JP3410523B2 - Manufacturing method of polarizing film - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-performance polarizing film having excellent heat and moisture resistance. [0002] Liquid crystal display devices are widely used in word processors, personal computers, color liquid crystal televisions and the like. 2. Description of the Related Art In recent years, liquid crystal display devices have been increasingly used under high-temperature and high-humidity conditions for use in vehicles and outdoors. In order to improve the reliability of a liquid crystal display device in a high-temperature and high-humidity atmosphere, a polarizing film having high moist heat resistance is strongly demanded. Recently, black-and-white display or color display of a liquid crystal display element has become popular. In order to cope with such a display, there is a demand for a polarizing film which is excellent in wet heat resistance and high in transmittance and degree of polarization. Conventionally, a polarizing film is produced by adsorbing iodine or a dichroic dye on a stretched and oriented polarizing film base film. As a polarizing film, a film which is optically transparent and has no anisotropy on both surfaces is generally used as a protective film. As a protective film, a triacetate film (triacetyl cellulose film) is used. As the base film of the polarizer, a polyvinyl alcohol-based polymer is generally used. The general method of making an iodine-based polarizing film is to adsorb iodine to a stretched polyvinyl alcohol-based film, treat it with a fixed aqueous solution containing boric acid, and then bond a protective film made of a triacetate film to both surfaces of the polarizer. General.
As a general method for producing a dye-based polarizing film, a method of adsorbing a dichroic dye on a uniaxially stretched film is used. As the polymer film used in this case, a polyvinyl alcohol-based film is suitably used.
In particular, a polarizing film in which iodine is adsorbed on a polyvinyl alcohol-based film has the best transmittance and the degree of polarization, and is most commonly used as a polarizing film. [0003] The polyvinyl alcohol-based polarizing film has a problem of poor wet heat resistance.
For example, when the moist heat resistance in an atmosphere of 80 ° C. and 90% relative humidity (RH) is tested with a polarizing film on which iodine is adsorbed (both polarizers are protected with a triacetate film on both sides), the polarizing film is cross-Nicol When placed in the state (degree of polarization: 99.9%) and observed under the above conditions, the degree of polarization decreased to 75% in 100 hours, and further decreased to 0.8% in 250 hours. As shown in the above results, the conventional polarizing film had very poor heat and moisture resistance. Various methods have been proposed to improve the moist heat resistance of the polarizing film. There is a method of installing a protective layer having low moisture permeability such as polyester or polymethyl methacrylate resin on the triacetate film, but there are practical problems such as a decrease in light transmittance and occurrence of interference fringes. there were. An object of the present invention is to solve such a problem and to provide a polarizing film having excellent polarization characteristics and wet heat resistance. Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above problems, and as a result, have found that a film made of a polyvinyl alcohol-based resin and an alkali metal iodide does not contain hydrochloric acid and sulfuric acid. pH 6 to 9 containing an oxidizing agent
The present inventors have found a method for producing an iodine-based polarizing film having a degree of polarization of 95% or more, which is characterized by performing an oxidation treatment in an oxidation bath at 15 ° C. or lower, and reached the invention. Hereinafter, the method for producing the polarizing film of the present invention will be described more specifically. The average degree of polymerization of the polyvinyl alcohol-based resin is preferably from 1500 to 4000, and the degree of saponification is preferably at least 85 mol%, more preferably at least 95 mol%. Next, the polyvinyl alcohol resin and the iodide of the alkali metal are both dissolved in the same solvent. As the solvent used in this case, water, an organic solvent or a mixed solvent of water and an organic solvent is preferably used. As the organic solvent, dimethyl sulfoxide, glycerin, ethylene glycol, propylene glycol and the like can be used. The concentration of polyvinyl alcohol is 1 to solvent.
The content is preferably up to 50% by weight, preferably 3 to 30% by weight. The concentration of the alkali metal to be dissolved is 0.5 to 20% by weight, preferably 1 to 15% by weight, based on polyvinyl alcohol.
