KR20150031519A - Cellulose diacetate phase diffeRence film - Google Patents

Abstract

The present invention relates to a cellulose diacetate phase difference film having the substitution degree of an acetyl group of 2.1 to 2.7. The cellulose diacetate phase difference film is capable of: uniformly giving double refraction to a cellulose ester film in an elongation process, providing high hydrophilicity with lots of hydroxyl groups and excellent compatibility with a water-based additive used for manufacturing a polarizing plate, and preventing bonding separation from a PVA film.

Description

Cellulose diacetate phase diffraction film < RTI ID = 0.0 >

The present invention relates to a cellulose diacetate film, and more particularly, to a cellulose diacetate retardation film that imparts birefringence to a cellulose diacetate film in a stretching process and does not cause separation of adhesion with a PVA film during the production of a polarizing plate .

In recent years, the development of thin and lightweight notebook computers is under way. Accordingly, a protective film of a polarizing plate used in a display device such as a liquid crystal display device has been increasingly demanded for further thinning and higher performance. Since a liquid crystal display device displays a display by polarization control by a liquid crystal, a polarizing plate is required, and a polarizing plate in which a PVA film containing iodine is stretched is usually used. Since the polarizing plate is fragile, a polarizing plate protective film is used to protect the polarizing plate. In general, triacetylcellulose film is widely used for a polarizing plate protective film. In addition to these polarizing plate protective films, a retardation film is also used to control the retardation of the polarized light. The retardation film used liquid crystal display device or the like may be used in order to solve the problems such as color compensation and wide viewing angle by using a retardation (R _th) in the thickness direction by the use in combination with a polarizing plate, and the phase difference in-plane direction (R _o) May have a function of converting linearly polarized light into circularly polarized light with respect to the wavelength of the visible light region or conversely converting circularly polarized light into linearly polarized light.

The polarizing plate protective film is intended to protect the polarizing plate and it is most preferable to use a film made of cellulose acetate in order to protect the polarizing plate made of PVA containing moisture, considering the production process of the polarizing plate. On the other hand, as a retardation film, materials other than cellulose acetate have been used to exhibit optical performance. That is, conventionally, as a material of the retardation film, for example, there are polycarbonate, polysulfone, polyethersulfone, amorphous polyolefin and the like. These polymer films have such characteristics that the longer the wavelength is, the smaller the retardation, and it is difficult to impart an ideal retardation property to the entire wavelength of the visible light region.

In the case where linearly polarized light is converted into circularly polarized light with respect to the wavelength of the visible light region or conversely circularly polarized light is converted into linearly polarized light, in order to obtain the above effect with one piece of retardation film, the retardation in the wavelength? 4 < / RTI > Such a retardation film can be obtained by using, for example, a retardation film having a phase difference of? / 4 and only one polarizing plate for a reflective liquid crystal display device having a back electrode as a reflective electrode, thereby obtaining a reflective display device having excellent image quality . Further, with respect to the observer of the guest-host type liquid crystal layer, the retardation film is used on the back side, or the circularly polarized light of the reflection type polarizing plate composed of cholesteric liquid crystal or the like that reflects only one of the left and right circularly polarized light is converted into linearly polarized light It is also used as an element.

Further, the retardation film has a function of converting linearly polarized light into elliptically polarized light or circularly polarized light, or converting linearly polarized light in a certain direction into another direction, and therefore, the viewing angle, contrast, etc. of the liquid crystal display device can be improved .

Generally, a retardation film is attached to a pair of polarizing plates, respectively. At present, N-TAC of Konica Minolta Holdings, Inc. of Japan is generally used as a phase difference film for a VA mode liquid crystal display. The NTAC retardation film was a cellulose acetate propionylate (CAP) having a retardation in the plane of 50 nm (retardation (R ₀ , relative to λ = 550 nm) and a retardation in the thickness direction (R _th , ) Film.

