Multi-color display device and process of fabricating same

A liquid crystal display having a multi-color display is fabricated with a plurality of display electrodes on a first substrate, on which a color filter is selectively formed by electro-depositing a polymer together with coloring matter and electroconductive particles so that the display electrodes have different colors and electroconductivity. A counter electrode is formed on a second substrate and a liquid crystal material is sandwiched between the first and second substrates, so that the material serves, in operation, as a light shutter controlled by the voltage applied between said color filter via said display electrodes and the counter electrode.

BACKGROUND OF THE INVENTION 
The present invention relates to a multi-color display device and a process 
of fabricating the multi-color display device using color filters and, 
more particularly, to a process of fabricating the multi-color display 
device provided with color filters which are made of polymer layers formed 
by electro-deposition. 
FIG. 1 shows one example of a conventional multi-color display device which 
makes use of color filters. In FIG. 1 numeral 1 indicates a transparent 
substrate; numeral 2 a display electrode made of a transparent conductive 
film; numeral 3 a color filter formed in close contact with the surface of 
the display electrode 2; numeral 4 a transparent counter electrode; and 
numeral 5 a transparent rear substrate. The space between the two 
substrates 1 and 5 is filled with a substance which functions as an 
optical shutter which can be opened or closed by the application of a 
voltage, such as a liquid crystal or an electrochromic material, and the 
color filters 3, 3' and 3" are formed to have different colors. Several 
colors can be displayed by selectively applying a voltage between the 
display electrodes 2, 2' and 2" and the transparent counter electrode 4. 
This multiplication of the colors of a display using color filters is very 
effective in practice because it is considered that the method is 
convenient, any color can be easily obtained, and this color 
multiplication can be used in combination with various display materials 
and systems. 
However, when fabricating this multi-color display device using color 
filters, no discrepancy can be allowed between the patterns on the display 
electrodes and the patterns on the color filters formed on the surfaces of 
the display electrodes. Especially when realizing a color graphic display 
using fine patterns in the three primary colors, duplicating the patterns 
of the display electrodes and the color filters presents a serious problem 
which makes the fabrication difficult. Another problem concerns color 
changes during the formation of the various colors, which complicates the 
process. In particular, if the coloration is effected by a dyeing method, 
resist-printing steps are needed, which make the fabrication process more 
complex. Namely parts which have already been dyed should not be dyed 
again in subsequent dyeing steps. Moreover, the resist printing technique 
itself presents difficult problems which must be solved for each dye. 
Methods of forming the color filters that have been considered, generally 
speaking, use means such as screen printing or photolithography. Screen 
printing does not require any resist printing, but has limitations 
concerning size reduction so that positional accuracy becomes worse as the 
number of colors increases, with resultant discrepancies in the display 
pattern. Photolithography can produce fine patterns but a 
photolithographic step is necessary for each change of color, and resist 
printing is also needed to prevent re-dyeing so that the process becomes 
very complicated and the advantage of a convenient color-increasing means 
is lost. 
The color filters obtained by the above mentioned methods are made of 
insulator layers, and the display device is formed in such a way that an 
insulator is sandwiched between a display electrode and display material. 
Losses in the drive voltage are caused by the drops in voltage across the 
color filters, thus obstructing the drive at low voltages. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to propose a 
convenient process for fabricating a multi-color display device in which 
no pattern shift occurs, even when the display electrode pattern is fine, 
in which many kinds of colors can be used without any special resist 
printing steps, and which uses durable color filters so that it is 
suitable for a low-voltage drive. To attain these objects, the color 
filters are fabricated by a method in which display electrodes on the 
substrate are utilized as the electrodeposition electrodes, and color 
layers are formed by electrodeposition from a solution containing 
polymers, a coloring matter and electroconductive particles. 
According to this method, the display electrode can be formed by any 
desired pattern-forming operation such as vacuum evaporation, sputtering 
or etching using a mask, and a display electrode with no pattern 
displacement can be formed by electrodepositing polymers, a coloring 
matter and electroconductive particles selectively onto the display 
electrode to which a voltage is applied. By repeating these steps, 
moreover, an increase in colors can be easily obtained. The substrate used 
in this method is not limited as far as its material and shape is 
concerned, so long as its surface is insulating, because a display 
electrode having excellent adhesion to the substrate is selected. 