It is good to make it by weight%. Examples of the polyvinyl alcohol-based resin in the present invention include polyvinyl alcohol, polyvinyl formal, polyvinyl acetal, polyvinyl butyral, and saponified ethylene-vinyl acetate copolymer. Particularly preferably used is polyvinyl alcohol. The alkali metal iodides used in the present invention include potassium iodide, sodium iodide,
Lithium iodide or the like is used. In the present invention, a polyvinyl alcohol-based resin in which an alkali metal iodide is dissolved is obtained by dissolving a polyvinyl alcohol-based resin and an alkali metal iodide in the same solvent and then performing casting. The obtained film is oxidized by immersing it in an oxidation bath containing an oxidizing agent such as hydrogen peroxide to generate iodine such as I ₃ ⁻ and I ₅ ⁻ in a polyvinyl alcohol film. In the present invention, selection of oxidation conditions is most important. That is, when a strong acid such as sulfuric acid or hydrochloric acid is added to the oxidation bath and the acidity is increased, the oxidation rate becomes uncontrollably high.As a result, the light transmittance of the obtained polyvinyl alcohol film dyed with iodine is reduced. There is a tendency that the film remarkably decreases and a film usable as a polarizing film cannot be obtained. In the present invention, the hydrogen ion concentration index (pH) in the oxidation bath is used.
Is controlled to 6 to 9, the degree of polarization of the polarizing film is improved. If the pH is lower than 6, it is difficult to control the oxidation rate, and a polarizing film with good performance cannot be obtained in many cases. pH
When the ratio is higher than 9, the oxidation rate becomes extremely slow, and the polarizing performance of the polarizing film tends to decrease. [0007] The temperature of the oxidation bath is 15 ° C or less . By lowering the temperature of the oxidation bath, the single transmittance and the degree of polarization of the polarizing film can be increased, which is good. The reason why the performance of the polarizing film is improved by lowering the temperature is not exactly understood, but it is considered that the reaction can be performed under milder conditions so that iodine molecules suitable for the polarizing film can be generated. The most suitable oxidizing agent is a mild oxidizing agent such as hydrogen peroxide. The thus-obtained raw film of polyvinyl alcohol dyed with iodine is stretched 2 to 6 times, preferably 3 to 5 times in water or air. After the stretching, the treatment is preferably performed in a solution containing a crosslinking agent while maintaining the tension state. As a crosslinking agent used in this case, boric acid, borax, a titanium compound, a zirconium compound, or the like is used. The crosslinking temperature is preferably 20 to 110 ° C. with a crosslinking agent. A protective layer is formed on one side or both sides of the obtained polarizing film to obtain a polarizing film. A transparent film such as a triacetyl cellulose film can be used as the protective film. Regarding the oxidation treatment time, the temperature is 1
When the temperature is 5 to 25 ° C, the time is preferably 30 seconds or less. When the temperature is 0 to 10 ° C, the time is preferably 1 minute or more, more preferably 2 minutes or more. The present invention will be described in more detail with reference to the following examples. Example 1 Polyvinyl alcohol having a polymerization degree of 1700 and a saponification degree of 99.7 mol% was dissolved in water at a ratio of 5% by weight.
The dissolution was performed at 80 ° C. After the temperature was cooled to room temperature, lithium iodide was added to and dissolved in polyvinyl alcohol at a ratio of 10% by weight. This solution was cast on a polyethylene terephthalate film,
A film of polyvinyl alcohol containing lithium iodide and having a thickness of 70 μm was obtained. This film is heated at 120 ° C for 1
Heat treatment was performed for 0 minutes. At a temperature of the film 0 ° C., 10 wt% aqueous hydrogen peroxide solution (p H6.4)
Oxidation treatment was performed by immersion for 3 minutes. Thereafter, the film was washed with water at 0 ° C. Then 50 ℃, 5% by weight
Was immersed in an aqueous solution of boric acid for 30 seconds and stretched 4 times in water. The film was immersed in a 50 ° C. aqueous solution containing 10% by weight of boric acid for 4 minutes while keeping the film under tension. Thereafter, by drying at 40 ° C. for 5 minutes,
A polarizing film was obtained. A thickness of 8 on both sides of this polarizing film
A polarizing film was obtained by bonding a 0 μm triacetyl cellulose film using a polyvinyl alcohol-based adhesive. The transmittance and the degree of polarization of this polarizing film were measured. The single transmittance was 40% and the degree of polarization was 99.9% (the transmittance in the vertical direction was 0.05% or less when installed in a crossed Nicols state. LD-201) (4)
(Calculated from the spectral transmittance measured every 10 nm in the wavelength range of 00 nm to 700 nm). The degree of polarization is a value obtained from the following equation. Degree of polarization (%) = (H ₁ −H ₂ ) / (H ₁ + H ₂ ) × 100 H ₁ : Parallel transmittance (transmittance when the orientation directions of two polarizing films are set to be the same direction) H ₂ : vertical transmittance (transmittance when the orientation directions of the two polarizing films are perpendicular to each other) This polarizing film (with a triacetate film adhered to both sides) has a moisture and heat resistance of 80 ° C. and 90% RH.
