KR101765793B1 - Manufacturing method of Cellulose ester Film - Google Patents

Abstract

The present invention relates to a method for producing a cellulose ester film as a protective film of a polarizing plate by introducing a longitudinal stretching process into a belt system. More specifically, the casting stock solution is extruded from the casting surface to the surface of the belt through a die and applied in the form of a sheet. After passing through the steel conveyor belt and the peeling roll, the longitudinal direction (MD) Is stretched in a first longitudinal direction in a preheating section before stretching in the width direction in the first drying section and stretched in a second longitudinal direction in a transverse stretching direction after the transverse stretching section. Method.
The present invention has the effect of minimizing the physical and physical properties of the film in longitudinal and transverse directions, and minimizing differences in properties in the longitudinal and transverse directions by maximizing the longitudinal stretching effect and alleviating the direction of the polymer width direction.

Description

Technical Field [0001] The present invention relates to a cellulose ester film,

The present invention relates to a method for producing a cellulose ester film which is a protective film of a polarizing plate. More particularly, the present invention relates to a method for producing a cellulose ester film in which longitudinal stretching is introduced into a belt system.

Polymer films represented by cellulose ester, polyester, polycarbonate, cycloolefin polymer, vinyl polymer and polyimide are used for the silver halide photographic photosensitive material, the retardation film, the polarizing plate and the image display device. From these polymers, a film superior in planarity and uniformity can be produced, and is widely employed as a film for optical use. For example, a cellulose ester film having an appropriate moisture permeability can be directly bonded on-line with a polarizing film made of the most common polyvinyl alcohol (PVA) / iodine.

Thus, cellulose esters, particularly cellulose acetate, have been widely adopted as protective films for optical applications.

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 in such a liquid crystal display device is used in combination with a polarizing plate to solve problems such as color compensation and view angle enlargement by using a retardation in the thickness direction (Rth), and also uses an in-plane retardation Ro And has a function of converting linearly polarized light into circularly polarized light or conversely converting circularly polarized light into linearly polarized light with respect to the wavelength of the visible light region.

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) film having a retardation in the plane (reference for R0,? = 550 nm) of 50 nm and a retardation in the thickness direction (Rth,? = Lt; / RTI >

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 obtain such a performance in a single retardation film, it is preferable that the in-plane direction retardation Ro of the wavelength? Incident on the retardation film be? / 4, but for this purpose, The in-plane directional phase difference Ro needs to be increased. 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 [?] = 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, 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 캜 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. When the thickness of the film is as large as about 200 mu m, a preferable retardation of about 80 to 150 nm is provided. In this case, the thickness direction retardation (Rth) is excessively larger than 350 nm, It did not function as a film and was not sufficient. Furthermore, the molecular weight distribution of the obtained cellulose acetate is not described, and the control of the retardation characteristics by controlling the molecular weight distribution is not described or suggested.

The cellulose ester film, which is a protective film for the polarizing plate, has a drum type and a belt type. In the drum type, it is easy to adjust the film width according to the drum size, and longitudinal direction (MD) stretching is introduced. In the belt type, belt width is limited and stretching in the width direction (TD) is required for a super wide film. At this time, the film has a modulus increase in the stretching direction and a CTE decrease, that is, a film property difference in the longitudinal direction and the width direction occurs depending on the stretching direction of the film.

Accordingly, the inventors of the present invention have confirmed that the physical property difference in the longitudinal direction and the width direction of the film can be minimized by the directional relaxation of the polymer through the longitudinal direction stretching in the belt system, thereby completing the present invention.

Korean Patent Registration No. 10-1272696 (Publication date: March 13, 2013) Korean Patent Registration No. 10-0860204 (Publication date: December 06, 2002)

It is an object of the present invention to provide a cellulose ester film which is a protective film of a polarizing plate that alleviates physical property differences in the longitudinal direction and the width direction.

The present invention is a method for producing a cellulose ester film in which longitudinal stretching is introduced into a conventional belt method in which a wide film is produced only by widthwise stretching in order to overcome differences in physical properties from the belt method to the drum method.

More specifically, the casting stock solution is extruded from the casting surface to the surface of the belt through a die and applied in the form of a sheet. After passing through the steel conveyor belt and the peeling roll, the pre- Stretching, stretching in the transverse direction, and stretching in the second longitudinal direction at the downstream end of the drying section.

