WO1999001788A1

WO1999001788A1 - An optically functional film, a method of producing same, and a liquid crystal display

Info

Publication number: WO1999001788A1
Application number: PCT/EP1998/004017
Authority: WO
Inventors: Masatoshi Ishimaru
Original assignee: Akzo Nobel N.V.; Sekisui Chemical Co., Ltd.
Priority date: 1997-07-04
Filing date: 1998-06-18
Publication date: 1999-01-14
Also published as: JPH1124078A

Abstract

The invention pertains to a method of producing optically functional film which gives excellent productivity and is low in cost without reducing the orientation of the substrate, by using a specific solvent and a specific film as the substrate. The method of producing the optically functional film comprises the steps of laminating a high-molecular liquid crystal material onto a rubbed cellulose derivative film with a substitution degree of 0.6 to 2.8 by casting a solution of the high-molecular liquid crystal material in an aromatic ether and/or an aromatic ester, and orienting it.

Description

AN OPTICALLY FUNCTIONAL FILM, A METHOD OF PRODUCING SAME, AND A LIQUID CRYSTAL DISPLAY

The present invention relates to an optically functional film in which use is made of the orientation of a high-molecular liquid crystal material, a method of producing same, and a liquid crystal display. More particularly, it relates to a method of obtaining an optically functional film for compensating or adjusting phase difference or optical rotatory power using the orientation of a high-molecular liquid crystal polymer, an optically functional film obtained by said method, and a liquid crystal display using said optically functional film.

Optically functional films in which use is made of the orientation of a high- molecular liquid crystal material phase difference plates, optical rotators, selective wave reflectors, etc. are under study. Such optically functional films have a structure consisting of a substrate made of a transparent isotropic material and a high-molecular liquid crystal material laminated onto the substrate.

For the substrate use is made of a glass sheet or a plastic sheet with little residual phase difference, such as a cellulose triacetate film. Moreover, to orient a high-molecular liquid crystal material usually the substrate itself is rubbed directly before lamination, or the substrate covered with an orienting film made of a polyimide, etc. is rubbed.

As a method of laminating a high-molecular liquid crystal material onto a substrate generally a solution for casting is used, and in this case the high- molecular liquid crystal material is oriented by heating up to higher than its clearing temperature (Tc), followed by cooling to a temperature between the glass transition temperature (Tg) and the clearing temperature (e.g., Japanese Patent Laid-Open No. 1991-87720).

Substrates available for laminating a high-molecular liquid crystal material are those rubbed directly themselves to be oriented, such as a cellulose triacetate film, and those covered with an orienting film made of a polyimide, etc. If the two types are compared, substrates rubbed directly themselves without using any orienting film provide better productivity and are low in cost. In addition, if the heat-resisting temperature of substrates such as cellulose triacetate film is lower than the baking temperature of the orienting film, a substrate covered with an orienting film such as a polyimide film cannot be used.

On the other hand, for use as an optically functional film it is required that the substrate be highly transparent and have as little residual phase difference as possible. Materials satisfying these conditions include glass, cellulose triacetate, etc., but considering impact resistance and the reduction of film weight, a plastic film such as cellulose triacetate film is preferred.

Japanese Patent Laid-Open No. 1994-75214 discloses a method comprising the steps of the surface of a plastic substance used as the transparent protective layer of a polarizing plate being directly rubbed and a high-molecular liquid crystal material being laminated onto the rubbed surface.

Japanese Patent Laid-Open No. 1994-75214 discloses a method comprising the steps of the surface of a transparent protective layer being directly rubbed and laminated with a high-molecular liquid crystal material, as described above. However, no clear mention is made of any solvent for dispersing the high-molecular liquid crystal material, nor of the lamination conditions. Furthermore, the method according to the prior art has the problems that if the surface of a plastic material is rubbed directly without using any orienting film, followed by a high-molecular liquid crystal material being laminated onto it by solution casting, the substrate may be eroded by the solvent used. Moreover, even if the surface of the substrate seems free from erosion to the naked eye, it may nevertheless be microscopically disturbed (disordered), and in such a case it is difficult to orient the high- molecular liquid crystal material uniformly. Besides, if the surface protecting layer is formed by a cellulose resin, it may be that the temperature for removing the solvent by drying a high-molecular liquid crystal polymer solution after lamination and the temperature necessary for orientation treatment cause the substrate to be deformed, impairing the smoothness of the film, thus preventing uniform orientation from being achieved, because the cellulose resin has lower heat resistance than ordinary engineering plastics.

