U.S. patent application number 09/877312 was filed with the patent office on 2002-04-18 for pdlc cell.
Invention is credited to Kloosterboer, Johan G., Serbutoviez, Christophe, Touwslager, Fredericus J..
Application Number | 20020045014 09/877312 |
Document ID | / |
Family ID | 8227956 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020045014 |
Kind Code |
A1 |
Serbutoviez, Christophe ; et
al. |
April 18, 2002 |
PDLC cell
Abstract
The invention provides a method of filing a PDLC cell, a
polymerizable mixture suitable for this purpose as well as a
display device provided with such a PDLC cell. The mixture in
accordance with the invention comprises two types of non-volatile
reactive monomers, the first type of monomer being readily miscible
with liquid crystalline material and the second type of monomer
being poorly miscible with the liquid crystalline material. Such
mixtures prove to be very stable. In addition, when such mixtures
are used in cells, problems regarding compositional drift do not
occur. Cells in which the inventive mixture is used demonstrate a
relatively low hysteresis as well as a relatively low switching
voltage. By virtue thereof, it is very attractive to use these
cells in a display device.
Inventors: |
Serbutoviez, Christophe;
(Voiron, FR) ; Kloosterboer, Johan G.; (Eindhoven,
NL) ; Touwslager, Fredericus J.; (Eindhoven,
NL) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8227956 |
Appl. No.: |
09/877312 |
Filed: |
June 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09877312 |
Jun 8, 2001 |
|
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|
09013546 |
Jan 26, 1998 |
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Current U.S.
Class: |
428/1.1 |
Current CPC
Class: |
G02F 1/1334 20130101;
Y10T 428/10 20150115; C09K 2323/00 20200801; C09K 19/544
20130101 |
Class at
Publication: |
428/1.1 |
International
Class: |
C09K 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 28, 1997 |
EP |
97200218.2 |
Claims
1. 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, characterized 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.
2. A method as claimed in claim 1, 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.
3. A method as claimed in claim 1, 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.
4. A method as claimed in claim 1, characterized in that the
mixture is introduced into the cell under the influence of a
reduced pressure.
5. A polymerizable mixture which can suitably be used in a
polymer-dispersed liquid crystal cell, which mixture comprises
reactive monomers and a photoinitiator, characterized in that the
mixture contains two types of non-volatile reactive monomers, the
first type of monomer being readily miscible with a liquid
crystalline material and the second type of monomer being poorly
miscible with said liquid crystalline material.
6. A polymerizable mixture as claimed in claim 5, 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.
7. A polymerizable mixture as claimed in claim 5, 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.
8. A polymerizable mixture as claimed in claim 5, characterized in
that a quantity of 70-90% by weight of a liquid crystalline
material is added to the mixture.
9. A display device comprising a polymer-dispersed liquid crystal
cell with a matrix of individually drivable rows and columns of
electrodes as well as means for driving these electrodes,
characterized in that a cell manufactured in accordance with the
method claimed in claim 1 is used in said display device.
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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 filing 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] An interesting embodiment of the method in accordance with
the invention, is characterized in that the mixture is introduced
into the cell by means of a reduced pressure. In experiments it has
been established that PDLC cells manufactured in accordance with
this embodiment of the invention exhibit very stable
electro-optical properties.
[0014] The invention also relates to a polymerizable mixture which
can suitably be used in a polymer-dispersed liquid crystal cell,
and which comprises reactive monomers and a photoinitiator. In
accordance with the invention, this mixture comprises two types of
non-volatile, reactive monomers, the first type of monomer being
readily miscible with liquid crystalline material and the second
type of monomer being poorly miscible with liquid crystalline
material. PDLC cells comprising this mixture exhibit good
electro-optical properties, such as, in particular, a uniform
electro-optical response.
[0015] A preferred embodiment of the polymerizable mixture 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
this connection, good results have been achieved with a mixture in
which 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.
[0016] Preferably, the ratio between both types of monomers is
approximately 1:2. The polymerizable mixture is optimally suitable
for use in a PDLC cell if 70-90 % by weight of a liquid crystalline
material of a customary type has been added. The polymerizable
mixture thus obtained can be directly used to fill PDLC cells.
