JP2003315538A - Retardation film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a retardation film of which the retardation is larger and the optical elasticity is lower as the wavelength gets longer with a single film. <P>SOLUTION: The retardation film is obtained by casting a solution prepared by dissolving cellulose acetate propionate in a single solvent wherein a degree of acetyl substitution (DSac) and a degree of propionyl substitution (DSpr) of the cellulose acetate propionate satisfy the following inequalities (I) and (II). 2.0≤DSac+DSpr≤3.0 (I) 1.0≤DSpr≤3.0 (II). <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a retardation film using a cellulose ester having propionic acid as a substituent, as an optical film for a liquid crystal display device.

[0002]

2. Description of the Related Art Retardation films have been widely used in liquid crystal display devices in recent years, as typically seen in reflective TFT liquid crystal display devices. Generally, a polycarbonate film is used as the retardation film.

The applications of retardation films are further expanding, and accordingly, more sophisticated functions are required. Among these requirements, what has a phase difference of ¼ with respect to an arbitrary wavelength in the visible light region is particularly important.

Retardation is 1/4 of wavelength 1
The retardation film used as the / 4 wavelength plate preferably has a retardation corresponding to ¼ wavelength for each wavelength of visible light. That is, for example, for a wavelength of 441.6 nm, 11
A quarter wave plate having a phase difference of 0.4 nm, a phase difference of 128.6 nm for a wavelength of 514.5 nm, and a phase difference of 158.2 nm for a wavelength of 632.8 nm is ideal. The linearly polarized light passing through the retardation film is all circularly polarized light regardless of the wavelength, and the all circularly polarized light passing therethrough is linearly polarized light. However, in the polycarbonate retardation film which is generally widely used at present, the shorter the wavelength, the larger the retardation, and the reflective TFT liquid crystal display device using the retardation film exhibiting the wavelength dependence of such retardation produces a black display. In such a case, the light from the backlight cannot be completely blocked, resulting in deterioration of contrast and gradation display.

On the other hand, for example, Japanese Patent Laid-Open No. 5-2711
No. 8, JP-A-5-27119, and JP-A 5-1.
According to Japanese Patent Application Laid-Open No. 001114 and Japanese Patent Application Laid-Open No. 10-68816, by bonding two retardation films at a predetermined angle, a polycarbonate film can also provide a retardation film exhibiting a higher retardation at longer wavelengths.

However, these methods require a film laminating step and have the following problems when applied to a polycarbonate retardation film. That is, a normal polycarbonate film having a bisphenol component of bis (4-hydroxyphenyl) propane has a large photoelastic coefficient of 70 × 10 ⁻¹² m ² / N, and therefore, a stretching step in manufacturing a retardation film. In the above, the slight fluctuation of the tension easily causes the variation of the phase difference. Further, not only the desired phase difference is displaced due to the tension at the time of laminating such a retardation film, but also the retardation value is changed due to contraction of the polarizing plate after laminating.

Therefore, various studies have been made on a retardation film using a cyclic polyolefin polymer having a relatively small photoelastic coefficient, and JP-A 2001-124925, JP-A 2001-126311, and JP-A-8-0759 have been studied.
21, JP-A-2-042441, JP-A-4
No. 245202 discloses a retardation film in which variations in retardation are suppressed. However, these retardation films have a problem that when they are bonded to a polarizing film or glass using an adhesive, the affinity with the adhesive is insufficient and peeling or microfoaming easily occurs under a long-term durability test. There is. Further, the retardation film requires a high degree of film thickness accuracy, and is generally formed into a film by a solvent casting method, but the solvent that can be used for the olefin-based polymer is limited, and particularly, toluene or xylene. When a solvent is used, it has various problems other than polycarbonate, such as poor productivity due to the high boiling point of the solvent.