To solve such problems as the heat resistance of the substrate to the orientation treatment temperature for the high-molecular liquid crystal material and the solvent resistance of the substance to the solvent used for the high-molecular liquid crystal material, Japanese Patent Laid-Open No. 1991-333313 proposes a method comprising the steps of coating a plastic film having sufficient heat resistance and solvent resistance with a high- molecular liquid crystal material, orienting the whole, and transferring it onto a highly transparent and optically isotropic substrate made of cellulose triacetate, etc. by using an adhesive. However, transfer by using an adhesive requires the extra step of transferring a high-molecular liquid crystal material, as well as a plastic film to be temporarily coated with the high-molecular liquid crystal material before the transfer is required, thus raising the production costs. The object of the present invention is to eliminate the problems of said prior art, i.e., to provide a method for producing an optically functional film which does not require any orienting film and which is low in cost and gives excellent productivity, and in which the smoothness of the substrate and the orientation of the high-molecular liquid crystal material are not impaired by the solvent. The other objects of the present invention are to provide an optically functional film obtained by said production process, and a liquid crystal display in which said optically functional film is used.

The inventors surprisingly have found that it is possible to obtain an optically functional film containing a high-molecular liquid crystal material laminated onto a substrate by solution casting and which is then oriented without any defect caused to the substrate by the solvent and with the high- molecular liquid crystal material uniformly oriented, viz. by using a film made of a cellulose derivative with a specific substitution degree as the substrate for laminating, rubbing the surface of the film, coating the film with a high-molecular liquid crystal material solution by using a specific solvent, and heat-treating at higher than the liquid crystal clearing temperature.

The version of the present invention is a method of producing an optically functional film, comprising the steps of laminating a high-molecular liquid crystal material onto a rubbed cellulose derivative film having a substitution degree of 0.6 to 2.8 by casting a solution of the high-molecular liquid crystal material in an aromatic ether and/or an aromatic ester, removing the solvent, and orienting it.

The cellulose derivative preferably is cellulose acetate, more preferably, said cellulose is formed as the protective layer of a polarizing plate. In the case of the method of producing optically functional film, preferably said aromatic ether is anisole or phenetole.

In the case of the method of producing optically functional film, preferably said aromatic ester is methyl benzoate or ethyl benzoate.

In the case of the method of producing optically functional film, a vitreous liquid crystal material is used as the high-molecular liquid crystal material, and in the case of the method of producing optically functional film, a polyether based high-molecular liquid crystal polymer is used as the high- molecular liquid crystal material.

Moreover, preferably the high-molecular liquid crystal material is a material showing twisted nematic orientation.

The optically functional film is obtained by the method of producing optically functional film according to this invention.

Another version of the present invention is a liquid crystal display comprising the optically functional film as described above and a liquid crystal display cell.

Substrate

As stated above, in the present invention the substrate onto which a high- molecular liquid crystal material is to be laminated is a film made of a cellulose derivative with a specific substitution degree, because it has excellent transparency and because the stress is hard to maintain. The substitution degree is represented by m in the general formula [C₆H₇0₂(OH)₃._m(OR)_m], which stands for a cellulose derivative in which the hydroxy groups of cellulose are substituted by hydrogen atoms, and the value should be 0.6 to 2.8. If the substitution degree is less than 0.6, the surface condition is changed by moisture because the number of hydroxy groups is large enough for the effect of rubbing to be lost, hence the high- molecular liquid crystal material cannot be uniformly oriented. If the substitution degree is more than 2.8, although the film's appearance does not show any sign of erosion attributable to the solvent used in the present invention, the surface nevertheless is eroded microscopically, the effect of rubbing thus being lost. Therefore, the high-molecular liquid crystal material cannot be oriented uniformly.

The substituent groups can be carboxylic acid groups such as acetyl groups and propionyl groups, nitro groups and alkyl groups. Said cellulose derivative can be selected, for example, from cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, nitrocellulose, methyl cellulose, ethyl cellulose, etc. Among these, cellulose acetate is most preferred.

