Note: Descriptions are shown in the official language in which they were submitted.
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GE-1004
~rT.E
PROCESS FOR THE PREPAR~TION OF OPTICAL COLOR FILTERS
~2~ 21!ND~ LMy~TION
The subject of the invention is a process for the
preparation of any type of patterned, monochromatic or
polychromatic, optical color fi.lter, such as is used,
for e~ample, for color separation in devices for making
color images or particularly in systems for the
reproduction of such images (displays).
Most of such color filtercs are finely patterned,
that is, the structural element:s are often only a few
microns large. Depending on the use, these elements are
in the form of stripes, circles or other geometric
figures. Most are re~uired for filters with closely
adjoining areas of different colors.
Many processes are known for the preparation of
such color filters. The methods used include screen
printing, color photography based on silver halides,
diazo processes, the transfer of preformed polychromatic
patterns onto a common substrate, imagewise ablation of
color layers by laser, washoff after photocrosslinking,
applying color to lithographically formed patterns from
colorless binders, and others.
Similarly known are processes in which finely
divided dyes or dyed compositions ars coated in the form
of the desired pattern onto the substrate. Thus,
European Patents 01 13 237 and 01 57 486 disclose the
preparation first of a patterned~ conductive layer on an
insulating substrate and then coating electro-
phoretically on the conductive areas a dyed polymer
layer from an aqueous solution containing a polymer and
a dye. Various conductive areas insulated from one
another can be provided successively with coatings of
various colors in suitable electrophoretic baths. If
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the conductive areas are later to serve as electrodes in
liquid crystal cells, then the electrical field is
partially shielded by the polymer layer, which decreases
the sensitivity of the cell.
German Patents ~7 58 209 and 29 29 615 disclose
negative-working light-sensitive layers, the exposed
areas of which are tonable with a powdered color to
produce an image. The layers described therein can also
contain a plasticizer. The use of negative-working
layers for polychromatic images offers the advantage
that, in the first step, the unexposed and untoned areas
retain their light-sensitivity and can be toned in a
succeeding step with another color after exposure of
another pattern. This process can be repeated until the
entire layer is toned. However, patterns prepared by
this process on transparent supports are not suitable as
color filters, because they scatter too much light. In
addi-tion, they are unstable to many solvents, so that
further processing to liquid crystal cells, for example,
the application of equalizing layers, is possible only
with limitations.
The problem involved in the invention is the
development of a process in which color filters with low
light scatter and good stability to mechanical and
chemical stresses can be prepared in a simple manner by
the use of negative-working, light-sensitive, tonable
layers.
The present invention is directed to a process
for the preparation of a monochromatic or polychromatic
color filter by
(a) applying a negative-working, light-
sensitive, tonable layer onto a transparent
support,
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~b) imagewise exposing in a pattern
corresponding to a spatial configuratLon of
a filter element color,
(c) toning the layer with a colored toner which
is capable of being substantially completely
volatilized in a heat treatment,
(d) subjecting the toned layer to a heat
treatment.
Pet a i le~l~s~:i}~;is~b~e~
Negative-working, light-3ensitive layers with 1,4-
dihydropyridine compounds, as are described, for
example, in German Patents 27 58 209 and 3g 29 615, are
preferred for performing the process of the invention.
Compounds of 2'-nitrophenol-1,4-dihydropyridine, for
example, the compounds cited on page 3, lines 51 to 60
of German Patent 34 29 615, are preferred. These
compounds can be coated on transparent supports by a
conventionai coating process, for example, spin coating,
singly or as mutual mixtures, either as such or
dissolved in a volatile solvent.
The light-sensitive layers can also be prepared
from 1,4-dihydropyridines mixed with hexaaryl
bisimidazoles. Suitable compounds are listed in Tables
I and II of German Patent 27 58 209.
The li~ht-sensitive layer~ can contain plasticizers
compatible with the toner binder. Examples of suitable
plasticizers are esters of aliphatic or aromatic di-~
tri-, and tetracarboxylic acids with two to four carbon
atoms and monofunctional alcohbls with 1 to 20 carbon
atoms, esters of di-, tri-, and tetrafunctional alcohols
with 2 to 20 carbon atoms and aliphatic or aromatic
monocarboxylic acids with 4 to 12 carbon atoms, and
ethers of any of the cited polyfunctional alcohols with
the cited monofunctional alcohols. Examples are
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diisohutyl phthalate, dibutyl adipate, dibutyl glycol,
and trioctyl mellitate. The necessary quantity o~
plasticizer can be determined by test; it is generally
between 2 and 15 percent by weight of the dry portion of
the light-sensitive layer.