The multi-color display device obtained in this way has electroconductive 
color films which are prone to little voltage drop when a voltage is 
applied, and is thus suitable for a low-voltage drive.

DETAILED DESCRIPTION OF THE INVENTION 
The method of forming color films by electrodepositing polymers in 
accordance with an important feature of the present invention will be 
described hereinafter. One means of electrodepositing polymers onto a 
display electrode is a method by which a monomer is electrochemically 
polymerized on the electrode. An example of this method has been reported 
(in Metal Surface Technology Vol. 19, No. 12, 1968) by which a variety of 
vinyl compounds can be electrochemically polymerized on an iron plate to 
form a polymer surface film. In recent years, moreover, research has been 
conducted into methods by which pyrrole or thiophene is electrochemically 
polymerized to form a conductive polymer such as polypyrrole or 
polythienylene on the electrode. However, these means for directly 
polymerizing monomers in an electrochemical manner are not yet efficient 
in practice. For use in the present invention, there are still problems in 
that the films obtained are already colored and that the coloring is not 
sufficiently varied. In another method of electrodepositing a polymer onto 
the electrode, the polymer is made insoluble and is deposited onto the 
electrode by a polymer solution. One example of this method is known in 
the industry as the so-called "electrocoating" method by which a pigment 
is dispersed in an aqueous solution of a polymer, and a piece of metal is 
dipped into the solution and is used as an electrode so that a color layer 
is electrodeposited onto the electrode. This electro-coating method is 
used for precoating automotive bodies. The principle of this method 
involves introducing a hydrophilic group such as a carboxyl group into a 
polymer, and then neutralizing and making water-soluble the carboxyl group 
by an inorganic alkali or organic amine. The electrode is dipped into an 
aqueous solution of this water-soluble polymer and a voltage is applied to 
the electrode. The carboxyl anions dissociated in the aqueous solution 
migrate toward the anode and react on the electrode with the protons which 
are generated by the electrolysis of the water, so that the polymer 
becomes insoluble to water and deposits. More specifically, the reactions 
expressed by the following formula occur on the anode so that the polymer 
deposits thereon: 
##STR1## 
On the other hand, if a basic group (such as a polyamine) is used as the 
hydrophilic group and is neutralized and made water-soluble by an acid, a 
polymer is found to deposit upon the cathode. 
In this electro-coating operation, a film thickness of 10 to 20 .mu.m is 
usually obtained by applying a voltage of between 100 to 200 V. For the 
color filter according to the present invention, however, thinner color 
films are preferable. For this purpose, it is necessary to determine the 
most suitable resin concentration, voltage and solvent composition, as 
will be described in examples given below. 
As the polymer for the anionic electrodeposition, an adduct of a natural 
drying oil and maleic acid, an alkyd resin into which a carboxyl group is 
introduced, an adduct of an epoxy resin and maleic acid, a polybutadiene 
resin into which a carboxyl group is introduced, or a copolymer of acrylic 
or methacrylic acid and its ester can be used. Depending on the 
characteristics of the electrodeposited surface film, another polymer or 
an organic compound having a functional group may be introduced into the 
framework of the first polymer. When light is observed through color 
filters, as in the present invention, the color films must be transparent, 
and a polymer such as an acrylic or polyester resin is suitable for 
meeting that requirement. Moreover, the quantity of hydrophilic functional 
groups such as carboxyl or hydroxyl groups in the polymer plays an 
important role. The electrodeposited film is not sufficiently insoluble 
and forms an irregular film if there are too many hydrophilic groups. The 
water-solubility during neutralization becomes insufficient if there are 
too few hydrophilic groups. Water is the main component of the solvent of 
the polymer, and it contains as the polymerizing solvent, a hydrophilic 
solvent such as isopropanol, n-butyl alcohol, t-butyl alcohol, methyl 
"Cellosolve", ethyl "Cellosolve", isopropyl "Cellosolve", butyl 
"Cellosolve", diethylene glycol methyl ether, diethylene glycol ethyl 
ether or diacetone alcohol. The type and quantity of the hydrophilic 
solvent have a strong influence upon the thickness and evenness of the 
electrodeposited film. 