The thickness of the substrate is not especially limited, but if the substrate is mounted as a member of an optical device such as a phase difference plate, etc., the thickness preferably is 10 μm or more in view of the handling convenience for mounting.

The substitution degree of the cellulose derivative can be measured by titrating the substituent groups. For example, a general method for cellulose acetate comprises the sample being dissolved in acetate and acetyl groups being saponified by a sodium hydroxide aqueous solution, in order to titrate the amount of sodium hydroxide used and determine the acetylation degree. The method for determining the substitution degree of a cellulose derivative as described above is described in detail in the New Handbook on the Analysis of High Polymers (written in Japanese) (Kinokuniya Shoten), pp. 748 ~ 753.

Rubbing

The surface of the substrate can be rubbed by any one of all publicly known means, and the method is not especially limited. For example, while a long substrate film is being driven to run, a rubbing roll installed so as to enable it to be set at an optional angle in the running direction of the substrate film is rotated in contact with the surface of the substrate, in order to rub it. The substrate feeding speed, the rubbing angle, the rotation speed of the rubbing roll, the rubbing pressure, etc. can be properly set to achieve the intended rubbing effect.

The material of the surface of the rubbing roll, etc. used is not especially limited. However, in view of the fact that the optically functional film obtained according to the method of producing optically functional film of the present invention is used for optical applications, it is preferable to select a material unlikely to be flawed, such as the felt of nylon, cotton or polyester, etc.

Lamination of the high-molecular liquid crystal material For laminating a high-molecular liquid crystal material onto the rubbed surface of the substrate, solution casting using any aromatic ether or aromatic ester or any one of their mixtures as a solvent can be employed. With a view to the heat resistance of the film at the time when the solvent is removed by drying, it is desirable that the boiling point of the solvent is lower than about 200°C. Therefore, anisole and/or phenetole is preferred as the aromatic ether, and methyl benzoate and/or ethyl benzoate is preferred as the aromatic ester.

To control the orientation of the rubbed substrate, preference is given to any one of the above solvents. In the case of the present invention, the high-molecular liquid crystal material must be dissolvable in the solvent.

The high-molecular liquid crystal material satisfying this condition which is used in the present invention usually has a molecular weight of 1 ,000 to

200,000, though it is not specifically limited thereto.

Since it is desired that the orientation will not be disturbed in the ordinary service environment of the optically functional film, the clearing temperature of the high-molecular liquid crystal material preferably is 50°C or higher.

Furthermore, considering that the substrate must not be deformed at the orientation temperature, it is desired that the clearing temperature of the high-molecular liquid crystal material is lower than 120°C.

To prevent the orientation from being disturbed in the service environment of the optically functional film, a high-molecular liquid crystal material with a glass transition temperature Tg of lower than 50°C can be used, and if such a material is used, the liquid crystal material can be cross-linked to stabilize the orientation.

The high-molecular liquid crystal material can be a vitreous liquid crystal material, or it can be a side chain-type high-molecular liquid crystal polymer based on a polyether, polysiloxane, polyacrylate, polymethacrylate or poly- vinyl ether, etc.

Preferably, the high-molecular liquid crystal material is a vitreous liquid crystal material which can be selected, for example, from those disclosed in WO 96/03476A1. WO 96/03476A1 discloses vitreous high-molecular liquid crystal polymers including the compounds represented by the following general formula (1):

wherein R., stands for an aromatic group with 5 to 24 carbon atoms, an aromatic group with an aliphatic group with 6 to 24 carbon atoms, a heterocyclic group with 4 to 24 carbon atoms, or an alicyclic group with 6 to 24 carbon atoms; R₂ stands for H bonded to a spacer group, a non- mesogen group, or a mesogen group; R₃, R₄, and R₅ can be independently selected from the groups enumerated for R₂; and 25% or less of all of R₂, R₃, R₄, and R₅ are H or non-mesogen groups.

More preferred is a polyether based high-molecular liquid crystal polymer obtained by radical polymerization, which can be obtained by polymerizing a monomer composition comprising a compound with a hydroxy group and a monoepoxide with a mesogen group, as disclosed in WO 96/06145A1.

