Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
1998-01-26
2001-10-23
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C349S086000, C349S088000, C349S092000, C349S093000, C349S187000
Reexamination Certificate
active
06306469
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of manufacturing a polymer-dispersed liquid crystal cell, in which method a mixture, which predominantly comprises a liquid crystalline material as well as reactive monomers and a photoinitiator, is sandwiched between two substrates, which are provided with an electrode layer, whereafter the mixture is polymerized under the influence of radiation. The invention also relates to a polymerizable mixture which can suitably be used in a polymer-dispersed liquid crystal cell as well as on a display device comprising such a cell.
Polymer-dispersed liquid crystal cells (abbr. PDLC cells) are increasingly being used in electro-optic devices, such as display devices, optical projectors and electrically drivable optical shutters. The optically active material of these cells is formed by liquid crystalline material which is dispersed in a matrix of a polymerized material. Such a material is referred to as a polymer-dispersed liquid crystalline material (abbr. PDLC material). This material is customarily prepared by providing a mixture of a liquid crystalline material (70-95% by weight), reactive monomers (5-30% by weight) and at least one photoinitiator, in the form of a layer, between two substrates of a cell and, subsequently, polymerizing this layer under the influence of radiation. During polymerization, phase-separation occurs, which leads to the formation of the desired optically active layer of polymer-dispersed liquid crystalline material. This layer can be switched between an optically transparent state (in the presence of a field) and an optically scattering or translucent state (in the absence of a field) by means of an electric field.
A method of the type mentioned in the opening paragraph is known per se, for example, from European Patent publication EP-A 575.791. More particularly, in examples 5-18 of said publication, a description is given of a prepolymer of reactive monomers composed of 2-ethylhexylacrylate (EHA) as well as one or two polyfunctional monomers. One part by weight of this prepolymer is mixed with four parts by weight of a non-reactive liquid crystalline material. Also a small quantity of a photoinitiator is added to this mixture. The resultant polymerizable mixture is subsequently provided between two substrates and polymerized by means of UV light so as to form an optically active layer.
The known method has an important drawback. It has been found that the electro-optical response of the PDLC cells thus manufactured is not uniform at all parts of the surface of the cell. For example, the switching voltage necessary to switch from transparent to scattering, and vice versa, is found to be different for different parts made of PDLC material. It has further been found that the electro-optical properties of the PDLC material are insufficiently stable with respect to time. Life tests show that these properties deteriorate relatively rapidly. For example, the hysteresis and the switching voltage increase rapidly.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to overcome the above-mentioned disadvantage. The invention more particularly aims at providing a method of manufacturing PDLC cells which exhibit a uniform electro-optical response, which is stable with respect to time. The PDLC cells manufactured by means of the inventive method should have a relatively low switching voltage, preferably, of approximately 6 V or less as well as a relatively low hysteresis, preferably, of approximately 3% or less. Another object of the invention is to provide a polymerizable mixture which is stable with respect to time and which can suitably be used in the method in accordance with the invention. The invention should also provide a display device having an improved PDLC cell.
These and other objects of the invention are achieved by a method of the type mentioned in the opening paragraph, which is characterized, in accordance with the invention, in that the mixture comprises two types of non-volatile, reactive monomers, the first type of monomer being readily miscible with the liquid crystalline material and the second type of monomer being poorly miscible with said liquid crystalline material.
The invention is based on the insight that in the case of the known cells a non-uniform electro-optical response is obtained because the composition of the PDLC material is not the same everywhere. This is attributed to the presence of EHA in the known polymerizable mixture. This compound has a relatively great volatility. During filling of the cell, this compound evaporates, which leads to concentration differences in the filled cell. This results in a non-uniform electro-optical response in the known cell. EHA exhibits the greatest volatility problems if the cells are filled under the influence of a reduced pressure.
It has been found that the problem cannot be solved by simply replacing the volatile EHA with a single, non-volatile acrylate compound having approximately the same molecular mass. The replacement of EHA of the known polymerizable mixture by a non-volatile, higher alkylacrylate, such as decylacrylate (DA) yields poor results. Various electro-optical properties, such as the switching voltages and the hysteresis of the switching curve, of a cell comprising such a polymerized mixture turn out to be considerably worse than those of the known cell comprising the EHA-containing mixture. It is noted that the term “non-volatile monomers” is to be understood to mean monomers whose vapor pressure is smaller than 1 Pa.
The invention is further based on the experimentally gained insight that the mixing properties of the non-volatile monomers to be polymerized, which contain the liquid-crystalline material, play an important role in the electro-optical properties of the ultimate PDLC cell. It has been found that a part of these monomers should be readily miscible with the liquid-crystalline material, whereas another part of these monomers should be poorly miscible with said liquid-crystalline material. Mixtures comprising these two types of non-volatile, reactive monomers can be used very successful in PDLC cells. The electro-optical properties of these cells range from good to very good.
A preferred embodiment of the method in accordance with the invention is characterized in that the first type of monomer is an ethoxylated alkyl-phenolacrylate whose alkyl group comprises at least five C-atoms, and in that the second type of monomer is an alkylacrylate whose alkyl group comprises at least 8 and maximally 18 C-atoms.
In experiments it has been established that ethoxylated alkyl-phenolacrylates of the above-mentioned type are very readily miscible with customary liquid crystalline material, provided that the number of C-atoms of the alkyl group is greater than four. It has also been found that alkylacrylates of the above-mentioned type are poorly, i.e. incompletely, miscible with customary liquid crystalline materials, provided that the alkyl group comprises at least 8 and maximally 18 C-atoms. If alkyl groups comprising fewer than 8 C atoms are used, then the alkylacrylate becomes too volatile. If alkyl groups comprising more than 18 C-atoms are used, the degree of miscibility of the alkylacrylate with the liquid crystalline material becomes too high.
A further preferred embodiment of the method in accordance with the invention is characterized in that the quantity of each of the two types of monomers is at least 20% by weight, calculated with respect to the overall quantity of both types of monomers. If the quantity of one of the two types of reactive monomers is smaller than 20% by weight, then the switching voltage and the hysteresis of the cell manufactured with said monomers is relatively high. Preferably, the ratio between both types of monomers is approximately 1:2. In this case, the lowest values as regards switching voltage and hysteresis of the PDLC cell are achieved.
An interesting embodiment of the method in accordance with the invention, is characterized in that the mixture is introduce
Kloosterboer Johan G.
Serbutoviez Christoph
Touwslager Fredericus J.
Ahmad Nasser
U.S. Philips Corporation
Waxler Aaron
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