To solve these problems, Japanese Unexamined Patent Publication No. 2000-1
In No. 37116 public information, we use cellulose acetate,
A retardation film having a higher retardation for a longer wavelength has been proposed with a single film. However, since cellulose acetate has low solubility in a solvent, the solvent that can be selected is limited when the solvent casting method is used. Further, since cellulose acetate is difficult to dissolve in methylene chloride alone which is generally used in the solvent casting method, it is necessary to use a mixed solvent of methylene chloride and methanol or to use a low concentration solution in the case of a single solvent. However, when a film is produced by the solvent cast method, it becomes difficult to recover the solvent when a mixed solvent is used, and in a low-concentration solution, the energy required for drying is large, which may lead to an increase in production cost. is there.

[0009]

DISCLOSURE OF THE INVENTION The present invention focuses on a single film obtained by a casting method using a single solvent, has a higher retardation in the visible light region at longer wavelengths, and has a low light intensity. The purpose is to provide a retardation film having an elastic modulus.

[0010]

In order to solve the above problems, the inventors of the present invention have conducted extensive studies and found that a cellulose ester having a specific substituent group can achieve the intended purpose, leading to the present invention. It was

That is, the present invention is a retardation film obtained by casting a solution prepared by dissolving cellulose acetate propionate in a single solvent, wherein the cellulose acetate propionate has a degree of acetyl substitution (DSac). ) And the propionyl substitution degree (DSpr) satisfy the following formulas (I) and (II). 2.0 ≦ DSac + DSpr ≦ 3.0 (I) 1.0 ≦ DSpr ≦ 3.0 (II) According to another aspect, the retardation of the retardation film with respect to monochromatic light of 441.6 nm (Re (441.
6)) and retardation for monochromatic light of 514.5 nm (Re (514.5)) and retardation for monochromatic light of 632.8 nm (Re (632.
8)) is Re (441.6) <Re (514.5) <R
The retardation film of the present invention is characterized by satisfying e (632.8). As a result, it is possible to obtain a desired retardation in a wide band even with a single film, and display with good contrast becomes possible.

According to another aspect, 514.5
Retardation (Re (51
The retardation film of the present invention is characterized in that 4.5)) is from 5 to 1000 nm and the photoelastic coefficient is 20 × 10 ⁻¹² m ² / N or less. This makes it possible to obtain a retardation film having a small change in retardation with respect to tension caused by bonding with a polarizing plate.

As the sole solvent, methylene chloride is preferable because it has a low boiling point, is easy to dry, and is highly safe against fire and the like.

Further, the retardation film of the present invention is capable of easily imparting a desired retardation by stretching it in at least a uniaxial direction.

The substitution degree in the present invention is the substitution degree measured by the following method.

<Method of measuring degree of substitution> Cellulose acetate propionate acetylation degree DSac and propionyl substitution degree DSpr are determined by A. Blumstein, J. Asrar, R.
B. Blumstein Liq. Cryst. Ordered Fluids 4. 311 (198
By applying the method for measuring the degree of substitution of cellulose acetate by ¹ H-NMR described in 4), it can be determined as follows.

The proton in the cellulose skeleton is represented by the following formula (1)

[0018]

[Chemical 1]

H defined by tetramethylsilane (TMS) and underlined a (hereinafter Ha) when defined as shown in
Is 0.8 to 1.4 ppm (region A), H underlined b (hereinafter Hb) and H underlined c (hereinafter Hc) is 2
To 2.5 ppm (region B), each H (hereinafter, H1 to H2) bonded to the carbons at the 1st to 6th positions is 3
To 5.4 ppm (region C), respectively. The areas of the peak groups of the region A, the region B, and the region C are A,
Assuming B and C, A = Ha B = 2Hb + 3Hc C = H1 + H2 + H3 + H4 + H5 + 2H6. _{-OCOCH 2 CH 3 / OCOCH 3 /} OH
= X / y / z, the above equation is replaced with 3x = A 2x + 3y = B x + y + z = 3C / 7, and DSac and DSpr can be obtained from x / y / z.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION As a typical molding method of an optical film, a melt extrusion method in which a resin is melted and extruded from a T die or the like to form a film, and a resin is dissolved in an organic solvent and cast on a support. Examples of the solvent casting method include drying the solvent by heating to form a film, but the retardation film of the present invention is a retardation film obtained by the solvent casting method because an optical film with good thickness accuracy can be produced relatively easily. It is a film. If the thickness accuracy is poor, the unevenness due to the thickness variation acts like a lens, and there is concern that image distortion (so-called lens effect) may occur when incorporated in a liquid crystal display device, and the retardation (phase difference) Since it is represented by the product of birefringence and thickness, it may lead to in-plane variation in retardation value.