When the hydroxy group-containing compound is an acrylate alcohol, a liquid crystal polyether containing acrylate groups can be obtained. The polyether, after having been oriented, can be UV polymerized in the presence of a UV initiator or UV cross-linked by adding a diacrylate or a triacrylate, in order to improve its stability. Of course, thermal polymeri- zation can also be adopted instead of UV polymerization.

In the polyether based high-molecular liquid crystal polymer disclosed in said WO 96/06145A1 , the molar ratio of epoxy groups to OH groups in the monomer composition preferably is in the range of 10:1 ~ 2:1. More preferably, the OH group-containing compound is a compound with one OH group represented by the following general formula (2):

HO-(Y)_m-Z (2)

wherein Z stands for H, -O-C(O)-CH=CH₂, -O-C(O)-C(CH₃)=CH₂, a cyclic, aromatic or heterocyclic compound with 4 to 10 carbon atoms, -CH(CH₂-O- C(O)-CH=CH₂)₂, -C(CH₂-O-C(O)-CH=CH₂)₃, -C(CH₂-O-C(O)-CH=CH₂)₂CH₃, -CH(CH₂-O-C(O)-C(CH₃)=CH₂)₂, -C(CH₂O-C(O)-C(CH₃)=CH₂)₃, or

-C(CH₂O-C(O)-C(CH₃)=CH₂)₂CH₃; each Y stands independently for -CH₂-, -C(CH₃)₂-, -CH(CH₃)-, -HC[-(CH₂)_m-O-φ₁-(Q)_n-φ₂-R₁]-; m stands for an integer of 0 to 6; a compound with an oxygen atom at the α position or β position to said OH group is excluded; Q for the Y group stands for -C(O)-O-, -C=C, -C=N-, -N=C-, -O-C(O)-, -C≡C- or -N-N-; R₁ stands for -O-R₂, -NO₂, -CN, -HC=C(CN)₂, -C(CN)=C(CN)₂, or -R₂; R₂ stands for an alkyl group with 1 to 15 carbon atoms, -(CH₂)_k-O-C(O)-CH=CH₂, -(CH₂)_k-O-C(O)-C(CH₃)=CH₂ or -(CH₂)_x-OH; x stands for an integer of 0 to 6; k stands for an integer of 0 to 6; if R^ stands for -O-R₂, k is neither 0 nor 1 ; φ., stands for a substituted or non-substituted cyclic, aromatic or heterocyclic compound with 4 to 10 carbon atoms; φ₂ stands for a cyclic, aromatic or heterocyclic compound with 4 to 10 carbon atoms; and n stands for 0 or 1.

The high-molecular liquid crystal material used in the optically functional film of the present invention allows homeotropic orientation and twisted nematic orientation. The best phase difference compensation effect can be obtained in combination with an STN-type liquid crystal display cell when the optically functional film is twisted at the same twist angle as the liquid crystal of the liquid crystal display cell, but in the reverse direction. Therefore, when the optically functional film of the present invention is used as a phase difference film in combination with an STN-type liquid crystal display cell, the twist angle of the high-molecular liquid crystal polymer of the optically functional film is kept equal to the twist angle of the liquid crystal of the liquid crystal display cell, but the direction is reversed.

In the optically functional film of the present invention, to control the twisting direction (right-hand or left-hand) of the high-molecular liquid crystal material, and to achieve the desired twist angle, an optically active substance called a chiral dopant can also be added to the high-molecular liquid crystal material. The optically active substance can be any optional optically active substance, and can be selected, for example, from cholesterol derivatives, 2-octyl 4-(4-hexyloxybenzoyloxy)benzoate, etc. The optically active substance is usually added in an amount of 10 wt.% or less, based on the total weight of the high-molecular liquid crystal material.

Optically active groups can also be introduced into the high-molecular liquid crystal material.

Furthermore, in the present invention, the solution casting can be by any publicly known method and is not specifically limited. For example, it can be achieved by roll coating, knife coating, die coating, gravure coating, offset gravure coating, microgravure coating, curtain coating, lip coating, immersion or spin coating, etc. The substrate is coated with a high- molecular liquid crystal material solution according to any one of these methods and dried by hot air drying or infrared heating, etc., to remove the solvent.