In order to improve the viewing angle characteristics (black display state (black characteristic), etc.) of the liquid crystal display device, wavelength dispersion and control techniques are required. In general, the N-TAC film exhibits an inverse wavelength dispersion characteristic in which the retardation value increases with increasing wavelength, and exhibits an excellent viewing angle characteristic improvement effect as compared with the retardation film having a regular wavelength dispersion characteristic in which the retardation value decreases with increasing wavelength. Further, in order to improve a liquid crystal display device, a retardation film having a specific retardation value and a combination thereof are used.

Conventional retardation films (PC, PSu, PA, etc.) have such characteristics that the longer the wavelength is, the smaller the retardation. It is difficult to impart an ideal retardation characteristic to the entire wavelength of the visible light region. So that necessary performance is obtained. In order to achieve such a performance in a single phase difference film, it is preferable that the in-plane direction retardation (R _o ) of the wavelength λ incident on the retardation film be λ / 4, Plane directional phase difference (R _o ) increases as the wavelength becomes longer. In the film made of cellulose acetate, if such a retardation property can be given, the polarizing plate protective film and the retardation film can be used together, the retardation film composed of a plurality of retardation films can be omitted, It is possible to improve the total light transmittance.

With respect to this problem, Japanese Patent Laid-Open No. 2000-137116 proposes to use an oriented film of cellulose acetate having a degree of substitution (acetylation degree) of 2.5 to 2.8 as a retardation film. According to this method, the longer the wavelength, the larger the phase difference, and the ideal phase difference characteristic is obtained with respect to the entire wavelength of the visible light region. That is, the above-mentioned patent discloses a phase difference plate in which the phase difference becomes smaller as the measurement wavelength becomes shorter with one film. A retardation film comprising a polymeric orientation film having a longer birefringence (? N) at a wavelength of 400 to 700 nm, wherein the polymeric orientation film is an orientation film of a polymeric film having an average refractive index at the above- And the like. As a means for solving this problem, a technique of orienting cellulose acetate having an acetylation degree of 2.5 to 2.8 by stretching is disclosed.

In the embodiment of the above-mentioned patent, 100 parts by weight of cellulose triacetate having an intrinsic viscosity [?] Of 1.335 and an acetylation degree of 2.917 obtained from Wako Junyaku Kogyo Co., Ltd. was dissolved in 500 parts by weight of methylene chloride, and thereto was added a 96% Hydrolysis of cellulose triacetate with acetic acid and water at 70 DEG C for 100 minutes while removing methylene chloride by depressurization and the reaction product was precipitated with a large amount of water and washed and dried , And a cellulose acetate having an acetylation degree of 2.661 was obtained. Then, a film was prepared from a solution prepared by dissolving 100 parts by weight of this polymer and 3 parts by weight of dibutyl phthalate as a plasticizer in 700 parts by weight of a mixed solvent of methylene chloride / methanol (weight ratio 9/1) by solvent casting method, And uniaxially stretched at a temperature of 170 占 폚 at 1.5 times. That is, in Embodiment 1 of the patent, the retardation film having the same wavelength characteristics (wavelength dispersion characteristics) as the latter is obtained by stretching. It is also disclosed that by adjusting the retardation value, it is possible to use? / 4 or another retardation film. In Example 4 of the aforementioned patent, cellulose acetate having an acetylation degree of 2.421 was obtained. When the retardation property of the film using the film is measured, the retardation is insufficient when the film thickness is about 100 mu m (50 to 150 mu m) and when the film thickness is preferable as the self-supporting film. Further, when thick enough and has a film thickness 200㎛ over to, the retardation (R _th) in the thickness direction at the time to provide a desired phase difference of about 80 to 150nm is more than 350nm, in the field of view when functioning as a λ / 4 phase difference film It did not function as an enlarged film and was not sufficient. Furthermore, the molecular weight distribution of the obtained cellulose acetate is not described, and the control of the retardation characteristic by controlling the molecular weight distribution is not described or suggested.