When the solvent cast method is adopted, it is preferable to collect and recycle the organic solvent that evaporates in the drying step. However, when the mixed organic solvent is used, distillation equipment is required in addition to the recovery equipment. Therefore, it is desirable to use a single solvent. The single solvent as used herein refers to a solvent that is industrially obtained as a single product, and does not indicate that there are no impurities mixed in.

The single solvent used for obtaining the retardation film of the present invention is not particularly limited, but a solvent having a lower boiling point is preferable from the viewpoint of drying efficiency, and specifically, 100 ° C.
The following low boiling point solvents are preferred. For example, ketones such as acetone (boiling point 57 ° C.) and methyl ethyl ketone (boiling point 80 ° C.), and ester solvents such as ethyl acetate (boiling point 77 ° C.) and ethyl propionate (99 ° C.) can be used. Further, halogenated hydrocarbon solvents such as methylene chloride (boiling point 40 ° C.) easily dissolve the resin material and have a low boiling point.
It is one of the suitable solvents. In addition, methylene chloride is also highly safe against fires and the like during drying, and is therefore particularly preferable as the sole solvent used when producing the retardation film of the present invention.

The retardation film of the present invention is a film obtained by casting a solution prepared by dissolving cellulose acetate propionate in a single solvent. The cellulose acetate propionate has a degree of acetyl substitution (DSac). ) And propionyl substitution degree (DSpr) satisfy the following formulas (I) and (II). 2.0 ≦ DSac + DSpr ≦ 3.0 (I) 1.0 ≦ DSpr ≦ 3.0 (II) The meaning of the formula (I) is as follows. DSa
c + DSpr represents how much the three hydroxyl groups present in the cellulose molecule are esterified on average. 3
In case of, all the hydroxyl groups are esterified. All the hydroxyl groups are esterified with either acetyl group or propionyl group, DSac + DSpr is 3
When the film made of cellulose acetate propionate is uniaxially stretched, a retardation film with negative optical anisotropy is obtained in which the direction orthogonal to the stretching direction is the slow axis direction.
Regarding the wavelength dependence of the retardation (retardation) of this retardation film, the retardation (absolute value) tends to increase as the wavelength becomes shorter. When DSac + DSpr is made smaller than 3, the ease of expression of retardation due to stretching decreases, and a film with almost no retardation even when stretched at about 2.8 to 2.9 is obtained, and DSac + DSpr is further reduced. Then,
The stretching direction becomes the slow axis direction, and the retardation film has positive optical anisotropy. Along with this, the wavelength dependence of the retardation of the retardation film shows that the retardation (absolute value) tends to be larger as the wavelength is longer, and when DSac + DSpr is further reduced, this tendency is lost and the retardation does not depend on the wavelength. The phase difference becomes constant. DSac + DSpr, which shows a constant phase difference regardless of such wavelength, is
Although it depends on the ratio of pr, it is generally in the range of 2.0 to 2.3. Then, when DSac + DSpr is further reduced, a retardation film having a larger retardation (absolute value) at shorter wavelengths is obtained, similar to the retardation film made of polycarbonate.

For the above reasons, DSac + DSpr does not exceed 3, and the display quality of the liquid crystal display device deteriorates when the retardation tends to increase as the wavelength becomes shorter. A more preferable numerical value range of DSac + DSpr is 2.3 or more and 2.9 or less, and further preferably 2.6 or more and 2.8 or less.

Next, the meaning of the formula (II) will be described. From the viewpoint of wavelength dependence described above, Japanese Patent Application Laid-Open No.
As disclosed in Japanese Unexamined Patent Publication No. 00-137116, the hydroxyl group of cellulose can be substituted with an acetyl group or a propionyl group to achieve the object.