At this time, the difference between the tensile strength in the longitudinal direction (MD) and the tensile strength in the transverse direction (TD) of the film is 10% or less of the tensile strength in the transverse direction, and the difference between the longitudinal direction thermal expansion coefficient and the lateral direction thermal expansion coefficient Or less.

When the difference in the tensile strength exceeds 10% of the tensile strength in the width direction or the difference in the thermal expansion coefficient exceeds 30% of the coefficient of thermal expansion in the width direction, there arises a problem that the viewing angle and contrast of the liquid crystal display device are lowered.

Wherein the residual solvent content in the end step after the first drying step is 0.5 to 30 wt%, and the film thickness deviation of the film is 0.1 to 2 탆. If the film thickness deviation of the film is more than 2.0, the uniformity of the film properties is lost and the merchantability is degraded.

The film is characterized in that the tensile strength in the longitudinal direction (MD) and the transverse direction (TD) is 80 N / mm < 2 > or more and the total thickness of the film is uniformed to 20 to 85 mu m. Particularly, when the film is thicker than 85 탆, the polarizing plate after the polarizing plate processing becomes too thick, and thus it is not particularly suitable for the purpose of thin and lightweight in the liquid crystal display used for a notebook computer or a mobile electronic apparatus. On the other hand, if the film is thinner than 20 占 퐉, the moisture permeability becomes high and the ability to protect the polarizer from humidity is deteriorated.

The tensile strength in the longitudinal direction and the width direction of the film is preferably 80 to 200 N / mm 2. When the tensile strength is less than 80, the durability of the product is low and the possibility of industrial use is low. When the tensile strength is more than 200, there is a problem that the production economical efficiency is low.

The present invention relates to a method of controlling longitudinal properties and widthwise physical properties of a film by controlling the lengthwise and transverse direction properties of the film by maximizing the longitudinal stretching effect and relaxing the orientation of the polymer in the width direction, There is an effect of minimizing the difference.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an overall schematic view of a film production apparatus that can be used in the production of a cellulose film according to the present invention. Fig.
Fig. 2 is a view showing a stretching step in the film production apparatus shown in Fig. 1. Fig.

Hereinafter, the present invention will be described in detail.

The cellulose ester used in the present invention is preferably a lower fatty acid ester of cellulose. The lower fatty acids used in the production of lower fatty acid esters of cellulose means fatty acids having 6 or less carbon atoms.

Examples of lower fatty acid esters are fatty acid esters of mixed cellulose such as cellulose acetate, cellulose propionate, cellulose butyrate and cellulose acetate propionate or cellulose acetate butyrate. Of the lower fatty acid esters of cellulose, cellulose triacetate or cellulose acetate propionate is particularly preferred.

In the present invention, cellulose ester synthesized from raw materials such as linter pulp, wood pulp or kenaf pulp may be used singly or in combination. It is particularly preferable to use cellulose acetate synthesized from a raw material such as linter pulp alone or in combination.

The stretched cellulose ester film of the present invention is preferably applied to a liquid crystal display member in terms of low moisture permeability and excellent numerical stability. The liquid crystal display member is a material used in a liquid crystal display, for example, a polarizing plate, a polarizing plate protective film, a retardation film, a reflector, a viewing angle enhancement film, an antiglare film, an antireflection film or an antistatic film.

The cellulosic ester film of the present invention is particularly preferably applied to a polarizing plate or a polarizing plate protective film. The polarizing plate can be produced by a conventional method. For example, the polarizing film prepared by alkaline saponification of the stretched cellulose ester film of the present invention and immersing the resulting film in a iodine solution of a polyvinyl alcohol (PVA) film and stretching the film, Alcohol solution is used to bond. Alkali saponification refers to a treatment in which the cellulose ester film is immersed in a high-temperature strong alkali solution in order to improve the wettability of the film to an aqueous adhesive and to provide good adhesion to the film.

Various additives such as UV blocking agents, plasticizers, deterioration inhibitors, fine particles, optical property adjusting agents and the like can be added to the preparation of the stretched cellulose ester film of the present invention. You can. Further, a step of preparing and adding an additive to the final preparation step of the dope-preparing step may be added.