On the other hand, although the protective film of the polarizer PVA layer of the polarizing plate was produced by using materials such as TAC and acrylic, the raw materials added to the CAP, COP, and TAC at the time of manufacturing the retardation film are expensive, .

DISCLOSURE Technical Problem The present invention has been conceived to solve the problems of the prior art as described above, and it is an object of the present invention to provide a cellulose ester film which uniformly imparts birefringence to a cellulose ester film in a stretching process, has a high hydrophilic property due to a large number of hydroxy groups, It is an object of the present invention to provide a cellulose ester optical film which is excellent in compatibility with a water-based adhesive and does not cause separation of adhesion with a PVA film.

In order to achieve the above object, the present invention provides a cellulose diacetate having an acetyl group substitution degree of 2.1 to 2.7, wherein the degree of molecular weight dispersion (weight average molecular weight: Mw / number average molecular weight: Mn) , An average molecular weight (Mw) in the range of 150,000 to 220,000, an acetyl group in the second, third and sixth positions, an average degree of substitution in the range of 70 to 90 mole%, and a hydroxyl group in the range of 10 to 30 mole% Acetate phase difference film.

According to an exemplary embodiment of the present invention, the cellulose diacetate phase difference film has a surface direction retardation ( _Ro ) of 20 to 100 nm and a thickness direction retardation ( _Rth ) of 50 to 250 nm, and the glass transition of the cellulose diacetate flake And the glass transition temperature (Tg) of the retardation film is in the range of 150 to 190 占 폚.

The present invention having such a constitution as described above is characterized in that the cellulose ester film uniformly imparts birefringence to the cellulose ester film in the stretching step and has a high hydrophilicity due to a large number of hydroxy groups and thus is excellent in compatibility with the water- Adhesion separation does not occur, and a low-cost raw material is applied, which makes it possible to secure price competitiveness and economic efficiency after production of the retardation film.

Hereinafter, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited thereto.

The cellulose ester optical film according to the present invention, in detail, a cellulose diacetate phase difference film is produced in a tenter through a preheating process, a stretching process and a heat setting process, wherein the cellulose diacetate phase difference film has an acetyl group substitution degree 2.1 (Mw / Mn) is in the range of 2.5 to 4.5, and the average molecular weight (Mw) is in the range of 150,000 to 220,000.

The cellulose diacetate phase difference film according to the present invention can be produced by solution casting method. In the solution film forming method, cellulose diacetate may be dissolved in an agitator using an additive such as a plasticizer, a UV absorber, a matting agent, etc. and a mixed solvent such as methylene chloride and methanol to prepare a dope, which may be filtered using a filtration apparatus.

The degree of substitution of cellulose diacetate which can be used in the present invention is within the range of 70 to 90 mole% substitution degree in the range of the second, third and sixth positions of the acetyl group.

In the present invention, the molecular weight range of cellulose diacetate is not limited, but it is preferable that the weight average molecular weight ranges from 150,000 to 220,000.

By reducing the molecular weight to a certain level or more, the strength of the film can be effectively prevented from being lowered.

Further, by keeping the molecular weight below a certain level, the viscosity of the cellulose acylate solution (dope) is maintained at a certain level or lower, thereby facilitating film production by the solution casting method.

The degree of molecular weight dispersion (weight average molecular weight Mw / number average molecular weight Mn) of the cellulose diacetate is preferably in the range of 2.5 to 4.5.

When a film is produced by a solution film forming method (or a solvent casting method), an organic solvent is preferable as the solvent for preparing the cellulose diacetate composition (dope). As the organic solvent, it is preferable to use halogenated hydrocarbons, and halogenated hydrocarbons include chlorinated hydrocarbons, methylene chloride and chloroform, among which methylene chloride is most preferred.