However, in order to form a film having a good thickness accuracy by the solvent casting method, it is desirable that a high concentration solution can be prepared, and furthermore, it is possible to dissolve it in a single solvent at a high concentration. desired. From such a viewpoint, the degree of propionyl substitution (D) is higher than that of cellulose acetate propionate having a high degree of acetyl substitution (DSac).
Cellulose acetate propionate having a high Spr) has much higher solubility in an organic solvent, and a remarkable difference is recognized particularly when methylene chloride is used.
Therefore, the higher the propionyl substitution degree (DSpr) is, the more suitable is 1.0 or more and 3.0 or less, more preferably 2.0 or more and 2.9 or less, and further preferably 2.5 or more and 2.8 or less. Is.

The cellulose acetate propionate used in the present invention can be produced by a method known per se. For example, cellulose is treated with a strong caustic soda solution to give alkaline cellulose, which is acylated with a mixture of acetic anhydride and propionic anhydride. The obtained cellulose ester has a substitution degree DSac + DSpr of about 3, but by partially hydrolyzing the acyl group, a cellulose acetate propionate having a desired substitution degree can be produced. Further, by changing the ratio of acetic anhydride and propionic anhydride during the acylation, the desired degree of propionyl substitution can be obtained.

Further, the cellulose acetate propionate used in the present invention may have a substituent other than the acetyl group and the propionyl group. Examples of such substituents include other ester groups such as butyrate, and ether groups such as alkyl ether groups and araalkylene ether groups.

The preferred number average molecular weight of the cellulose acetate propionate used in the present invention is 5,000 to 10.
It is 10,000, and more preferably 10,000 to 70,000. An unnecessarily high molecular weight lowers the solubility in a solvent, and the viscosity of the obtained solution is too large to be suitable for the solvent casting method, and causes problems such as difficulty in thermoforming. On the other hand, too low a molecular weight is not preferable because it lowers the mechanical strength of the obtained film.

The cellulose acetate propionate constituting the retardation film of the present invention may be of one type, or may be a blend of two or more types with another polymer, if necessary. . As such another polymer, other cellulose esters such as cellulose butyrate and cellulose ethers such as methyl cellulose and ethyl cellulose can be preferably used.

The presence of an ester group in the molecule increases the hydrophilicity of the polymer, and if water is left present during film formation, it may have an unfavorable effect on the strength of the obtained film. It is preferable to dry the resin, pellets, solvent, etc. used for the above in advance.

Further, when forming a film, a small amount of additives such as a plasticizer, a heat stabilizer and an ultraviolet stabilizer may be added if necessary. Particularly when the obtained film is brittle, it is effective to add a plasticizer for the purpose of improving processing characteristics such as stretching.

When forming a film by the solvent casting method, the cellulose ester of the present invention is dissolved in the solvent, cast on a support and dried to form a film. The viscosity of the solution is preferably 10 poise or more and 50 poise or less, more preferably 15 poise or more and 40 poise or less. As a preferable support, a stainless steel endless belt, a film such as a polyimide film, a biaxially oriented polyethylene terephthalate film, or the like can be used.

Drying after casting can be carried out while being supported on the support, but if necessary, the film preliminarily dried until it has self-supporting property is peeled from the support and further dried. You can also The film is dried
Generally, a float method, a tenter or a roll transfer method can be used. In the case of the float method, the film itself is subjected to complicated stress, and uneven optical properties are likely to occur. Also, in the case of the tenter method, it is necessary to balance the width shrinkage of the film due to solvent drying and the tension to support its own weight, depending on the distance between the pins or clips supporting both ends of the film. There is a need. on the other hand,
In the case of the roll transfer method, the tension for stable film transfer is applied in the film flow direction (MD direction) in principle, so that it has a feature that the direction of stress can be easily made constant. Therefore, it is most preferable to dry the film by the roll transportation method. Further, drying in an atmosphere in which humidity is kept low so that the film does not absorb water when the solvent is dried is an effective method for obtaining the film of the present invention having high mechanical strength and transparency.

The thickness of the retardation film of the present invention is 10 μm.
m to 500 μm is preferable, more preferably 30 μm
To 300 μm. The light transmittance of the film is 85%
The above is preferable, and 90% or more is more preferable.
The haze of the film is preferably 2% or less, more preferably 1% or less.