Satisfactory results are obtained without a
particular plasticizer with cert:ain combinations of
light-sensitive material and toner bindex, for example,
dihydropyridine compounds, particularly 2'-nitrophenyl-
1,5-dihydropyridine derivatives, preferably the ester of
2'-nitrophenyl-1,5-dihydropyrid~ne-3,5-dicarboxylic acid
and cellulose acetate, polymethyl methacrylate or poly-
alpha-methyl styrene. In this case, lt is assumed that
the light-sensitive material itself or its photolysis
products act simultaneously as plasticizers. Thus, the
plasticizers are present in the layer only after
imagewise exposure. Consequently, with such systems,
the whole surface of the backside of the toned, light-
sensitive layer is exposed through the transparent
support to increase plasticizer concentration in the
layer.
It is not absolutely necessary to add high
molecular weight binders to the light-sensitive layers.
Adhesion of the filter areas o~ the invention to the
support is generally adequate without a binder.
The thickness of the light-sensitive layer can be
selected freely within certain limits. Consideration
should be given to the fact that thicker layers have
lower resolution capability, whereas thi~ner layers have
lower color density. Good results are generally
obtained with layers 0.5 to 5 microns thick.
The toners to be used in the in~ention contain
requisite color agents (pigments or dyes), which can be
selected according to the desired properties ~f the
color filter. Suitable color agents are cited, for
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example, on page 5,lines 57 onwards ln German Patent
29 45 569. Furthermore, it is advantageou~ for the
toners to contain a thermoplastic binder, which must be
compatible with the plasticizer of the :Light-sensitive
layer. Suitable examples thereof are polyvinyl
chloride, polyvinyl alcohol, polystyrene, and vinyl
chloride/vinyl acetate copolymers; cellulose acetate,
poly-alpha-methyl styrene and polymethyl methacrylate
are preEerred. If polyamide acids are used as binders
for the toner, the subsequent heat treatment results in
very resistant, chemically and mechanically, filter
elements as a result of conversion to polyimides. If
the color agent itself is fusible or is thermoplastic
without decomposition, the toner does not need a binder.
The toners are used in the form of powders with a
particle size between 0.2 and 30 microns and preferably
with at least 50% of the particles having a spherical
diameter between 2 and 10 microns. Use can also be made
of toners specified for electrostatic copying processes
with thermal fixing.
For the preparation of filters by the process of
the invention, a pattern in the form of the desired
filter element of one color is exposed through an
appropriate mask on the light-sensitive layer of the
transparent support and toned with the corresponding
colored toner powder. This toning can be accomplished
by various known methods, for example, with a pad or a
brush, by strewing the powder and letting it trickle
off, optionally with the support beiny vibrated, or by
contact with an auxiliary surface on which the toner
powder is loosely bonded. The pattern can also be toned
with the aid of a color-bearing transfer layer, for
example, in accordance with German Patents 29 49 ~62 or
36 25 01~. The light-sensitive layer is brought into
contact with the transfer layer, optionally with the
application of heat and pressure, and then peeled away.
These process steps of exposure and toning are repeated
for each color.
In many instances, it is necessary to have on the
support, in addition to the color areas, achromatic,
highly light-absorbing areas that form a so-called black
matrix. Such a matrix can be prepared like the filter
element by exposing a pattern and toning with a toner
containing a black color agent. Preferably, this is
done in a first step before the preparation of the color
filter.
It is advantageous to treat light-sensitive layers
containing dihydropyridines with a strong acid after
each toning, as described in German Patent 35 40 804.
This prevents areas colored with a specific toner from
being contaminated with other colors in subsequent
toning steps.
Whenever all of the desired colors are applied, the
system of support, light-sensitive layer, and toners
that have been applied patternwise is subjected to a
heat treatment. This accomplishes two objectives.
First, the light-sensitive layer's volatile portion,
which is no longer needed and which could adversely
affect the characteristics of the finished display, is
volatilized. Second, the toner particles coalesce and
the filter elements lose most o~ the disrupting property
of high light scattering. In addition, the filter
elements become significantly more stable to solvents
used in further processing, for example, phenoxy ethanol
or N-methyl pyrrolidone.
Heat treatment is accomplished at the lowest
possible temperature, preferably below 240C. Its
duration is preferably lO to 60 minutes. ~owever,
duration and temperature should always be selected so
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that no visible decomposition of the support and toners
occurs.
It is not completely clear why the heat treatment
results }n such a sharp reduction of li~ht scattering in
the toned filter elements. It is assumed that the
number of scattering interfaces is drastically reduced
by the coalescence of the toner particles. However, it
is surprising that this effect doe snot occur if the
toner particles actually melt. The effect of the
invention is observed even if the toner contains only
such color agents and binders that cannot melt at the
temperature of the heat treatment, as well as if the
polymer binder of the toner has a melting point lower
than this temperature. It is similarly surpxising that
performing the process of the invention does not produce
a noticeable change in the form of the filter elements.
This applies independently of whether the toner contains
components that melt at the temperature of the heat
treatment.