If necessary, organic solvents other than halogenated hydrocarbons may be mixed and used. Organic solvents other than halogenated hydrocarbons include esters, ketones, ethers, alcohols and hydrocarbons. Examples of the ester include methyl formate, ethyl formate, propyl formate, pentyl formate, methyl acylate, ethyl acylate, and pentaacetate. Examples of the ketone include acetone, methyl ethyl ketone, diethyl ketone, di Isobutyl ketone, cyclopentanone, cyclohexanone, methylcyclohexanone and the like can be used. As the ether, diisopropyl ether, dimethoxymethane, dimethoxyethane, 1,4-dioxane, Ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, t-butanol, 1-pentanol, 2-butanol and the like can be used. Methyl-2-butanol, cyclohexanol, 2-fluoroethanol, 2,2,2-trifluoroethanol and 2,2,3,3-tetrafluoro-1-propanol.

More preferably, methylene chloride may be used as the main solvent, and alcohol may be used as the minor solvent. Specifically, methylene chloride and alcohol may be mixed at a weight ratio of 80:20 to 95: 5.

The cellulose diacetate composition can be prepared by room temperature, high temperature or low temperature dissolution.

Next, the additive used in the production of the cellulose diacetate film will be described. The cellulose diacetate solution (dope) used in the solution casting method may contain various additives depending on the application such as a plasticizer, a deterioration inhibitor, a matte fine particle, a stripping agent, a UV stabilizer, an ultraviolet absorber, An additive such as a wavelength dispersion adjusting agent, an optical anisotropy adjusting agent and the like may be added. The specific kind of such additives can be used without limitation as long as they are commonly used in the field, and the content thereof is preferably used within a range that does not deteriorate the physical properties of the film. The timing of adding the additives depends on the type of additive. A step of adding an additive to the end of the doping treatment may be performed.

The cellulose diacetate phase difference film has a surface direction retardation ( _Ro ) of 20 to 100 nm and a thickness direction retardation ( _Rth ) of 50 to 250 nm.

R _o and R _th are surface retardation value thickness direction retardation values respectively, and are expressed by the following equations (1) and (2).

(1) R _o = (nx-ny) x d

(2) R _th = (nx + ny) / 2-nz x d

(Where d is the thickness of the film in nm, nx is the maximum refractive index in the plane of the film, ny is the refractive index in the direction perpendicular to nx in the film plane, and nz is the refractive index of the film in the thickness direction)

In addition, the cellulose diacetate film contains a plasticizer for improving mechanical strength, good castability, imparting water absorbency, reducing water permeability, and the like. As the plasticizer, any conventionally used plasticizer can be used without limitation, and examples thereof include carboxylic acid esters selected from phosphoric acid esters and phthalic acid esters or citric acid esters.

Examples of phosphoric acid esters include triphenyl phosphate (TPP), biphenyl diphenyl phosphate, and tricresyl phosphate (TCP). Examples of phthalic acid esters include dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), dioctyl phthalate (DOP), diphenyl phthalate (DPP) and diethylhexyl phthalate have.

Examples of citric acid esters include o-acetyltriethyl citrate (OACTE) and o-acetyl tributyl citrate (OACTB). Examples of other carboxylic acid esters include butyl oleate, methyl acetyl ricinoleate, dibutyl sebacate and various trimellitic acid esters. Two or more kinds of plasticizers may be used in combination. The content of the plasticizer is 2 to 20 parts by weight, more preferably 5 to 15 parts by weight, based on 100 parts by weight of cellulose diacetate.

The cellulose diacetate solution thus obtained is cast on a support through a casting die to form a cellulose diacetate sheet.

The cellulose diacetate sheet thus formed is subjected to a stretching step in a tenter. The preheating step is carried out such that the glass transition temperature (Tg) of the cellulose diacetate flake is 185 to 200 ° C, the glass transition temperature (Tg) of the retardation film is 150 To 190 < 0 > C.

The cellulose diacetate film of the present invention can be completed through a step of drying in a dryer after removing the left and right ends of a film whose surface is damaged by a clip or pin of a tenter after being subjected to a stretching step in a tenter under the above conditions .