Orientation

The high-molecular liquid crystal material is oriented by heat treating to higher than the temperature required to transform the high-molecular liquid crystal material from the vitreous (glassy) state to the liquid crystal phase, and cooling. The treatment temperature must of course be within the heat resisting temperature range of the substrate.

Optically functional film The optically functional film of the present invention is obtained by the method of producing optically functional film according to the present invention and has a high-molecular liquid crystal material laminated onto a substrate and oriented as described above.

Liquid crystal display

The liquid crystal display of the present invention is composed of the optically functional film of the present invention and a liquid crystal display cell. While the liquid crystal display cell is not specifically limited, for compensating phase difference an STN-type liquid crystal display cell, etc. can be used.

Application

The optically functional film obtained by the method of producing optically functional film according to the present invention can be used as a phase difference plate for compensating phase difference in said liquid crystal display, optical rotor, or selective wave reflector, etc.

Action

In the method of producing optically functional film according to the present invention a film made of a cellulose derivative with a substitution degree of 0.6 to 2.8 is used as the substrate. Thus, the orientation effected by rubbing can be sufficiently controlled, and even when a high-molecular liquid crystal material is laminated by solution casting using an aromatic ether and/or aromatic ester as the solvent, the substrate is unlikely to be eroded by the solvent. Therefore, the high-molecular liquid crystal material can be oriented as desired, and the oriented state is not disturbed.

In the method of producing optically functional film according to the present invention, a film made of a cellulose derivative with a specific substitution degree is used as the substrate, and a high-molecular liquid crystal material is laminated onto it, with said aromatic ether and/or aromatic ester being used as the solvent. Therefore, damage to the substrate by a solvent is prevented, and the high-molecular liquid crystal material can be oriented uniformly.

The present invention is described below based on examples, but is not limited thereby or thereto.

Examples

Preparation of the high-molecular liquid crystal material Epoxide of a methoxyphenylbenzoate and a methoxyphenyl(2,3-dihydroxy- propyloxy)benzoate were mixed at a ratio by weight of 5:1 , to prepare a polyether based high-molecular liquid crystal polymer. In this case, the preparation of the respective monomers and the polymerization were as described in Example 1 of WO 96/06145A1.

The polyether based high-molecular liquid crystal polymer obtained as above had a weight average molecular weight of 2984, a glass transition temperature of 45 to 51 °C, and a transition point Tc from liquid crystal to isotropic phase of 146°C.

Example 1

A 80 μm thick film of cellulose acetate with a substitution degree of 2.5 was rubbed using nylon felt, continuously coated with an anisole solution containing 23 wt.% of said polyether based high-molecular liquid crystal polymer using a microgravure coater, to form a thickness of 5 μm, and retained in a 110°C heating zone provided on the line for 3 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Example 2

A 80 μm thick film of cellulose acetate with a substitution degree of 1.8 was rubbed using polyester felt, continuously coated with a methyl benzoate solution containing 19 wt.% of said polyether based high-molecular liquid crystal polymer using an offset gravure coater, to form a thickness of 4 μm, and retained in a 120°C heating zone provided on the line for 5 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Example 3

A 60 μm thick film of cellulose acetate with a substitution degree of 0.9 was rubbed using polyester felt, coated with a phenetole solution containing 26 wt.% of a mixture consisting of said polyether based high-molecular liquid crystal polymer and 5 wt.% of a chirality agent (CB15 produced by Merck) using a spin coater, to form a thickness of 6 μm in batch operation, and retained in a 115°C heating dryer for 4 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Example 4

A 70 μm thick film of cellulose propionate with a substitution degree of 2.1 was rubbed using polyester felt, continuously coated with an anisole solution containing 23 wt.% of said polyether based high-molecular liquid crystal polymer using a microgravure coater, to form a thickness of 4 μm, and retained in a 110°C heating dryer for 3 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Example 5 A 80 μm thick film of cellulose acetate with a substitution degree of 2.5 was rubbed using nylon felt, continuously coated with a solution with 23 wt.% of said polyether based high-molecular liquid crystal polymer dissolved in a mixed solvent consisting of anisole and phenetole mixed at a ratio by weight of 2:1 , using a microgravure coater, to form a thickness of 5 μm, and retained in a 110°C heating zone provided on the line for 4 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Comparative Example 1 A 80 μm thick film of cellulose acetate with a substitution degree of 3.0 was rubbed using nylon felt, continuously coated with an anisole solution containing 23 wt.% of said polyether based high-molecular liquid crystal polymer using a microgravure coater, to form a thickness of 5 μm, and retained in a 110°C heating zone provided on the line for 3 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer. Comparative Example 2