In this coloring method, a pigment is used in the electro-coating operation 
and the charged pigment is subjected to electrophoresis together with the 
polymer annd is included in the film. To obtain the transparent color 
filters of the present invention, the obscuring ability processed by most 
pigments is not required, and since the number of transparent pigments is 
limited, a free choice of colors is difficult to obtain. 
Therefore, the present invention has devised a method of electrodepositing 
a dye as the coloring matter together with a polymer. In order to 
electrodeposit the dye with the polymer, the dye molecules must be charged 
and subjected to electrophoresis. With a water-soluble dye, the 
dissociated dye ions have the effect of adding to the supporting 
electrolyte, which produces increases in the current and the film 
thickness, and irregularities in the film. A dye which is barely or not 
soluble in water usually coagulates in the water, but the electrodeposited 
polymer can act as a kind of soap with hydrophobic and hydrophilic groups, 
and can exhibit some dispersion of the organic dye molecules so that they 
can separate into fine particles and be electrodeposited together with the 
polymer, as has been found in the present invention. In this case, it is 
necessary to make the rates of electrodeposition of the dye and the 
polymer substantially equal to each other. This can be controlled by the 
composition of the solution. 
The electroconductive particles making the electrodeposited color films 
conductive are dispersed as charged particles attracted by the polymers in 
the solution, like the pigment used in the electrocoating, and are 
subjected to electrophoresis by the applied voltage so that they are taken 
into the color films. The diameter and quantity of the conductive 
particles strongly influence the electrical resistance and transparency of 
the color films. The conductive particle is made of tin oxide, indium 
oxide, antimony oxide, zinc oxide, cadmium oxide, gold, silver and nickel. 
EXAMPLES 
The process for fabricating the multi-color display device using color 
filters will now be specifically described in connection with examples 
thereof. 
(Example 1) 
FIG. 2 shows an example of the multi-color display device to which the 
color filter fabrication process according to the present invention is 
applied. 
In the following, the process of fabricating the multi-color display 
device, as shown in FIG. 2, will be specifically described. 
(1) Patterning Step 
Reference numeral 1 denotes a transparent substrate onto which a 
transparent conductive film of tin oxide is formed by a spray-coating 
method. The transparent conductive film is patterned into a striped shape 
by an etching method to form display electrodes 2. 
(2) Electrodepositing Step 
Next, paint (i.e., S-Via ED-3000, produced by Shinto Toryo KK) of the 
following composition: 
______________________________________ 
S-Via ED-3000 
Water-soluble polyester resin 
70% by weight 
Water-soluble melamine resin 
Butyl Cellosolve 
Ethyl Cellosolve 30% by weight 
n-butanol 
______________________________________ 
is used to prepare an electrodeposition bath of this composition: 
______________________________________ 
Substance Wt. Ratio 
______________________________________ 
S-Via ED-3000 10 
Water 108 
Tin oxide powder 7 
Silver pigment 3 
Methyl Cellosolve 
12 
Oil-soluble dye x 
______________________________________ 
The oil-soluble paint used is limited to one that is soluble in a 
hydrophilic solvent and preferably has a metal complex salt structure 
which has an extremely good light resistance. The oil-soluble paint has a 
molecular structure that is expressed by the following formula, for 
example (known under the tradenames Aizen Spilon, Oleosol Fast or the 
like): 
##STR2## 
The bath-preparing procedure involves adding the tin oxide powder and the 
silver pigment to the S-Via ED-3000, and dispersing them to obtain a paste 
by means of a pigment dispersion mixer. Water is added to that paste and 
is blended therewith to provide a well-mixed solution. 
The dye is then dissolved in the methyl Cellosolve. The weight ratio x of 
the dye can be any value selected so that it does not exceed the range of 
solubility of the dye in methyl Cellosolve. The methyl Cellosolve in which 
is dissolved the dye is added to the first solution so that the dye is 
uniformly dispersed. The methyl Cellosolve acts as a dispersion medium for 
the dye, but it would make the film become thicker or irregular if too 
much of it were added, or if the number of carbon atoms in the alkyd 
groups of the Cellosolve is increased. 
