JP3285437B2 - Method for producing protective film for polarizing plate and polarizing plate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to a method for producing a protective film for a polarizing plate comprising a cellulose triacetate film.

[0002]

2. Description of the Related Art Cellulose triacetate film is
Since it is transparent, has excellent physical and mechanical properties, and has a small dimensional change with respect to temperature and humidity, it is used in a wide range of fields such as photographic film bases, drafting tracing films, and electrical insulating materials. In recent years, it has been used as a protective film of a polarizing plate constituting a liquid crystal display panel used for a personal computer, a word processor, a television, and the like.

[0003] The protective film of the polarizing plate has optical transparency, optical non-orientation, adhesiveness to the polarizing film, flatness, ultraviolet absorption characteristics, and can withstand a severe environment such as mounted on a car. In addition, characteristics such as high-humidity heat durability and moisture absorption dimensional stability are required. In addition, the ultraviolet absorption property is an important element for preventing deterioration of the liquid crystal due to ultraviolet light. For example, as long as optical transparency can be ensured, the ultraviolet absorption property is improved by adding an ultraviolet absorber. Was.
Also, the film surface may be scratched during the processing and transporting process such as saponification, hard coating, and adhesive coating when assembling the protective film to the polarizing film, and such scratches may be fatal for a liquid crystal display panel. Because of the defects, the sliding properties of the film surface were also required. In order to improve the sliding properties of the film, a method of forming irregularities on the film surface with fine particles or the like is usually employed.

Further, with the development of full-color, high-gradation, and high-contrast liquid crystal display panels, measures have been taken to increase the single transmittance and the degree of polarization of the polarizing plate. Here, the degree of polarization is defined by the following equation. Polarization degree = (Hp−Hc) / (Hp + Hc) × 100 (%) In the formula, Hp is the average parallel transmittance, and Hc is the average orthogonal transmittance.

The polarizing plate comprises a polarizing film and a protective film. The polarizing film is generally obtained by impregnating polyvinyl alcohol with an aqueous solution of iodine or a dichroic dye, and further uniaxially stretching the film. Can be When the stretching ratio is increased to increase the degree of polarization, the single transmittance decreases. Therefore, it is necessary to select an appropriate stretching ratio in consideration of the balance between the degree of polarization and the single transmittance. As a method of increasing the degree of polarization, a technique has been proposed in which a polyvinyl alcohol-based film is dehydrated to form polyethylene, and then a dichroic dye is adsorbed and oriented (Japanese Patent Application Laid-Open No. 58-1983).
-44407).

[0006]

As described above, a technique for improving the degree of polarization of the polarizing film has been proposed, and the degree of polarization of the polarizing film has been improved. However, the protective film is
Since the polymer is stretched in the film forming direction during the production process to cause polymer orientation, there is a problem that birefringence is developed and the degree of polarization as a polarizing plate does not sufficiently increase even if the degree of polarization of the polarizing film is high. In particular, JP-A-3-148603, JP-A-2-189505
As described in the publication, when the protective film is produced by melt film formation, uniaxial or biaxial stretching is performed to improve the flatness, so large birefringence is developed, and the polarizing plate is substantially formed. Could not be used. In addition, even when a solution is formed by dissolving a polymer in a solvent to form a film, the polymer is oriented in the film forming direction due to the tension imparted during drying and transported, and birefringence is developed. I couldn't get a degree.

An object of the present invention is to solve the above problems and to provide a method for producing a protective film for a polarizing plate, which can produce a protective film having small birefringence.

[0008]

Means for Solving the Problems In order to solve the above problems, the present inventors have intensively studied the relationship between the production conditions of a cellulose triacetate film and the birefringence, and extremely reduced the birefringence. The present invention has been completed by finding the conditions under which the conditions can be satisfied, and finding the range of birefringence in which a suitable degree of polarization can be obtained.

That is, in the method for producing a protective film for a polarizing plate of the present invention, a dope in which cellulose triacetate is dissolved in a solvent is cast on a band or a drum, peeled off from the band or the drum, and then dried. In the method for producing a protective film for a polarizing plate to be prepared, the volatile matter in the cellulose triacetate film is X [%] and the film surface temperature is Y
When [° C.] is set, the film is dried in a region where X and Y are represented by the following formulas. When 0 <X <20 ... Y> -6X + 180 When 20≤X≤60 ... Y> -0.5X + 70

In the method for producing a protective film for a polarizing plate of the present invention,
By drying under the above-described predetermined conditions, a protective film for a polarizing plate having a predetermined quality can be obtained.

That is, the drying is performed under such a condition that the volatile matter X and the surface temperature Y fall within a region shown by oblique lines in FIG. In this area, during the entire drying process (that is, from about 75% of the solvent at the beginning of drying,
It is not necessary to enter the solvent at the end of drying (until the solvent reaches 0%), but it is preferable that the solvent enter at least 10 seconds or more, more preferably 15 seconds or more. By drying the region of the present invention, the orientation of the polymer in the film is relaxed,
In addition, since randomly oriented crystals are formed, the birefringence of the film is reduced as a result.