A 80 μm thick film of cellulose acetate with a substitution degree of 0.4 was rubbed using nylon felt, continuously coated with a methyl benzoate solution containing 19 wt.% of said polyether based high-molecular liquid crystal polymer using a microgravure coater, to form a thickness of 4 μm, and retained in a 120°C heating zone provided on the line for 5 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Comparative Example 3

A 80 μm thick film of cellulose acetate with a substitution degree of 2.5 was rubbed using nylon felt, continuously coated with a cyclohexanone solution containing 30 wt.% of said polyether based high-molecular liquid crystal polymer using a microgravure coater, to form a thickness of 5 μm, and retained in 110°C heating zone provided on the line for 3 min to evaporate the solvent, in order to orient the high-molecular liquid crystal polymer.

Evaluation

The optically functional films of the examples of the present invention and the comparative examples obtained as described above were evaluated for (1) solvent resistance and (2) uniformity of orientation according to the following procedures.

(1) Solvent resistance of film In the examples of the present invention and the comparative examples, the surface of each substrate (cellulose derivative film) was visually observed immediately after coating with a high-molecular liquid crystal material solution, to examine whether the substrate had deteriorated. (2) Uniformity of orientation

An optically functional film was inserted between a pair of polarizing plates arranged as crossed Nicols to form an angle of 45 degrees between the orienting axis of the high-molecular liquid crystal material and the absorbing axis of the polarizing plates, to evaluate the coloring uniformity. More specifically, in a range of 25 x 25 mm, the phase difference values were measured at 5 mm pitch, and the rate (%) of the standard deviation to the mean value was used as the uniformity of orientation.

Table 1

Example 1 Example 2 Example 3 Example

Material Cellulose Cellulose Cellulose Cellulose acetate acetate acetate propionate

Substitution degree 2.5 1.8 0.9 2.1 Solvent Anisole Methyl Phenetole Anisole benzoate

Concentration of solution 23 wt.% 19 wt.% 26 wt.% 23 wt.% Coating method Microgravure Offset Spin coat

Microgravure gravure

Drying and orienting 110°C, 3 mm 120°C, 5 min 115°C, 4 min 110°'C, 3 mm conditions

Solvent resistance Not Not Not Not deteriorated deteriorated deteriorated deteriorated Uniformity of orientation 0.2% 0.3% 0.2% 0.2% Table 2

Example 5 Comparative Comparative Comparative

Example 1 Example 2 Example 3

Material Cellulose Cellulose Cellulose Cellulose acetate acetate acetate acetate

Substitution degree 2.5 3.0 0.4 2.5

Solvent Anisole: Anisole Methyl Cyclohexanone phenetole benzoate

=2:1

Concentration of solution 23 wt.% 23 wt.% 19 wt.% 30 wt.%

Coating method Microgravure Microgravure Microgravure Microgravure

Drying and orienting 110°C, 4 mm 110°C, 3 min 120°C, 5 mm 110°C, 3 mm conditions

Solvent resistance Not Not Not Not deteriorated deteriorated deteriorated deteriorated

Uniformity of orientation 0.3% 4.6% 9.7% 20.4%

As can be seen from Table 2, Comparative Example 3, the use of solution casting with cyclohexanone as the solvent to laminate the high-molecular liquid crystal material resulted in the cellulose acetate substrate being corroded at the surface immediately after coating. Furthermore, the use of cyclohexanone as the solvent resulted in a uniformity of orientation as poor as 20.4%, showing the orientation to be highly disturbed.

In Comparative Example 1 , the use of anisole as the solvent resulted in the cellulose acetate substrate not being corroded at the surface immediately after coating, but since the cellulose acetate film used had a substitution degree of 3.0, the uniformity of orientation was as comparatively poor as 4.6%.