According to one embodiment of the cellulose ester film according to the present invention, it has a film thickness of about 20 탆 to about 80 탆 which is obtained by adding about 2% by weight to about 15% by weight of a plasticizer among various additives.

As the plasticizer in the present invention, a plasticizer such as a phosphate ester, a phthalate ester, a trimellitate ester, a pyromellitic acid, a polyhydric alcohol ester, a glycolate, a citrate ester, a fatty acid ester, a polyester and a polycarboxylic acid Ester plasticizers. Among these, a polyhydric alcohol ester plasticizer, a polyester plasticizer and a polycarboxylic acid plasticizer are preferred.

When the plasticizer is contained, the content thereof is preferably about 2% by weight to about 15% by weight based on the cellulose ester resin, considering the dimensional stability and processability. If the content of the plasticizer is too small, There is little effect of reducing the moisture permeability of the substrate. When slitting or punching is performed, a smooth cutting surface can not be obtained, and the occurrence of cutting residue tends to increase. That is, the effect of containing the plasticizer can not be sufficiently exhibited. If too much, the plasticizer tends to bleed out from the resin film to deteriorate the physical properties of the film.

On the other hand, as the UV screening agent according to the present invention, a benzotriazole-based UV screening agent or a triazine-based UV screening agent having a high transparency and excellent in preventing deterioration of a polarizing plate or a liquid crystal element is preferable, and a spectral absorption spectrum UV blocking agents are particularly preferred.

Conventionally known benzotriazole-based UV screening agents which are particularly preferably used together with the UV screening agent according to the present invention may be bisated, for example, 6,6'-methylenebis (2- (2H-benzo [d Yl]) - 4- (2,4,4-trimethylpentan-2-yl) phenol, 6,6'- methylenebis (2- (2H- benzo [ d] [1,2,3] triazol-2-yl)) - 4- (2-hydroxyethyl) phenol.

In the present invention, the UV blocking agent is preferably added in an amount of about 0.1% by mass to about 20% by mass, more preferably about 0.5% by mass to about 10% by mass, more preferably about 1% by mass to about 5% . These may be used in combination of two or more.

According to the present invention, the thickness of the stretched cellulose ester film is preferably about 20 占 퐉 to about 85 占 퐉. Particularly, when the film is thicker than 85 탆, the polarizing plate after the polarizing plate processing becomes too thick, and thus it is not particularly suitable for the purpose of thin and lightweight in the liquid crystal display used for a notebook computer or a mobile electronic apparatus. On the other hand, if the retardation film is thinner than 20 占 퐉, the moisture permeability of the film as the retardation film becomes high, and the ability to protect the polarizer from humidity is deteriorated.

Such a stretched cellulose ester film is preferably produced by a solution casting film-forming method. The solution casting film forming method of the present invention can produce a film having excellent physical properties such as optical properties as compared with other production methods such as a melt film forming method. In the solution casting method, various additives such as a cellulose ester and a UV absorber, a matting agent, a retardation control agent and a plasticizer are mixed with a mixed solvent containing dichloromethane or methyl acetate as a main solvent to prepare a dope. Subsequently, A flexible film is flexibly formed on the support from a die or T-die. After the flexible film is made to have a self-supporting property on a support such as a drum or a steel belt, the flexible film is peeled from the support as a wet film, dried to volatilize the solvent, .

According to the present invention, the film is stretched by about 130% in the longitudinal direction within the temperature range of Tg to Tg-50 ° C to Tg + 50 ° C, and further stretched to about 150% in the transverse direction and then the film is stretched for about 0.01 minute to about 5 minutes . Thereafter, the film is further stretched in the longitudinal direction by about 120% at the downstream end of the drying section, heat-settled, and the final stretched film is dried at a temperature not higher than Tg and then wound.

At this time, it is preferable to relax about 0.1% to about 10% in the transverse direction and / or the longitudinal direction within the temperature range of not more than the final heat fixing temperature and not less than Tg. Further, it is preferable that the cooling is progressively cooled from the final heat fixing temperature to Tg at a cooling rate of 100 DEG C or less per second. The means for cooling and relaxation is not particularly limited and can be carried out by conventionally known means. In particular, it is preferable to perform these treatments while sequentially cooling in a plurality of temperature ranges from the viewpoint of improving the dimensional stability of the film. The cooling rate is a value obtained by (T1-Tg) / t when the final heat fixing temperature is T1 and the time from when the film reaches the final heat fixing temperature to Tg is t. More optimal conditions for these heat fixing conditions, cooling, and relaxation treatment conditions depend on the cellulose ester constituting the film. Therefore, the properties of the biaxially stretched film obtained may be measured and appropriately adjusted so as to have desirable characteristics.

