Film

Details Film Film

2000-08-31

2003-07-29

Huff, Mark F. (Department: 1756)

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

C428S001400

Reexamination Certificate

active

06599443

ABSTRACT:

The invention relates to an optical retardation film comprising a layer of an anisotropic polymer material with an optical axis substantially parallel to the plane of the layer, said optical retardation film being obtainable by a process comprising the following steps
A) coating a polymerizable mesogenic composition essentially consisting of
a) 10 to 50% by weight of one or two polymerizable mesogenic compounds of formula la and 5 to 35% by weight of one or two polymerizable mesogenic compounds of formula Ib
 wherein
W is H or CH
3
,
n is an integer from 3 to 6, and
R is alkyl or alkoxy with 1 to 8 C atoms,
b) 15 to 60% by weight of a polymerizable mesogenic compound of formula II
 wherein
W is H or CH
3
,
n is an integer from 3 to 6,
Z
1
and Z
2
are each independently —COO— or —OCO—, and
X
1
and X
2
are each independently H or CH
3
, and
c) 0.1 to 8% by weight of a photoinitiator, said mixture optionally being dissolved in an organic solvent in a concentration of up to 50% by weight, on a substrate in form of a layer,
B) aligning the polymerizable mesogenic composition into a homogeneous orientation,
C) polymerizing the polymerizable mesogenic composition by exposure to UV light, and
D) optionally removing the substrate from the polymerized material,
 characterized in that
I) 50 to 2500 ppm of a mixture of non-ionic fluoroalkyl-alkoxylate surfactants selected from the following formulae
C
n
F
2n+1
SO
2
N(C
2
H
5
)(CH
2
CH
2
O)
x
CH
3
  III
C
n
F
2n+1
(CH
2
CH
2
O)
x
H  IV
with n being an integer from 4 to 12 and x being an integer from 5 to 15,
are added to the polymerizable mesogenic composition prior to or during step A) of said process, and
II) a PET or TAC film is used as the substrate onto which said polymerizable mesogenic composition is coated in step A) of said process, and
III) prior to step A) of said process the surface of said PET or TAC film adjacent to said polymerizable mesogenic composition is rubbed unidirectionally or is covered by a polyimide layer which is rubbed unidirectionally.
The invention furthermore relates to a process of preparing an optical retardation as described above, to the use of such an optical retardation film in liquid crystal displays, and to a liquid crystal display device comprising a liquid crystal cell and such an optical retardation film.
Optical retardation films are used for light modulation in various optical applications. For example, a quarter wave retardation film (QWF) can transform circularly polarized light into linearly polarized light. Furthermore, optical retardation films can be used as compensators to improve the optical properties of liquid crystal displays, such as the contrast ratio or gray scale at large viewing angles. For specific applications, birefringent optical films with an extraordinary optical axis parallel to the plane of the film are needed, which are also known as A-plates.
For use as A-plates, uniaxially stretched films of a prefabricated isotropic or LC polymer have been suggested in prior art. PCT International Application No. PCT/EP97/03676, which was published as WIPO International Publication No. WO 98/04651, describes an optical retardation film comprising a layer of anisotropic polymers material with an optical axis that has a low tilt angle relative to the plane of the layer, which is prepared by coating a polymerizable mesogenic mixture onto a substrate, aligning and curing the mixture.
However, when preparing such an optical retardation film it is often difficult to achieve planar alignment of the anisotropic polymer material with a low tilt angle between the optical axis and the plane of the film, especially with a tilt angle that is close to or substantially zero.
Thus, it is still desirable to have available an A-plate optical retardation film that can be used to improve the optical properties of a liquid crystal display, is easy to fabricate in large scale as a flexible film with a large area, and has an optical axis substantially parallel, with substantially zero tilt angle, to the plane of the film.
One of the aims of the present invention is to provide such an optical retardation film. Another aim of the invention is to provide a process of preparing such an optical retardation film. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It has been found that these aims can be achieved by providing an optical retardation film according to the present invention.
One object of the present invention is an optical retardation film comprising a layer of an anisotropic polymer material with an optical axis substantially parallel to the plane of the layer, said optical retardation film being obtainable by a process comprising the following steps A) to D) as described above.
Preferred embodiments of the present invention relate to
an optical retardation film, wherein the tilt angle between the optical axis and the plane of the layer of said anisotropic polymer material is from 0 to 1 degrees.
an optical retardation film obtainable by a process as described above, wherein said mixture of non-ionic fluoroalkyl-alkoxylate surfactants is comprising 50 to 2500, preferably 100 to 1200 ppm of at least two compounds of formula III.
an optical retardation film obtainable by a process as described above, wherein said mixture of non-ionic fluoroalkyl-alkoxylate surfactants is comprising 50 to 2500, preferably 800 to 1800 ppm of at least two compounds of formula IV.
an optical retardation film obtainable by a process as described above, wherein in the compound of formula II X
1
is H, X
2
is CH
3
, Z
1
is —COO— and Z
2
is —OCO—.
an optical retardation film obtainable by a process as described above, wherein in the compounds of formula Ia, Ib and II W is H.
an optical retardation film obtainable by a process as described above, wherein in step A) of said process a PET film is used as a substrate, the surface of which being adjacent to the polymerizable mesogenic composition is rubbed unidirectionally prior to step A).
Another object of the present invention is a process of preparing an optical retardation film comprising a layer of an anisotropic polymer material with an optical axis substantially parallel to the plane of the layer, wherein said process comprises steps A) to D) as described above and below.
Another object of the present invention is the use of an optical retardation film as described above and below in liquid crystal displays.
Yet another object of the present invention is a liquid crystal display device comprising a liquid crystal cell and an optical retardation film as described above and below.
The retardation of the inventive optical retardation film is preferably ranging from 20 to 600 nm. Particularly preferred are films with a retardation from 25 to 170 nm, furthermore films with a retardation from 300 to 600 nm.
The optical retardation film according to the present invention is suitable for use as phase retarder or as compensator in electrooptical displays, in particular for displays based on the twisted nematic (TN) effect, such as TN, STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated birefringence), DAP (deformation of aligned phases), VAN/VAC (vertically aligned nematic/cholesteric), OMI (optical mode interference), or SBE (super birefringence effect), including displays of the active and passive matrix type, furthermore in displays of the guest-host-type, the IPS (in plane switching) type, the ferroelectric or antiferroelectric type.
It is also possible, for example, to use an inventive optical retardation film as a QWF together with a broadband reflective polarizer, wherein the retardation of the optical retardation film is substantially 0.25 times the central wavelength of the wavelength band reflected by the reflective polarizer.
The inventive optical retardation film comprises a layer of a polymerized mesogenic material and is characterized by a significantly high birefringence. Furthermore

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