer is less than 4% by weight, the effect of addition is small and the cost is high. If it exceeds 20% by weight, a so-called crying phenomenon in which the plasticizer in the film is deposited on the surface occurs. The value of the product may be significantly reduced.

Examples of the phosphoric acid ester include triphenyl phosphate (TPP) and tricresyl phosphate (TCP), cresyl diphenyl phosphate, octyl diphenyl phosphate, diphenyl biphenyl phosphate, trioctyl phosphate, tributyl. Phosphate.

Examples of the phthalic acid ester include dimethyl phthalate (DMP) and diethyl phthalate (DE
P), dibutyl phthalate (DBP), dioctyl phthalate (DOP), diphenyl phthalate (DPP),
And diethylhexyl phthalate (DEHP).

The polarizing plate is formed by attaching the above polarizing plate protective film to one or both surfaces of the polarizing film with an adhesive. As a polarizing film used for a polarizing plate, a film obtained by adsorbing iodine to PVA is generally used, but a film using a dichroic organic dye instead of iodine can also be preferably used.

[0029]

Example [Preparation of Raw Material Dope] (Preparation of Fine Particle Dispersion a) Silica (“Aerosil R972”, manufactured by Nippon Aerosil Co., Ltd.) 6.00% by weight Cellulose Acetate (Acetylation degree: 60.8%) 79% by weight Triphenyl phosphate 0.14% by weight Biphenyl diphenyl phosphate 0.07% by weight Methylene chloride 84.64% by weight Methanol 7.36% by weight A solution having a total of 100.00% by weight was prepared, and was treated with an attritor. Volume average particle size
Dispersion was performed to 5 μm. Here, as the volume average particle size, a value measured by "particle size distribution measuring device LA920" manufactured by Digyard Seisakusho was used. Further, a dispersion was prepared at a ratio of 88.04% by weight of dispersion, cellulose acetate (acetation degree: 60.8%), 10.68% by weight, triphenylphosphate 0.85% by weight, and biphenyldiphenylphosphate 0.43% by weight. And thoroughly mixed to obtain a solid content of 19.
A fine particle dispersion a of 00% by weight was obtained.

(Preparation of Raw Material Dope A) A solution of cellulose triacetate (acetylation degree: 60.8%) 19.00% by weight, dichloromethane: 74.52% by weight, methanol: 6.48% by weight was prepared, and sufficiently mixed by stirring. The minute is 19.0
0% by weight of dope A was obtained.

(Preparation of Plasticizer Solution b) Triphenyl phosphate 12.67% by weight Biphenyl diphenyl phosphate 6.33% by weight Dichloromethane 74.52% by weight Methanol 6.48% by weight is prepared and mixed sufficiently. And the solid content is 19.0
A 0% by weight plasticizer solution was obtained.

(Preparation of Dope B for Film Formation)
After mixing the fine particle dispersion liquid a and the plasticizer solution b at an appropriate ratio, the mixture is sufficiently stirred, and filtered with a filter paper (manufactured by Toyo Roshi Kaisha, Ltd., “# 6
3)) to obtain a film-forming dope B.

(Preparation of UV absorber solution c) 2 (2'-hydroxy-3 ', 5'-di-tert-butylphenyl) -5-chlorobenzotriazole 5.83% by weight 2 (2'-hydroxy- 3 ', 5'-di-tert-amylphenyl) benzotriazole 11.66% by weight Cellulose acetate (degree of acetylation 60.8%) 1.35% by weight triphenylphosphate 0.11% by weight biphenyldiphenylphosphate 0 0.055% by weight Methylene chloride 74.52% by weight Methanol 6.48% by weight A solution having a total of 100.00% by weight was prepared and thoroughly mixed by stirring to obtain a solid content of 19.00.
% By weight of the ultraviolet absorbent solution c was obtained. This solution was filtered with "Astrophore 10 filter" manufactured by Fuji Photo Film Co., Ltd.

(Preparation of film-forming dope C) The above-mentioned film-forming dope B, the above-mentioned ultraviolet absorbent solution c, the value of the amount of the ultraviolet absorbent relative to the solid content in the dope expressed in terms of% by weight, and the finished film Is 83.2, the product of the values of the average thickness expressed in μm.
And mixed well with stirring.