Example 1

A mixed solution of methylene chloride: methanol = 9: 1 was added to a cellulose diacetate having an acetyl group affinity of 2.7, and the mixture was stirred to prepare a completely dissolved cellulose diacetate solution. Then, the prepared solution was cast on a support, and the solvent was evaporated to form a film. The film thus formed was stretched at a draw ratio of 135% at a stretching temperature of 170 占 폚, and the drying temperature was 110 占 폚 and the drying time was 30 minutes, and the thickness of the produced film was 40 占 퐉.

Example 2

A cellulose diacetate film having a thickness of 40 탆 was prepared in the same manner as in Example 1 except that cellulose diacetate having an acetyl group affinity of 2.56 was used.

Example 3

A cellulose diacetate film having a thickness of 40 탆 was prepared in the same manner as in Example 1 except that cellulose diacetate having an acetyl group affinity of 2.41 was used.

Example 4

A cellulose diacetate film having a thickness of 40 탆 was prepared in the same manner as in Example 1 except that cellulose diacetate having an acetyl group affinity of 2.30 was used.

Comparative Example 1

The same procedure as in Example 1 was carried out except that a film was prepared by using cellulose acetate propionate (CAP) as a flake, and optical characteristics were analyzed.

Comparative Example 2

The same procedure as in Example 1 was carried out except that flakes were made of cellulose acetate propionate (CAP) and cellulose diacetate having an acetyl group affinity of 2.95 to 2.7 or more, and optical properties were analyzed .

Comparative Example 3

The same procedure as in Example 1 was carried out except that flakes were made of cellulose acetate propionate (CAP) and cellulose diacetate having an acetyl group affinity of 2.91 to not less than 2.7 and the optical characteristics were analyzed .

Comparative Example 4

The same procedure as in Example 1 was carried out except that flakes were made of cellulose acetate propionate (CAP) and cellulose diacetate having an acetyl group affinity of 2.86 to not less than 2.7, and the optical characteristics were analyzed .

Sample Optical characteristic Solubility Acetyl substitution degree R _o R _th R _th / R _o Example 1 2.70 23.1 75.0 3.2 O Example 2 2.56 40.0 95.9 2.4 O Example 3 2.41 52.0 126.0 2.4 O Example 4 2.30 61.0 132.0 2.2 O Comparative Example 1 - 50.1 124.2 2.5 O Comparative Example 2 2.95 - - - X Comparative Example 3 2.91 - - - X Comparative Example 4 2.86 2.2 51.5 23.9 O

As shown in Table 1, Comparative Example 2 and Comparative Example 3 had a problem that cellulose diacetate did not dissolve in a mixed solution of methylene chloride: methanol = 9: 1, and that Comparative Example 4 had a problem that R _th / R _o 'Is 23.9, which is a problem in that the phase difference manifestation is insufficient.

Examples 1, 2, 3 and 4 having an acetyl group degree of substitution in the range of 2.1 to 2.7 show excellent results as a retardation film because of excellent phase retardation, and have performance equivalent to that of the comparative CAP .

The cellulose diacetate of the present example has merits of a simple manufacturing process and excellent performance compared to a CAP raw material substituted with an acetyl group and a propionate group.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. It will be easy for anyone to know.

Claims (3)

(Weight average molecular weight: Mw / number average molecular weight: Mn) in the range of 2.5 to 4.5 and an average molecular weight (Mw) in the range of 150,000 to 220,000, wherein the cellulose diacetate has an acetyl group substitution degree of 2.1 to 2.7, , The acetyl group has an average degree of substitution in the range of 70 to 90 mole%, and the hydroxyl group in the range of 10 to 30 mole% in the second, third, and sixth positions.
The method according to claim 1,
Wherein the cellulose diacetate retardation film has a retardation in the plane direction ( _Ro ) of 20 to 100 nm and a retardation in the thickness direction ( _Rth ) of 50 to 250 nm.
The method according to claim 1,
Wherein the glass transition temperature (Tg) of the cellulose diacetate flake is in the range of 185 to 200 占 폚, and the glass transition temperature (Tg) of the retardation film is in the range of 150 to 190 占 폚.