In Comparative Example 2, the use of methyl benzoate as the solvent resulted in the cellulose acetate film substrate not being corroded at the surface immediately after coating, but since the cellulose acetate film used had a substitution degree of 0.4, the uniformity of orientation was poor as 9.7%.

By contrast, in Examples 1 to 5 the cellulose derivative film substrates used conformed to the present invention in substitution degree, and an aromatic ether and/or aromatic ester was used as the solvent. Therefore, the substrates were not corroded at their surfaces immediately after coating, and the uniformity of orientation was very good (0.3% or less). Thus it can be seen that the high-molecular liquid crystal material was uniformly oriented.

Effects of the invention

In the case of the version of the present invention, a film made of a cellulose derivative with a substitution degree of 0.6 to 2.8 is rubbed, and a high-molecular liquid crystal material is laminated onto the substrate by solution casting, using an aromatic ether and/or aromatic ester as the solvent, and it is oriented. Therefore, it is unlikely that the substrate will be eroded by the solvent and the orientation of the high-molecular liquid crystal material disturbed (disordered). Consequently, an optically functional film of excellent quality can be produced by a method giving excellent productivity at low cost and without requiring any orienting film.

In the case of the version of the present invention, the cellulose derivative is cellulose acetate, and since it is high in transparency and has little residual phase difference, an optically functional film of higher quality can be provided.

If the cellulose derivative is the protective layer of a polarizing plate, said protective layer of a polarizing plate can be used as the substrate of the optically functional film of the present invention when it is laminated onto the polarizing plate. So, a structure with excellent optical properties can be obtained by a simpler process.

If anisole or phenetole with a boiling point of lower than 200°C is used as the aromatic ether, deforming of the cellulose derivative film can be reliably prevented when the solvent is removed by drying.

Similarly, if methyl benzoate or ethyl benzoate with a boiling point of lower than 200°C is used as the aromatic ester, deforming of the cellulose film can be reliably prevented when the solvent is removed by drying.

If the high-molecular liquid crystal material is a vitreous liquid crystal material, the orientation is not easily disturbed.

If the high-molecular liquid crystal material is a polyether based high- molecular liquid crystal polymer, it is easily dissolved in the solvent specified in the present invention. Furthermore, since it is relatively low in Tg, its orientation can easily be achieved using a substrate with low heat resistance, to provide an optically functional film which is stable during the orientation of the high-molecular liquid crystal polymer.

If the high-molecular liquid crystal material is a material showing torsional nematic orientation, an optically functional film suitable as a phase difference plate for compensating the phase difference of an STN-type liquid crystal display, etc. can be obtained.

If the method according to the present invention is used to obtain an optically functional film, the optically functional film obtained can be uniform and stable during the orientation of the high-molecular liquid crystal polymer, and hence suitable as a phase difference plate for compensating the phase difference of an STN-type liquid crystal display, etc.

A liquid crystal display capable of effectively compensating the phase difference of the liquid crystal display cell by an optically functional film can be provided.

Claims

1. A method of producing an optically functional film, comprising the steps of laminating a high-molecular liquid crystal material onto a rubbed cellulose derivative film having a substitution degree of 0.6 to 2.8 by casting a solution of the high-molecular liquid crystal material in an aromatic ether and/or an aromatic ester, and orienting it.

2. The method according to claim 1 wherein the cellulose derivative is cellulose acetate.

3. The method according to claim 1 or 2 wherein the cellulose derivative film is used as the protective layer of a polarizing plate.

4. The method according to any one of claims 1 to 3 wherein the high- molecular liquid crystal material is dissolved in anisole or phenetole, or a mixture thereof.

5. The method according to any one of claims 1 to 4 wherein the high- molecular liquid crystal material aromatic ester is dissolved in methyl benzoate or ethyl benzoate, or a mixture thereof.

6. The method according to any one of claims 1 to 5 wherein as high- molecular liquid crystal material a vitreous liquid crystal material is used.

The method according to any one of claims 1 to 5 wherein as high- molecular liquid crystal material a polyether based high-molecular liquid crystal polymer is used.

8. The method according to any one of claims 1 to 7 wherein a high- molecular liquid crystal material with torsional nematic orientation is used.

9. An optically functional film obtainable by the method of claim 1..

10. A liquid crystal display comprising the optically functional film of claim 9 and a liquid crystal display cell.