It measured in the atmosphere of. The result is shown in FIG. Compared with the conventional polarizing film, the polarizing film of the present invention has significantly improved wet heat resistance. Comparative Example 1 10 parts of polyvinyl alcohol having a polymerization degree of 1700 and a saponification degree of 99.7 mol% were dissolved in 90 parts of water to obtain a solution having a polyvinyl alcohol concentration of 10% by weight. This solution was cast on a polyethylene terephthalate film and dried to obtain an original polyvinyl alcohol film having a thickness of 70 μm. This raw film was dipped in water and stretched four times. Further, while keeping the film under tension, the film was immersed in an aqueous solution consisting of 0.5% by weight of iodine and 5% by weight of potassium iodide to adsorb iodine. In addition, boric acid 10
Cross-linking treatment was carried out for 5 minutes with an aqueous solution of 50% by weight and 10% by weight of potassium iodide at 50 ° C. This crosslinking treatment was performed under tension. Thereafter, drying was performed at 40 ° C. for 5 minutes. A triacetylcellulose film (Fuji Photo Film Co., Ltd., Fujitac) was bonded to both surfaces of this polarizing film using a polyvinyl alcohol-based adhesive in the same manner as in Example 1. Thus, a polarizing film was obtained. This polarizing film had a single transmittance of 41% and a degree of polarization of 99.9%. 80 ° C., 90% RH
It measured in the atmosphere of. The result is shown in FIG. Example 2 Polyvinyl alcohol having a polymerization degree of 1700 and a saponification degree of 99.7 mol% was dissolved in water at a ratio of 5% by weight.
The dissolution was performed at 80 ° C. After the temperature was cooled to room temperature, potassium iodide was added to and dissolved in polyvinyl alcohol at a ratio of 10% by weight. This solution was cast on a polyethylene terephthalate film,
A polyvinyl alcohol film containing potassium iodide and having a thickness of 70 μm was obtained. This film is heated at 120 ° C for 1
Heat treatment was performed for 0 minutes. At a temperature of the film 0 ° C., 10 wt% aqueous hydrogen peroxide solution (p H6.0)
It was immersed for a certain period of time (shown in FIG. 2) to perform oxidation treatment. Thereafter, the film was washed with water at 0 ° C. Then 5
After immersion in a 5% by weight aqueous solution of boric acid at 0 ° C. for 30 seconds, the film was stretched four times in water. The membrane was immersed in a 50 ° C. aqueous solution containing 10% by weight of boric acid for 4 minutes while keeping the membrane under tension. Thereafter, by drying at 40 ° C. for 5 minutes, a polarizing film was obtained. An 80 μm-thick triacetyl cellulose film was adhered to both surfaces of the polarizing film using a polyvinyl alcohol-based adhesive to obtain a polarizing film. FIG. 2 shows the relationship between the degree of polarization of the polarizing film and the oxidation time. By setting the oxidation time to 30 seconds or longer, a polarizing film having a high degree of polarization could be obtained. Comparative Example 2 An experiment was performed in the same manner as in Example 2 except that the oxidizing temperature was 25 ° C. The result is shown in FIG. When the oxidation temperature was 25 ° C., the degree of polarization greatly decreased as the oxidation time increased. The reason why the degree of polarization was greatly reduced was that the polarizing film was greatly colored. When the oxidation temperature was raised, it was necessary to control the oxidation time to a short time in order to obtain a good polarizing film. According to the present invention, a polarizing film having excellent wet heat resistance is provided.

BRIEF DESCRIPTION OF THE DRAWINGS [Figure 1] Example 1 (□) and showing the wet heat resistance of about the Comparative Example 1 (○). FIG. 2 shows the relationship between the oxidation time and the degree of polarization for Example 2 (○) and Comparative Example 2 (△).

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 5/30

Claims (1)

(57) [Claim 1] A film comprising a polyvinyl alcohol resin and an alkali metal iodide is treated with hydrochloric acid and sulfuric acid.
During oxidation bath pH 6 to 9 containing an oxidizing agent containing no acid, 15 ° C.
A method for producing an iodine-based polarizing film having a degree of polarization of 95% or more, characterized by oxidizing below .