The transparent substrate 1 on which is formed the display electrodes 2 is 
dipped into the electrodeposition bath thus prepared. Those of the 
stripe-shaped display electrodes which are to be dyed the same color are 
selected, and the group of electrodes thus selected are used as anodes to 
which a voltage of 20 V is applied for 3 minutes. After this, the 
transparent substrate 1 is pulled out of the bath and well rinsed to wash 
away the solution which adhere to the portions to which no power was 
supplied. After this rinsing operation, the transparent substrate 1 is 
dried, leaving very transparent color films formed over the group of 
electrodes to which the voltage was applied. 
(3) Hardening Step 
Next, the polyester resin and the melamine resin in the color films formed 
by the electrodeposition are baked and set by a condensation reaction. If 
the baking is conducted in air at 175.degree. C. for 30 minutes, the color 
films are completely hardened. These hardened color films will not be 
re-dyed, even if they are dipped into the electrodeposition bath again. 
Thereafter, second or subsequent formations of color films are conducted 
by again selecting groups of display electrodes to be dyed the same color, 
and repeating the electrodeposition and hardening steps with an 
electrodeposition bath in which a dye of a different color has been 
dispersed. 
In the present example, striped color filters 3 having red, blue or green 
stripes of 200 .mu.m are formed very conveniently by a process consisting 
of a patterning step, a step of electrodepositing red film, a hardening 
step, a step of electrodepositing blue film, a hardening step, a step of 
electrodepositing green film, and a final hardening step. The color filter 
thus fabricated has no color deterioration and has uniform characteristics 
that are resistant to attack by acids, alkalis, a variety of organic 
solvents, and hot water. Moreover, the dye of the metal complex salts used 
in the color films is very stable, and has an excellent light-resistance 
such that at least 95% of the initial light absorptivity, even after a 
carbon arc test of 360 hours. 
Using the method thus far described, the color filters 3 are formed on the 
display electrodes 2, and the transparent substrate 1 is integrated to 
form a cell with a spacer 6 and a transparent rear substrate 5 on which 
transparent counter electrodes 4 are formed in stripes so that the stripes 
of the display electrodes 2 and the counter electrodes 4 intersect at 
right angles. The cell is filled with TN-FEM liquid crystal to provide the 
display material 7, thus completing the multi-color liquid crystal 
display. In this case, a voltage is applied between the display electrodes 
2 and the counter electrodes 4, and the cell is sandwiched between a pair 
of polarizer which have parallel axes of transmission. The color of the 
transparent color filters 3 can be seen if observed from the transparent 
substrate 1 side or the transparent rear substrate 5 side, and go black if 
the applied voltage is cut. When light is illuminated thereon from the 
direction of the rear substrate 5, the colors of the color filters 3 are 
more effectively displayed because the cell is so transparent. 
Because of the conductivity imparted to the color filter films, moreover, 
the voltage-transmissivity characteristics of the electro-optical 
characteristics of a multi-color display of the present example were found 
to be substantially equal to those of the liquid crystal material used 
therein. 
Thus, the process of fabricating the multi-color display device of the 
present example has been found to be suitable for providing a color filter 
with a fine pattern without any reduction in the display quality even 
although this process is simple and convenient, as well as a colorgraphic 
display which can be driven by a very reliable low-voltage matrix. 
(Example 2) 
The display material 7 of Example 1 was replaced with a negative guest-host 
liquid crystal using a black dichromatic dyestuff, and the substrate 1 was 
made of a white material (e.g., white ceramic). Except for these details, 
a multi-color liquid crystal display device was fabricated in the same way 
as in Example 1. In this case, when a voltage was applied between the 
display electrodes 2 and the counter electrodes 4 and the display was 
observed from the transparent rear substrate 5 side through a polarizing 
plate, the colors of the color filters 3 were displayed brightly. If the 
voltage was interrupted, the liquid crystal looked black, or the color of 
the dichromatic dyestuff thereof. Effects similar to those of the Example 
1 were also obtained by the present example. 