In a particularly preferred embodiment o~ the
process of the inven~ion, light scattering by the filter
elements can be reduced even further. For this, the
finished filter after heat treatment is placed in a
solvent selected according to the characteristics of the
toner binder. In the case of cellulose acetate toners,
good results are obtained with acetone. Butanone and
dichloromethane are also suitable. The solvent is used
preferably in the vapor phase.
The color filters of the invention generally do not
have completely ~lat surfaces, because the filters
consist of individ~al, adjoining polychromatic areas
adhering to the substrate. However, they can be coated
by a simple method with an equali~ing layer of
polyimide. A solution of the polyimide in a suitable
solvent, for example, phenoxy ethanol, can be applied
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and dried. Such a coating is needed anyway as the so-
called orientation coating in liquid crystal displays.
In another version, a patterned, conductive layer, for
example, of indium tin oxide, can be coated on this
layer to serve as the control electrode in the display.
The process of the invention is different from the
state of the art in that it can be performed simply and
inexpensively. Thus, for example, only one light-
sensitive layer is required. Special measures to insure
dimensional stability, like tho~se necessary with
transfer layers, are eliminated, because the colored
filter elements are produced directly on the support.
The hue of the filter elements can be matched to the
goal value by choice of color agent or also by simple
mixture of different toners. Color density can also be
controlled in a simple manner, for exampler by the
exposure.
The color filters prepared in the invention are
outstandingly suitable for incorporation in liquid
crystal displays. They have accurate dimensions, very
uniform layer thickness, and are compatible with
solvents and assembly materials. Pigments with improved
aging stability can be used, because the type of color
agent can be selected freely within wide limits. The
~5 low scattering of the filter elements requires high
contrast in the finished display, because the light
becomes only very slightly depolarized in the filter
elements.
The invention can be used to prepare ~olor filter
elements for devices that are specified for reproducing
or recording monochromatic or polychromatic images, for
example, video cameras. To further illustrate the
present invention the following examples are presented.
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A glass plate was coated with a solution of 3.75 g
dimethyl ester of 2,6-dimethyl-9-~2'-nitrophenyl)-1,4-
- dihydropyridine-3,5-dicarboxylic acid and 11.25 g
diethyl ester of 2r6-dimethyl~4-~2'-nitrophenyl)-1,4-
dihydropyridine-3,5-dicarboxylic acid in 190 ml methyl
ethyl ketone and dried. The dried layer was 1.2 microns
thick. It was exposed at 160 mJ/cm2 through a mask with
clear, square areas of 250 microns in size by means of a
1000 watt mercury vapor lamp provided with a filter
having maximum transparency at 360 nm. Because the
light-sensitive layer was sliyhtly tacky, direct contact
with the mask was avoided.
The exposed layer was toned with a cellulose
acetate powder 154% acetylated, two microns average
particle size) incorporating 30 percent by weight
particles of Perylene Pigment Red 123 (CAS Number 24108-
83-2), and excess tonex was wiped off with a dust cloth.
For the determina~ion of scattering behavior, the
glass plate with the filter element pattern was laid on
the stage of an o~erhead projector. The filter elements
appeared black in the projection without a noticeable
polychromatic portion.
Now, the glass plate was heated in an oven for 60
minutes at 225C. After the plate was cooled, the test
was repeated with the overhead projector. The filter
elements now appeared brown in the projection.
Finally, the plate was placed in a covered beaker
for 10 minutes over acetone at a temperature of 50C.
The test in the projector now showed a slightly
blackened red color.
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Example 1 was repeated, except that the toner was a
preparation from 18 parts by weight Pigment Blue 15:4
~CAS Number 147-14-8) and 8~ parts by weight polymethyl
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methacrylate with a glass transition temperature of
105~C ("Elvacite" 2008 from the Du Pont Company).
Although testing the freshly prepared filter element in
the overhead projector showed a black image, a pure blue
green projection was observed after a heat treatment for
60 minutes at 225C.
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Example 2 was repeated, except that the toner
contained poly-alpha~methyl styrene as a polymeric
binder, instead of polymethyl methacrylate. Here, too,
a pure blue green image was observed in the projection
after the heat treatment.
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Example 2 was repeated, except that, instead of
polymethyl methacrylate, the toner contained a binder of
a copolymer of alpha-methyl styrene and vinyl toluene
with a softening temperature of 100C.
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Example 4 was repeated, except that the exposure
was at 240 mJ/cm2. Color filter elements were obtained
with a visually obvious, higher color density. This was
also shown by a 65% transmission at 500 nm (instead of
75% as in Example 4), as measured by a
spectrophotometer.
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Example 3 was repeated, except that, before the
heat treatment, the light-sensitive, toned layer was
exposed through the glass plate at 500 mJ/cm2. After
the heat treatment, the filter;elements showed a pure,
blue green image on projection.
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