[Formation of Polarizing Plate Protective Film] The dope C for film formation was cast on a casting support, dried until it had self-supporting properties, and peeled off as a film. The peeled film was further dried in an atmosphere at a maximum of 120 ° C.

[Measurement of Light Transmittance] The light transmittance of one sample of the produced polarizing plate protective film and, as shown in FIG. 1, four polarizing plate protective films 1 were measured by PVA (manufactured by Kuraray Co., Ltd.). , “PVA-117H”) An adhesive 2 composed of a 4% aqueous solution is applied so that the thickness after drying becomes about 1 μm, and is adhered so that the films are oriented in the same direction.
The light transmittance of the laminate 3 obtained by drying at a temperature of ° C. was evaluated by the following method. Measuring instrument: “AKA Phototube Colorimeter 5” manufactured by Kotaki Seisakusho
E "Measurement wavelength: 630 nm

Table 1 shows the measurement results of the addition ratio of the raw material dope A, the fine particle dispersion liquid a, and the plasticizer solution b and the light transmittance of the prepared sample.

[0038]

[Table 1]

[Preparation of Polarizing Plate] "PV" manufactured by Kuraray Co., Ltd.
A film ”(75 μm) is immersed in an aqueous solution of 0.3 g / L of iodine and 18.0 g / L of potassium iodide at 25 ° C., and further 80 g / L of boric acid and 30 g / potassium iodide
L, ferrous chloride 10 g / L, in an aqueous solution at 50 ° C.
It was stretched 0 times. Then, it dried at 60 degreeC for 5 minutes.
The light transmittance of the polarizing film alone was 44.0%.

The polarizing film thus produced is coated with PVA.
(Kuraray Co., Ltd., “PVA-117H”) The polarizing plate protective films of Examples 1 to 4 and Comparative Examples 1 to 3 which were subjected to saponification treatment using a 4% aqueous solution as an adhesive were supported by casting at the time of film formation. The surface in contact with the body was bonded to both surfaces as a bonding surface, and dried at 80 ° C. to obtain a polarizing plate.

[Measurement of Light Transmittance of Polarizing Plate] The light transmittance of one obtained polarizing plate, and the two polarizing plates are superimposed on each other using the PVA aqueous solution as an adhesive so that the polarization directions are the same. And the parallel transmittance was measured. Table 2 shows the results.

[0042]

[Table 2]

Each of the films of Examples and Comparative Examples has the same light transmittance of 92.5% (Table 1). However, the light transmittance of the polarizing plate using these films is as follows. Is 42% or more by itself and a good light transmittance of 35% or more in parallel transmittance when two sheets are bonded together. On the other hand, in each of the comparative examples, the light transmittance was small. Thereby, the effect of the present invention was confirmed.

[0044]

The present invention is configured as described above.
The light transmittance of the polarizing plate can be improved, and the luminance of the liquid crystal display can be improved and the power consumption can be reduced.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view showing a layer structure of a laminate obtained by laminating four polarizing plate protective films when measuring light transmittance of a polarizing plate protective film according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Polarizer protective film 2 ... Adhesive 3 ... Laminate

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H049 BA02 BB22 BB33 BC09 BC14 BC22 2H091 FA08X FA08Z FB02 GA17 KA10 LA16 LA30

Claims (6)

[Claims] 1. The light transmittance measured for one film and the laminate obtained by laminating four films so as to be in the same direction and using an adhesive to be used for producing a polarizing plate were measured. A polarizing plate protective film comprising a film having a difference from light transmittance of 1.0% or less. 2. The polarizing plate protective film according to claim 1, wherein the film is a cellulose acetate film. 3. The cellulose acetate, which is a raw material of the cellulose acetate film, has an acetylation degree of 55.0%.
The polarizing plate protective film according to claim 2, wherein the content is at least 62.5%. 4. The thickness of the film is 30 μm or more and 15
4. The structure according to claim 1, wherein the thickness is 0 μm or less.
3. The polarizing plate protective film according to item 1. 5. The polarizing plate protective film according to claim 1, wherein the film contains a phosphate or a phthalate as a plasticizer. 6. A polarizing plate, wherein the polarizing plate protective film according to claim 1, 2, 3, 4, or 5 is attached to at least one surface.