Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing...
Reexamination Certificate
2001-05-18
2003-10-21
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
C349S115000, C349S185000, C349S176000, C252S299700
Reexamination Certificate
active
06636291
ABSTRACT:
BACKGROUND AND SUMMARY
1. Technical Field
The invention relates to a method of manufacturing a liquid crystal display device comprising liquid crystalline material dispersed between a first and a second substrate, the first substrate comprising a patterned layer of a polymerized material having a cholesteric order wherein the axis of the molecular helix extends transversely to the layer.
The invention further relates to a method of manufacturing a liquid crystal display device comprising a layer of a polymerized material having a cholesteric order wherein the axis of the molecular helix extends transversely to the layer and wherein the pitch of the helix shows exhibits a gradient in the direction along the axis.
The invention also relates to a liquid crystal display device, such as one obtainable by one of the methods above.
2. Description
A method as mentioned in the opening paragraph is known per se. For example, United Kingdom patent specification GB 2,314,167 describes a liquid crystal (LC) display for stereoscopic vision comprising a patterned layer of a cholesterically ordered material. In accordance with this patent specification, such a layer may be manufactured by first providing a uniform layer of a cholesteric material on a substrate. By polymerizing areas of this layer at different temperatures, a patterned cholesteric layer is obtained. Use is made of the fact that the pitch of the molecular helix of the cholesterically ordered material is temperature-dependent. By polymerizing areas of the layer at a given temperature, the pitch associated with this temperature is, as it were, frozen in these areas.
The known method has drawbacks. For example, in practice it has been found that the known method is difficult to implement. This notably applies to the case where more than two areas having mutually different pitches must be provided in the layer. In that case, the method of manufacture is elaborate in that a relatively large number of masking steps is necessary and the precision with which the masks are adjusted is very critical. Moreover, the maximum difference in pitch which can be realized between the different areas by means of the known method appears to be relatively small. Patterning at different temperatures also appears to be difficult in practice.
It is an object of the invention to obviate these drawbacks. More particularly, it is an object of the invention to provide a method of manufacturing a liquid crystal display comprising a patterned layer of a polymerized material having a fixed cholesteric order wherein the patterned layer is manufactured using a simple and cost-effective method. The patterning step(s) of said the method should not necessarily involve the use of different temperatures and allow relatively large pitch differences between the different regions of the patterned layer to be obtained.
These and other objects achieved by a method of manufacturing a liquid crystal display device comprising a liquid crystal layer dispersed between a first and a second substrate, the first substrate comprising a patterned layer of a polymerized material having a fixed cholesteric order wherein the axis of the molecular helix extends transversely to the patterned layer and the patterned layer has at least a first and a second region in which the pitch of the molecular helix is mutually different, in which method the patterned layer of a polymerized material having a fixed cholesteric order is manufactured in accordance with a method comprising the steps of:
a. providing a layer of a polymerizable and/or crosslinkable cholesterically ordered material comprising a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion of said compound being inducable by radiation,
b. irradiating the layer in accordance with a desired pattern so that at least in a first region the convertible compound is converted to a different extent than in a second region,
c. polymerizing and/or crosslinking the irradiated polymerizable and/or crosslinkable cholesterically ordered material to form a three-dimensional polymerized cholesterically ordered material in which the cholesteric order is fixed.
It has been found that, using the method according to the invention, the patterned layer of cholesterically ordered material can be manufactured in a simple and cost-effective manner. The method is performed at the same temperature throughout, with the maximum pitch difference between the areas being relatively large. In any case, the pitch difference is sufficient to provide the patterned layer with colored regions spanning the entire visible range thus allowing full-color LC displays to be manufactured in a simple manner.
Due to the presence of the molecular helix, the patterned layer of cholesterically ordered material has regions which each selectively reflect circularly polarized electromagnetic radiation of a band of wavelengths. The central wavelength &lgr; of the band of reflected wavelenghts is determined by the pitch p of the molecular helix, according to &lgr;=p.n, where n is the average refractive index of the cholesterically ordered material. The bandwidth &Dgr;&lgr; is given by &Dgr;&lgr;=p. &Dgr;n, where &Dgr;n is the birefringence of the uniaxially oriented phase corresponding to the cholesterically ordered phase. In the visible range, the regions selective reflect circularly polarized light of a particular color. Typically, with &Dgr;n being less than about 0.15 and n of the order 1, the bandwidth in the visible range of the spectrum is 60 to 90 nm.
Because the patterned layer does not absorb any radiation incident upon it, it is not only a color and circular light selective reflector, but also a filter which selectively transmits light of the opposite handedness within the reflection band. Outside its reflection band, the cholesterically ordered material is transparent and transmits both polarization components.
By (partially) converting the convertible compound in the irradiated regions of the layer, the pitch of the molecular helix in the layer, and thus the color, is altered in these regions. The difference in pitch between the first and the second region is proportional to the difference in the amount of convertible compound in the converted state and/or the non-converted state between the first and the second region.
The conversion of the convertible compound is effected by irradiation with energy in the form of, for example, electromagnetic radiation, nuclear radiation or an electron beam. Preferably said conversion is effected by means of UV radiation. Being polymerized and/or crosslinked, the cholesteric order of the pattern-wise irradiated layer of polymerizable cholesterically ordered material is fixed. Being fixed, the cholesterically ordered material has lost its liquid crystalline character and is not capable any more to respond to an electric field in a manner typical of liquid crystalline materials.
Having a fixed cholesteric order, the patterned layer is capable of withstanding high temperatures in particular those temperatures which are used during the manufacture of (other parts of) the liquid crystal display device and those temperatures typically experienced during its service life. Also, the patterned layer is resistant to prolonged UV exposure. Resistance to UV exposure is improved if the patterned cholesteric layer is cross-linked.
In U.S. Pat. No. 5,555,114 a method of manufacturing a passive matrix LC display comprising a layer of cholesterically ordered material is disclosed. The known method does not involve the use of convertible compounds to control the pitch. Also, it does not even disclose how a patterned multi-color cholesteric layer is to be manufactured.
It is to be noted that, preferably, the cholesteric layer has a low absorbance for the radiation used in step b, and the radiation intensity along the axis of the helix (i.e. transverse to the layer) is relatively constant within each region. Con
Pitt Michael Geoffrey
Van De Witte Peter
Bram Eric M.
Chowdhury Tarifur R.
Koninklijke Philips Electronics , N.V.
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