(Example 3) 
The display material 7 of Example 1 was made of DSM liquid crystal, and the 
substrate 1 was patterned with aluminum by mask evaporation to form the 
display electrodes 2. Otherwise this multi-color liquid crystal display 
was fabricated in the same way as in Example 1. In this case, when a 
voltage was applied between the display electrodes 2 and the counter 
electrodes 4, and the display was observed from the transparent rear 
substrate 5 side, the DSM liquid crystal achieved a light-scattering state 
so that the colors of the color filters 3 were displayed against opaque 
white. When the voltage was interrupted, the light-scattering state 
disappeared so that the DSM liquid crystal became a dark color. 
Incidentally, in order to efficiently establish the light-scattering state 
of DSM liquid crystal, it was necessary to pass an ion current to some 
extent, but the high resistance of the color filters 3 hindered this. By 
providing extra transparent electrodes on the color filters, and by using 
them as the voltage-applying electrodes, therefore, the drive voltage 
could be reduced, and effects similar to those of Example 1 could still be 
obtained. 
(Example 4) 
The electrodeposition bath of Example 1 was replaced by paint (i.e., 
POWERMITE 3000-10, produced by Nippon Paint KK) of the following 
composition: 
______________________________________ 
POWERMITE 3000-10 
Water-soluble acrylic resin 
60% by weight 
Water-soluble melamine resin 
Butyl Cellosolve 
40% by weight 
Isopropyl alcohol 
______________________________________ 
was used to prepare an electrodeposition bath of this composition: 
______________________________________ 
Substance Wt. Ratio 
______________________________________ 
POWERMITE 3000-10 10 
Water 113 
Indium oxide powder 
5 
Nickel powder 2 
Ethylene glycol 20 
Dispersion dye x 
______________________________________ 
The dispersion dyes used are available on the market and frequently contain 
an anionic dispersant, which would be ionized in the bath to cause the 
current to increase. Therefore, it is preferable that the dispersion dye 
contains no dispersant. 
The bath was made up by adding the indium oxide and nickel powders to the 
POWERMITE 3000-10, and dispersing them by means of a pigment dispersion 
mixer to provide a paste. Water was added to this paste and blended to 
form a solution. The dispersion dye was then dispersed uniformly in the 
ethylene glycol within a range of x&lt;1.5, and was added to and mixed with 
the first solution. 
In the same way as in Example 1, a multi-color liquid crystal display was 
fabricated, and effects similar to those of Example 1 are obtained. It 
was, however, found that only a limited number of dyes could be used as 
the oil-soluble dye of a metal complex salt to provide the good 
light-resistance characteristics of the color filter. 
(Example 5) 
The electrodeposition bath of the Example 5 was changed so that it had the 
following composition: 
______________________________________ 
Substance Wt. Ratio 
______________________________________ 
S-Via ED-3000 20 
Water 105 
Tin oxide powder 10 
Indium oxide powder 
5 
Oil-soluble dye x 
______________________________________ 
This bath was made up by adding the tin oxide powder and the indium oxide 
powder to S-Via ED-3000, and the oil-soluble dye was then added within a 
range of x&lt;1.0. These were mixed by a dispersion mixer to provide a paste. 
Water was then added to the paste, and the mixture was blended to form the 
electrodeposition bath. A multi-color display was fabricated similar to 
that of Example 1, and effects similar to those of Example 1 could be 
obtained. 
It is quite obvious that the oil-soluble dye used in the present example 
need not be limited to one that is soluble in a hydrophilic solvent, but 
it should preferably have a good light-resistance. 
As has been specifically described hereinbefore in the examples, the 
process of fabricating a multi-color display according to the present 
invention is so simple and convenient that it can provide color filters 
withouth the use of any special resist printing means for separating the 
colors. Moreover, the color filters are strong and have no pattern 
distortion, so that they can provide a high display quality and 
reliability, even if they are combined with a display material such as a 
liquid crystal. Since the color filters are conductive, moreover, the 
multi-color display fabricated by this process has only small losses in 
the drive voltage, so that it can be driven at a low voltage.