[0017] The invention also relates to a display device comprising a
polymer-dispersed liquid crystal cell. In this case, the electrode
layers of the substrates of the cell are constructed so as to form
rows and columns, each row or column being individually drivable.
The rows of one substrate and the columns of the other substrate
are oriented so as to extend at right angles to each other. The
presence of the matrix of electrodes formed by said columns and
rows enables pixels of the PDLC material of the display device to
be driven locally by means of an electric voltage.
[0018] Preferably, each one of the pixels is provided with a
solid-state switch in the form of a thin-flm transistor or a
thin-film diode. By virtue thereof, it becomes possible to form
images. The PDLC cells manufactured in accordance with the
inventive method can very suitably be used in such a display
device.
[0019] These and other aspects of the invention will be apparent
from and elucidated with reference to the embodiments described
hereinafter.
[0020] In the drawings:
[0021] FIG. 1 is a schematic, sectional view of a PDLC cell,
[0022] FIG. 2 shows the electro-optical curve of a PDLC cell,
[0023] FIG. 3 shows a number of structural formulas of chemical
compounds,
[0024] FIG. 4 shows graphs in which the hysteresis and the V90 or
the V50-values of a few mixtures of non-volatile monomers are
plotted as a function of their mixing ratio.
[0025] It is noted that, for clarity, the Figures may not be drawn
to scale.
[0026] FIG. 1 is a schematic, sectional view of a cell which can be
used in the manufacture of a PDLC cell in accordance with the
invention. This cell comprises two predominantly parallel
substrates (1, 2) which are provided with an electrode layer (3, 4)
on the surfaces facing each other. An optically active layer (5) of
a polymerized PDLC material is situated in the space between the
electrode layers. The distance between the electrode layers ranges
between 4 micrometers and 20 micrometers, preferably between 5 and
10 micrometers. This distance is maintained by the presence of
spacers (not shown) in the optically active layer, for example in
the form of small balls or fibers of glass. The space between the
electrode layers is closed by means of a seal (6), for example in
the form of a seal line.
[0027] At least one of the electrode layers is made of a
transparent, electroconductive material, such as indium-tin oxide
(ITO). In order to be suitable for use in a display device
operating in the reflection mode, at least one of the substrates of
the cell is transparent to the light used. In order to be suitable
for use in a display device operating in the transmission mode,
both substrates and both electrode layers of the cell have to be
transparent.
[0028] The PDLC cell shown in FIG. 1 was manufactured as follows.
Two transparent substrates, for example of glass, which are
provided on a main surface with an electrode layer of ITO are
positioned substantially parallel to each other by means of spacer
balls (7 micrometers across), said electrode layers facing each
other. Subsequently, the side faces of the space thus formed are
sealed by means of a seal line, in such a manner that one or two
flitg holes are preserved.
[0029] The cell can be filled in two different ways, i.e. at a
reduced pressure or by means of capillary action. If the cell is
filled at a reduced pressure, said cell is evacuated by means of
one or more fiing holes and, subsequently, arranged in the PDLC
material to be polymerized. Subsequently, the vacuum in the cell is
removed. The reduced pressure in the cell, which is created in the
above-described manner, is used to fil the cell space with the PDLC
material to be polymerized. By contrast, if a cell is filled using
capillarity, it is filled by capillary action instead of a
reduction in pressure. If the cell is filled by capillary action,
it should comprise at least two fig holes, which must be provided
in two oppositely arranged parts of the cell.
[0030] After the fig holes have been closed, the PDLC mixture is
polymerized by subjecting it to radiation with UV light (300-400
nm; 7 mW/cm.sup.2) at a temperature of 30 .degree. C. for
approximately 5 minutes. During said polymerization process, a
phase separation between the liquid crystalline material and the
polymer being formed takes place. The intended polymer-dispersed
liquid crystaline phase is obtained by polymerizing.