The volatile component X in the film is represented by the following formula. Volatile content X = residual volatile content weight / film weight x 100 (%)

[0013] The weight of the remaining volatile components was 115
The value is obtained by subtracting the weight of the film after the heat treatment from the weight of the film before the heat treatment when the heat treatment is performed at 1 ° C. for 1 hour.

The film surface temperature Y is measured by various measuring devices such as an infrared surface thermometer or a contact surface thermometer.

The means for drying the film is not particularly limited, and is generally performed using hot air, infrared rays, a heating roll, or the like.

As the cellulose triacetate used for the protective film for a polarizing plate, a known cellulose triacetate can be used.
The acetic acid degree of cellulose triacetate is preferably 50 to 70%, more preferably 55 to 65%. The weight average molecular weight is 70
It is preferably from 000 to 120,000, more preferably from 8000 to 100,000.

The cellulose triacetate may be partially esterified with a fatty acid such as propionic acid or butyric acid, as long as the cellulose triacetate satisfies the acetylation degree as well as acetic acid. As long as it contains cellulose esters such as cellulose propionate and cellulose butyrate.

The cellulose triacetate film constituting the protective film for a polarizing plate generally contains a plasticizer. Examples of the plasticizer include phosphates such as triphenyl phosphate, tricresyl phosphate, and cresyl diphenyl phosphate, and phthalic esters such as diethyl phthalate and dimethyl phthalate.

The solvent used in the present invention is not particularly limited as long as it can dissolve cellulose triacetate, and any solvent that cannot be dissolved alone but can be dissolved by mixing with another solvent. Can be used.

Examples of the solvent include aliphatic hydrocarbons such as pentane, hexane, heptane, octane, isooctane and cyclohexane; aromatic hydrocarbons such as benzene, toluene and xylene; methyl chloride, methylene chloride (methylene chloride); Examples include halogenated hydrocarbons such as carbon chloride and trichloroethane, alcohols such as methanol, ethanol, isopropyl alcohol and n-butyl alcohol, and esters such as methyl formate, ethyl formate, methyl acetate and ethyl acetate.

Generally, a mixed solvent of methylene chloride and methanol is used, but other solvents such as isopropyl alcohol and n-butyl alcohol may be used as long as cellulose triacetate is not precipitated.

The weight ratio of cellulose triacetate and solvent in the dope is preferably in the range of 10:90 to 30:70.

The production apparatus for carrying out the method for producing a protective film for a polarizing plate of the present invention employs, for example, a drum casting method shown in FIG. 2 or a band casting method shown in FIG.

The production apparatus of the drum casting system shown in FIG.
A casting die 2 is provided near the casting drum 1, and a tenter drying unit 4, a roll drying unit 5, and a winding roll 6 are provided via a peeling roll 3. Then, after the dope is cast from the casting die 2 to the casting drum 1, the film is peeled off from the casting drum 1 by the peeling roll 3, and the first drying zone 41 and the second drying zone 42 of the tenter drying unit 4. The drying is performed in the third drying zone 43 and the fourth drying zone 44, and further, the drying is completely performed in the roll drying unit 5.

The band casting type production apparatus shown in FIG.
A casting die 8 is provided in the vicinity of the casting band 7, and a roll drying unit 10 and a take-up roll 11 are provided via a stripping roll 9. After the dope is cast from the casting die 8 to the casting band 7, the dope is completely dried in the roll drying unit 10. In the production apparatus of the band casting system, the drying rate of the dope is high in the casting band, so that there is no need to provide a tenter drying section unlike the drum casting system.

The drying conditions in the method for producing a protective film for a polarizing plate of the present invention may be such that, even if the drying is performed on the casting support of the drum or the band, the tenter drying section after being peeled off from the casting support, It may be carried out in a roll drying unit or both, but from the viewpoint of simplicity, it can be carried out with hot air, and thus it is preferably carried out after peeling off from the support.

According to the method for producing a protective film for a polarizing plate as described above, a cellulose triacetate film having a birefringence of 5 nm or less can be produced. Such a cellulose triacetate film having a birefringence of 5 nm or less was extremely suitable for the degree of polarization of the polarizing plate.

That is, the polarizing plate of the present invention has a birefringence of 5
It is preferable to use a cellulose triacetate film having a thickness of not more than nm as the protective film.

The birefringence of the cellulose triacetate film needs to be 5 nm or less, preferably 4 nm or less, more preferably 3 nm or less.

As the polarizing film, for example, iodine or a dichroic dye is adsorbed on a hydrophilic polymer film such as a polyvinyl alcohol-based film, a partially formalized polyvinyl alcohol-based film, and an ethylene / vinyl acetate copolymer-based partially saponified film. A film that has been subjected to a dehydration treatment of polyvinyl alcohol and a dehydrochlorination treatment of polyvinyl chloride can be used.

[0031]

According to the method for producing a protective film for a polarizing plate of the present invention, the birefringence of the produced cellulose triacetate film is reduced by carrying out the drying process of the cellulose triacetate film under a predetermined range of drying conditions.