Hereinafter, the present invention will be described in detail with reference to the following examples and comparative examples, but the following examples are for illustrative purposes only and are not intended to limit the present invention.

Example: Casting → 110% stretching in the longitudinal direction at 120 ° C for 3 minutes → 120% stretching in the width direction for 5 minutes at 180 ° C → 105% stretching in the longitudinal direction at 140 ° C for 1 minute → 15 minutes drying at 120 ° C

≪ Comparative Example 1 > Casting: 110% stretching in the longitudinal direction at 120 DEG C for 3 minutes> 120% stretching in the width direction at 180 DEG C for 5 minutes> Drying for 15 minutes at 120 DEG C

≪ Comparative Example 2 >: Casting → 120% stretching in the width direction at 180 ° C for 5 minutes → drying at 120 ° C for 15 minutes

As can be seen from the measurement values in the following Table 1, the tensile strength in the longitudinal direction could be improved by introducing the longitudinal stretching as in the examples, and the difference in physical properties and thermal expansion coefficient from the width direction could be alleviated.

At this time, the difference between the tensile strength in the longitudinal direction (MD) and the tensile strength in the transverse direction (TD) of the film is 10% or less of the tensile strength in the transverse direction, and the difference between the longitudinal direction thermal expansion coefficient and the lateral direction thermal expansion coefficient Or less. When the difference in the tensile strength exceeds 10% of the tensile strength in the width direction or the difference in the thermal expansion coefficient exceeds 30% of the coefficient of thermal expansion in the width direction, there arises a problem that the viewing angle and contrast of the liquid crystal display device are lowered.

The tensile strength in the longitudinal direction and the width direction is preferably 80 to 200 N / mm 2. When the tensile strength is less than 80, the durability of the product is low and the possibility of industrial use is low. When the tensile strength is more than 200, there is a problem that the production economical efficiency is low.

The coefficient of thermal expansion is suitably 30 to 100 占 퐉 / m 占 폚. Likewise, when the coefficient of thermal expansion is more than 100, the possibility of industrial use is lowered. If the coefficient of thermal expansion is less than 30, the economical efficiency of production is deteriorated.

Example Comparative Example 1 Comparative Example 2 Longitudinal tensile strength (N / mm2) 180 160 150 Tensile strength in the transverse direction (N / mm2) 180 190 200 The longitudinal thermal expansion coefficient (탆 / m · ° C) 50 65 100 The coefficient of thermal expansion in the width direction (占 퐉 / m 占 폚) 60 45 30

≪ Method for measuring physical properties &

1) Tensile strength: The sample was cut into 15 mm x 10 cm and allowed to stand at constant temperature and humidity (25 ° C, 55%) for 6 hours, and then the tensile strength was measured using a tensile strength meter (Instron).

2) Thermal Expansion Coefficient: The sample is cut into 6mm x 15mm, and the temperature is raised by 3 ℃ per 1min at 20 ~ 125 ℃ with 0.0016N load per 1mm thickness using TMA (TMA Q400).

10: Cellulose film
20: Die
30: Belt
32: Peel-off Roller
40: Tenter
50: Dryer
60: Winder

Claims (3)

In the method for producing a cellulose ester film,
The casting stock solution is extruded from the casting surface onto the surface of the belt through a die and applied in the form of a sheet. After passing through the belt and the peeling roll, stretching in the longitudinal direction (MD) and the transverse direction (TD) In the first drying section, the first longitudinal direction stretching is performed in the preheating section before the transverse direction stretching, the second longitudinal direction stretching is performed after the transverse direction stretching,
Wherein the difference between the longitudinal direction tensile strength and the transverse direction tensile strength of the film is 10% or less of the transverse tensile strength and the difference between the longitudinal direction thermal expansion coefficient and the lateral direction thermal expansion coefficient is 30% or less of the lateral direction thermal expansion coefficient. Lt; / RTI >
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