In order to obtain a retardation film, the above-obtained film is stretched in at least uniaxial direction by a known stretching method to carry out orientation treatment. As the stretching method, a uniaxial or biaxial thermal stretching method can be adopted. The film of the present invention is less likely to exhibit retardation during stretching, and unlike conventional polycarbonates, it is necessary to have a large stretching ratio, so longitudinal uniaxial stretching is preferable. When the optical uniaxiality of the obtained retardation film is important, free end uniaxial stretching is a particularly preferred stretching method. In addition, JP-A-5
It is also possible to control the three-dimensional refractive index of the film by performing special biaxial stretching as disclosed in Japanese Patent Publication No. 157911. Even when the retardation is applied, the retardation due to the orientation hardly appears, so that a film having a small retardation variation in the film plane can be obtained.

The retardation value of the retardation film is in the range of more than 5 nm to 1000 nm and can be selected according to the purpose. The retardation value can be set to a desired value by controlling the film thickness, the stretching temperature and the stretching ratio. Generally, the stretching ratio is 1.05 to 4 times, and the stretching temperature is selected in the range of (Tg-30) ° C to (Tg + 30) ° C with respect to the glass transition temperature Tg. A particularly preferred stretching temperature is in the range of (Tg-20) ° C to (Tg + 20) ° C. By setting this temperature range, it is possible to prevent whitening of the film during stretching, and to reduce variations in retardation of the obtained retardation film.

Further, the photoelastic coefficient, that is, the change in the phase difference value due to the rate of change in birefringence when subjected to a stress load, is 70 × 10 ⁻¹² m ² / N in the known polycarbonate, whereas 20 for the cellulose ester film
It is not more than × 10 ^-12 m ² / N. Therefore, it is caused by the unevenness of bonding when bonded together with the liquid crystal layer or the polarizing plate, the difference in thermal expansion between the constituent materials due to the heat from the backlight or the external environment, and the influence of the stress caused by the contraction of the polarizing film. It has a small phase difference change and is particularly suitable for a large-screen liquid crystal display device.

The wavelength dependence of the phase difference is Re (514.
5) / Re (441.6). Where Re
(441.6) represents the retardation of the film measured with monochromatic light having a wavelength of 441.6 nm, and Re (514.5) represents the retardation of the film measured with monochromatic light having a wavelength of 514.5 nm. This value is 1.06 for a polycarbonate using bis (4-hydroxyphenyl) propane as a bisphenol component, and the retardation film of the present invention shows a lower retardation at longer wavelengths, whereas the retardation film of the present invention has a degree of substitution of ester substituents. Although it depends also on, 0.8 or more and 0.99 or less, More preferably, it is 0.84 or more and 0.90 or less. And Re
(441.6) <Re (514.5) <Re (632.
8) is satisfied. Such a retardation film according to the present invention is preferable in wavelength dependency of retardation, as compared with a known retardation film made of polycarbonate, and has a reflection type T
It can be suitably used for FT liquid crystal display devices and the like.

[0040]

EXAMPLES Examples of the present invention will be described below, but the present invention is not limited to these examples.

Example 1 The degree of substitution of the acetyl group was 0.
1, the degree of substitution of propionyl group is 2.6, the number average molecular weight is 7
Cellulose acetate propionate (Eastman Chemical CAP482-20) of 5000 was dissolved in methylene chloride as a solvent to a solid concentration of 15%, cast on glass and dried to a thickness of 130 μm. A uniform film was obtained. The obtained film was heated at 152 ° C. to 95
% To obtain a retardation film. The phase difference at 441.6 nm, 514.5 nm and 632.8 nm of this retardation film was measured by a microscopic polarization spectrophotometer (Oak Seisakusho).
TFM-120AFT) was used for measurement by a rotation analysis method to determine the wavelength dependence of the phase difference. In addition, the photoelastic coefficient is a microscopic polarization spectrophotometer (TFM-120 manufactured by Oak Manufacturing Co., Ltd.).
AFT) using a measurement wavelength of 514.5 nm. It was determined by fixing one of the films cut into strips having a width of 1 cm in the optical axis direction, applying a weight of 500 g to the other, and calculating the amount of change in birefringence due to unit stress.
The measurement results are summarized in Table 1.