In the polarizing plate of the present invention, a protective film made of a cellulose triacetate film having a birefringence of 5 nm or less increases the degree of polarization.

[0033]

【Example】

Example 1 A drum casting type production apparatus shown in FIG. 2 was used. A dope consisting of 100 parts by weight of cellulose triacetate, 13 parts by weight of TPP (triphenyl phosphate), 360 parts by weight of methylene chloride, 40 parts by weight of methanol and 20 parts by weight of n-butanol was cast on a casting drum, and the volatile content was 75%. At the stage. Thereafter, drying was performed in four stages while being conveyed in the tenter drying section, and after passing through the tenter drying section, drying was performed in the roll drying section. Drying was performed using hot air, and the drying pattern was adjusted to be the drying pattern A in FIG.

Example 2 The same method as in Example 1 was used, except that drying was performed according to the drying pattern B in FIG.

Comparative Example 1 The same procedure as in Example 1 was carried out, except that drying was performed according to the drying pattern C in FIG.

Comparative Example 2 The same procedure as in Example 1 was carried out except that drying was performed in the drying pattern D in FIG.

The drying pattern shown in FIG. 4 is divided into five drying sections, each of which has a first drying zone 41,
Second drying zone 42, third drying zone 43, fourth drying zone
44 and the roll drying section 5.

Example 3 A band casting type manufacturing apparatus shown in FIG. 3 was used. A dope consisting of 100 parts by weight of cellulose triacetate, 13 parts by weight of TPP (triphenyl phosphate), 440 parts by weight of methylene chloride, and 40 parts by weight of methanol was cast on a casting band, dried in two stages, and dried. Was stripped off from the casting band when the volatile content became 20% or less. Thereafter, drying was performed in a roll drying section. Drying was performed using hot air, and the drying pattern was adjusted to become the drying pattern E in FIG.

Example 4 The same procedure as in Example 3 was carried out except that drying was performed according to the drying pattern F in FIG.

Comparative Example 3 The same procedure as in Example 3 was carried out except that drying was performed in the drying pattern G in FIG.

Comparative Example 4 The same procedure as in Example 3 was carried out except that drying was performed in the drying pattern H shown in FIG.

The drying pattern shown in FIG. 5 is divided into three drying sections, corresponding to the drying on the upper side of the belt 7, the drying on the lower side of the belt 7, and the roll drying section 10, respectively. are doing.

The birefringence of the obtained film was measured. Further, the obtained film is uniaxially stretched iodine-based PVA.
(Polyvinyl alcohol) A polarizing plate was prepared by bonding to both surfaces of a polarizing film via an adhesive, and the degree of polarization was measured. Table 1 shows the results. The single transmittance of the polarizing plate was 43%.

In addition, the obtained films all have a thickness of
At 80 μm, the visible light transmittance was 92.4%.

[0045]

[Table 1]

In the present invention, the birefringence can be made extremely small by drying the cellulose triacetate film under predetermined drying conditions.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a film volatile content and a film surface temperature in a method for producing a protective film for a polarizing plate of the present invention.

FIG. 2 is a conceptual diagram of an apparatus for producing a cellulose triacetate film by a drum casting method.

FIG. 3 is a conceptual diagram of an apparatus for producing a cellulose triacetate film using a band casting method.

FIG. 4 is a view showing a drying pattern when a cellulose triacetate film is formed by a drum casting method.

FIG. 5 is a view showing a drying pattern when a cellulose triacetate film is formed by a band casting method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Casting drum 2 ... Casting die 3 ... Peeling roll 4 ... Tenter drying part 5 ... Roll drying part 6 ... Winding roll 7 ... Casting band 8 ... Casting die 9 ... Peeling roll 10 ... Roll drying part 11 ... Winding roll

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-1009 (JP, A) JP-A-4-286611 (JP, A) JP-A-64-55214 (JP, A) JP-A-4-5521 176613 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B29C 41/26 B29C 41/28 C08J 5/18 CEP

Claims (2)

(57) [Claims] 1. A method for producing a protective film for a polarizing plate, comprising: casting a dope in which cellulose triacetate is dissolved in a solvent onto a band or a drum; stripping the dope from the band or the drum; X% volatile content in the triacetate film, Y
A method for producing a protective film for a polarizing plate, characterized by drying a film in a region where X and Y are represented by the following formulas when the temperature is 0 ° C .: 0 <X <20: Y> −6X + 180 20 ≦ X When ≤60 ... Y> -0.5X + 70 2. A method for producing a protective film for a polarizing plate, comprising: casting a dope in which cellulose triacetate is dissolved in a solvent onto a band or a drum; stripping the dope from the band or the drum; X% volatile content in the triacetate film, Y
A method for producing a protective film for a polarizing plate, characterized in that a film is dried for at least 10 seconds or more in a region where X and Y are represented by the following formulas at 0 ° C. 0 <X <20: Y>- 6X + 180 When 20 ≦ X ≦ 60 …… Y> −0.5X + 70