[0031] FIG. 2 shows an example of an electro-optical curve of a
PDLC cell. In this curve, the transmission T (%) is plotted as a
function of the electric voltage V (volt) which is applied across
the PDLC layer of the cell by means of the ITO electrodes. The
arrows (a) and (b) of FIG. 2 show the trend of the curve if the
voltage increases and decreases, respectively. The VIO and
V90-values are also indicated in the curve. The VIO value of the
curve is the voltage at which the transmission of the ascending
curve amounts to 10 % of the transmission which can be maximally
achieved with the cell. The V90 value of the curve is the voltage
at which the transmission of the ascending curve is 90 % of the
transmission which can be maximally achieved with the cell. The
hysteresis of the curve is determined at 50 % of the maximum
transmission. Said hysteresis is indicated as the difference (mV)
between the ascending and the descending curve. This value
multiplied by hundred and divided by the average value of the
ascending and the descending curve at 50 % transmission is
indicated as the percentage (%) of hysteresis.
[0032] Comparative experiment.
[0033] In a first series of experiments, a number of PDLC cells
were manufactured, using a polymerizable mixture in accordance with
the state of the art. The mixture used was composed of 20 % by
weight of PN393 (Merck) and 80 % by weight of a non-reactive liquid
crystalline material of the type TL205 (Merck). This comprises a
mixture of a non-reactive, low-molecular liquid crystalline
material. PN393 predominantly comprises the reactive monomer EHA,
the structural formula of which is shown in FIG. 3. It also
comprises a few monomers and oligomers containing two or more
reactive groups. These compounds serve as a cross-linkig agent in
the polymeric material to be prepared. PN393 also comprises two
photoinitiators (Darocur 1173 and Lucirine TPO) to polymerize the
reactive groups. In a first series of cells, the mixture was
introduced into the PDLC cells by means of capillary action, and in
a second series of cells the mixture was introduced via
vacuum-fifling, whereafter the mixture was polymerized under the
above-mentioned conditions.
[0034] Visual inspection revealed that the cells which were filled
at a reduced pressure exhibited clearly visible ring-shaped
structures around the filling hole. Closer inspection also revealed
that these cells demonstrated a poor electro-optical response. For
example, it was found that the voltage necessary to switch from
transparent to scattering, and conversely, was dependent on the
location and hence not the same throughout the surface of the
optically active layer of the cell. This phenomenon is ascribed to
the so-called "compositional drift", which can occur as a result of
evaporation of EHA during filling of the cell. When the cell is
filled by means of capillary action, said evaporation occurs to a
lesser degree. The cells of the second series also exhibited
ring-shaped structures around the filling hole. However, these
structures were less clearly visible than those of the
vacuum-filled cells.
[0035] Experiments in accordance with the invention.
[0036] In a subsequent experiment, the EHA of the polymerizable
mixture PN393 was replaced by a mixture of 37.5 parts by weight of
ethoxylated nonyl-phenolacrylate (ENPA, see FIG. 3) and 62.5 parts
by weight of tridecylacrylate (DA, see FIG. 3). This mixture is
referred to as PN393'. A quantity of 20 parts by weight of this
polymerizable mixture were mixed with 80 parts by weight of the
liquid crystalline material TL205 (Merck). In a first series of
cells, the mixture thus obtained was introduced into the PDLC cells
via capillary-filling, and, in a second series, via vacuum- filing,
whereafter said mixture was polymerized under the above-mentioned
conditions.
[0037] Visual inspection of the PDLC cells manufactured in
accordance with the invention revealed that there were no
ring-shaped structures around the filling opening. In the case of
both the vacuum-filed cells and the capillary-filled cells, said
ring-shaped structures were absent. Table 1 lists seven different
PDLC cells, which are filled, either via vacuum-filling or
capillary action, with one of the two above-described mixtures,
i.e. of PN393 or PN393'. To determine the stability of the
mixtures, a number of the cells were subjected to an accelerated
life test after the polymerization process ("aftertreatment").
Table 2 lists some electro-optical properties of these cells, i.e.
the value of the switching voltages V10 and V90 as well as the
hysteresis (%). These properties were measured immediately after
the manufacture of the cells (cells 1, 3 and 5) or after the cells
had been subjected to life tests (cells 2, 4, 6 and 7).