(Example 2) The substitution degree of the acetyl group was 0.
1, the degree of substitution of propionyl group is 2.4, the number average molecular weight is 2.
Cellulose acetate propionate of 5,000 (Eastman Chemical CAP482-0.5) was dissolved in methylene chloride as a solvent to a solid concentration of 15%, cast on glass and dried to a thickness of 99 μm. A uniform film was obtained. 95% of the obtained film at 150 ° C
It stretched and the retardation film was obtained. The retardation and photoelastic coefficient of the obtained film were measured in the same manner as in Example 1. The measurement results are summarized in Table 1.

(Comparative Example 1) A polycarbonate containing bisphenol A as a bisphenol component (Teijin Kasei
C-1400) with methylene chloride as a solvent and solids concentration of 1
After melting so as to be 5%, it was cast on glass and dried to obtain a film having a thickness of 65 μm. The obtained film was stretched 10% at 160 ° C. to obtain a retardation film.
The retardation and photoelastic coefficient were measured in the same manner as in Examples 1 and 2. The measurement results are summarized in Table 1. The film is Re
(441.6) <Re (514.5) <Re (632.
Since 8) is not satisfied and the photoelastic coefficient is large, it is impossible to use an ideal wavelength-dependent λ / 4 retardation film with this film alone.

(Comparative Example 2) The substitution degree of the acetyl group was 2.4.
Cellulose Acetate (Daicel Chemical Industries L-3
When (0) was dissolved using methylene chloride as a solvent to a solid content concentration of 15%, a part of the solid remained undissolved. Even after 10 hours from the start of stirring, an undissolved portion remained, and when only methylene chloride was used as a solvent, a uniform film could not be obtained.

(Comparative Example 3) The substitution degree of the acetyl group is 0.0.
3, degree of substitution of propionyl group 1.9, number average molecular weight 1
Cellulose acetate propionate (CAP504-0.2 manufactured by Eastman Chemical Co., Ltd.) of 5,000 was dissolved in methylene chloride as a solvent to a solid concentration of 15%, cast on glass and dried to a thickness of 70 μm. To obtain a uniform film. The obtained film was stretched at 160 ° C. by 95% to obtain a retardation film. The retardation and photoelastic coefficient of the obtained film were measured in the same manner as in Example 1. The measurement results are summarized in Table 1. The film had Re (441.6) <Re (514.5) <Re (63
Since 2.8) is not satisfied and the photoelastic coefficient is large, it is impossible to obtain an ideal wavelength-dependent λ / 4 retardation film by this film alone.

[0046]

[Table 1]

[0047]

According to the present invention, it is possible to obtain a retardation film having a large retardation as the wavelength becomes longer and a low photoelasticity with a single film of cellulose acetate. Further, the film can be produced at a low cost by using a solvent casting method using methylene chloride.

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Claims (5)

[Claims] 1. A retardation film obtained by casting a solution prepared by dissolving cellulose acetate propionate in a single solvent, the degree of acetyl substitution (DSac) and propionyl substitution of the cellulose acetate propionate. The retardation film whose degree (DSpr) satisfies the following formulas (I) and (II). 2.0 ≦ DSac + DSpr ≦ 3.0 (I) 1.0 ≦ DSpr ≦ 3.0 (II) 2. Retardation for monochromatic light of 441.6 nm (Re (441.6)) and retardation for monochromatic light of 514.5 nm (Re (514.
5)) and the retardation (Re (632.8)) for monochromatic light of 632.8 nm is Re (441.
The retardation film according to claim 1, wherein 6) <Re (514.5) <Re (632.8) is satisfied. 3. The retardation film according to claim 1, wherein the film is stretched in at least a uniaxial direction. 4. A retardation (Re (514.5)) for monochromatic light of 514.5 nm is 5 to 1000 n.
m, and and a phase difference film according to any one of claims 1 to 3, photoelastic coefficient is equal to or less than ^{20 × 10 -12 m 2 / N} . 5. The retardation film according to claim 1, wherein the single solvent is methylene chloride.