1TABLE 1 Cell Mixture Filling method After treatment 1. PN393/TL205
Capillary -- 2. PN393/TL205 Capillary 5 min, 90.degree. C. 3.
PN393'/TL205 Capillary -- 4. PN393'/TL205 Capillary 60 min,
90.degree. C. 5. PN393'/TL205 Vacuum -- 6. PN393'/TL205 Vacuum 60
min, 100.degree. C. 7. PN393'/TL205 Vacuum 960 min, 100.degree.
C.
[0038]
2TABLE 2 Cell 1 2 3 4 5 6 7 V10 (mV) 5.0 5.7 3.3 3.6 3.1. 3.2. 3.2
V90 (mV) 8.8 10.6 6.0 6.4 5.9 6.0 6.1 hysteresis(%) 3.0 7.1 2.6 4.4
2.9 2.9 2.8
[0039] The Tables show that the cells comprising the polymerized
mixtures in accordance with the invention exhibit a substantially
lower V90 value as well as a substantially higher stability than
cells comprising the known mixture. A comparison between the cells
1 and 2 (not in accordance with the invention) shows that after a
life test of 5 minutes at 90 .degree. C. the hysteresis of the
known mixture has already more than doubled.
[0040] The hysteresis of cells comprising the mixtures in
accordance with the invention increases less than the hysteresis of
cells comprising the known mixture. The hysteresis of the mixture
in accordance with the invention remains substantially constant if
it has been introduced into the PDLC cell via vacuum-filling. Cells
which are filled in this manner also demonstrate the lowest
hysteresis and the greatest stability.
[0041] A comparison of the cells 1 and 2 shows that the V10 and V90
values of the known material increase substantially after a short
treatment at an elevated temperature. The cells filled by means of
the method in accordance with the invention prove to be much more
stable in this respect. The stablest cells are those which are
vacuum-filed with the mixture in accordance with the invention.
[0042] In a number of further experiments, the ratio in which the
two non-volatile reactive monomers occur in the polymerizable
mixture was varied. In this case, the two non-volatile reactive
monomers include the above-mentioned ENPA with one of the following
alkylacrylates: decylacrylate (DA), dodecylacrylate (DDA),
tridecylacrylate (TDA) or octodecylacrylate (ODA).
[0043] The chemical structural formulas of these compounds are
shown in FIG. 3. ENPA is a monomer which is very readily miscible
with customary liquid crystalline materials, such as PN393. The
above-mentioned alkylacrylates, however, are poorly (i.e.
incompletely) miscible with customary liquid crystaline
materials.
[0044] A quantity of 80 parts by weight of TL205 (Merck) were added
to 20 parts by weight of the polymerizable mixtures comprising one
of the above-mentioned combinations of non-volatile monomers. The
mixtures thus formed were introduced into PDLC cells at a reduced
pressure. After polymerization of the mixture under the
above-mentioned conditions, the hysteresis as well as the V90 or
the V50 value of the cells were determined. The measured values are
indicated in the graphs of FIG. 4.
[0045] FIG. 4A-C show that the lowest values for the hysteresis and
for V90 or V50 are obtained with mixtures comprising at least 20 %
by weight of one of the two non-volatile reactive monomers. If use
is made of mixtures comprising less than 20% by volume of one of
the two monofunctional monomers, either the hysteresis or the V90
or V50 values demonstrate an unacceptably large increase. The
lowest values are achieved at mixing ratios of the non-volatile
monomers of approximately 1:2.
[0046] The invention provides a method of filling a PDLC cell, a
polymerizable mixture suitable for this purpose as well as a
display device provided with such a PDLC cell. The mixture in
accordance with the invention comprises two types of non-volatile
reactive monomers, the first type of monomer being readily miscible
with liquid crystalline material and the second type of monomer
being poorly miscible with the liquid crystalline material. Such
mixtures prove to be very stable. In addition, when such mixtures
are used in cells, problems regarding compositional drift do not
occur. Cells in which the inventive mixture is used demonstrate a
relatively low hysteresis as well as a relatively low switching
voltage. By virtue thereof, it is very attractive to use these
cells in a display device.
* * * * *