Note: Descriptions are shown in the official language in which they were submitted.
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ENCAPSULATED ELECTROPHORETIC DISPLAYS HAVING A
MONOLAYER OF CAPSULES
Technical Field
The present invention generally relates to materials and methods for forming a
monolayer of capsules for an encapsulated electrophoretic display.
Background Information
Current electrophoretic display technology produces a display that neither is
as
bright nor has as much contrast as is desired. Current displays are not
capable of
achieving uniform brightness or good contrast due to limitations in their
construction.
Thus, new materials and methods of construction are needed to provide
electrophoretic
displays with acceptable brightness and contrast.
Summary of the Invention
A bright, high-contrast encapsulated electrophoretic display is disclosed.
Such a
display can be achieved with various materials and methods that allow the
display to be
constructed such that a monolayer of capsules on a substrate is formed. The
capsules
may contain at least an electrophoretically mobile particle and a suspending
fluid. In
addition to forming a monolayer, materials and methods may allow the capsules
in the
monolayer to pack together and/or deform in certain, useful configurations.
For
example, capsules can be non-spherical.
Throughout the Specification, the invention is described as a display for ease
of
description. However, the compositions and processes disclosed herein are
equally
applicable to "elements." A display is one example of the broader concept of
an
element. One or more elements can be ordered into a display or other articles
of
manufacture. Elements can include any of the features described for a display.
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Generally, particles can move within the capsule under the influence of a
voltage.
Depending upon the location of the particles and the composition of the
suspending
fluid, various visual states are available. In one highly generalized example,
reflecting
particles, located at the front of the capsule (towards a viewer) in a colored
dye, will
reflect light and appear "white." When the reflecting particles move towards
the rear of
the capsule (opposite the viewer) upon application of an electric, the
particles will be
obscured by the dyed fluid and will appear "dark" to a viewer.
The successful construction of an encapsulated electrophoretic display
requires
the proper interaction of several different types of materials and processes.
Materials
such as a polymeric binder, a capsule membrane, and electrophoretic particles
and fluid
must all be chemically compatible. The capsule membranes may engage in useful
surface interactions with the electrophoretic particles, or may act as an
inert physical
boundary between the fluid and the binder. Polymer binders may set as
adhesives
between capsule membranes and electrode surfaces.
In some cases, a separate encapsulation step of the process is not necessary.
The
electrophoretic fluid may be directly dispersed or emulsified into the binder
(or a
precursor to the binder material) to form what may be called a "polymer-
dispersed
electrophoretic display." In such displays, the individual electrophoretic
phases may be
referred to as capsules or microcapsules even though no capsule membrane is
present.
Such polymer-dispersed electrophoretic displays are considered to be subsets
of
encapsulated electrophoretic displays.
In an encapsulated electrophoretic display, the binder material can surround
the
capsules and separate the two electrodes. This binder material should be
compatible
with the capsule and electrodes and should possess properties that allow for
facile
printing or coating. It may also possess barrier properties for water, oxygen,
ultraviolet
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light, the electrophoretic fluid, or other materials. Further, it may contain
surfactants and
cross-linking agents, which could aid in coating or durability. The polymer-
dispersed
electrophoretic display may be of the emulsion or phase separation type.
In an embodiment, an encapsulated electrophoretic element has a plurality of
non-spherical capsules disposed substantially in a single layer on a
substrate.
In another embodiment, an encapsulated electrophoretic element has a plurality
of
capsules disposed substantially in a single layer on a substrate and in
association with a
binder, thereby to form a film. The binder can include a binder solid, and a
ratio of a
mass of the binder solid to a mass of the capsules of at least a portion of
the element can
be from about 1:2 to about 1:20.
In another embodiment, an encapsulated electrophoretic element has a plurality
of
capsules disposed substantially in a single layer on a substrate and
associated with a
binder, thereby to form a film. At least a portion of the element has an
optically active
fraction of at least 70%.
According to one aspect of the invention, there is provided an encapsulated
electrophoretic element comprising an electrophoretic ink, the electrophoretic
ink
comprising a plurality of non-spherical capsules dispersed in a binder, at
least one of the
capsules being enclosed by a deformable membrane deformed into a non-spherical
shape, wherein the plurality of capsules form substantially a single layer
when the ink is
disposed on a substrate.
According to another aspect of the invention, there is provided an
encapsulated
electrophoretic element comprising an electrophoretic ink, the electrophoretic
ink
comprising a plurality of non-spherical capsules dispersed in a binder
comprising a
binder solid, at least one of the capsules being enclosed by a membrane,
wherein the
plurality of capsules form substantially a single layer when the ink is
disposed on a
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substrate, and wherein a ratio of a mass of the binder solid to a mass of the
capsules is
between about 1:2 and about 1:20.
According to another aspect of the invention, there is provided an
encapsulated
electrophoretic element comprising an electrophoretic ink, the electrophoretic
ink
comprising a plurality of non-spherical capsules dispersed in a binder, at
least one of the
capsules being enclosed by a membrane, and at least one of the capsules
comprising at
least one electrophoretic particle dispersed in at least one suspending fluid,
wherein the
plurality of capsules form substantially a single layer when the ink is
disposed on a
substrate and the element has an optically active fraction of at least 70%.
Various aspects of the present invention can have any of the following
features.
Additionally, elements of these aspects or those described below, along with
any of the
features described below, can be used alone or in combination to form a
display. A
plurality of capsules can be disposed on the substrate and can be in
association with a
binder, thereby to form a film. The film can have a binder that includes a
binder solid
and a ratio of a mass of the binder solid to a mass of the capsules, of at
least a portion of
the element, can be from about 1:2 to about 1:20. At least a portion of the
element can
have an optically active fraction of at least 70%. The capsules can be non-
spherical
and/or substantially planar on at least one side proximate the substrate. The
film can
include closely-packed capsules. At least one of the capsules can include a
suspending
fluid and at least one species of particle, or at least one of the capsules
can include at
least two species of particles such that an optical property of at least two
of the particle
species is different. The capsules can be a polymer matrix having fluid-
containing (such
as oil) cavities. A capsule wall defines the capsule and can have a thickness
from about
0.2 m to about 10 m. The substrate can include a polymeric material, a
polyester film,
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and/or at least one electrode (such as indium tin oxide). The substrate can be
from about
25 m to about 500 m thick.
An element also can have a layer of material that substantially fills
interstices
formed within the film that also can be substantially planar or a side
opposite the film. A
rear substrate can be disposed adjacent the layer of material. The layer of
material can
be initially associated with the film or with the rear substrate. The
capsules, binder, and
layer of material can form a stratum having a substantially uniform thickness
and/or a
stratum that is substantially free from voids. The stratum can be from about
10 m to
about 500 m thick, preferably about 50 m to about 300 m thick. The capsules
can be
of substantially uniform size. The layer of material can be the binder. The
layer of
material can include an insulator, conductor, or semiconductor. The layer of
material can
be tacky or liquid prior to, during, and/or after substantially filling the
interstices within
the film. The layer of material can have a thickness of less than or equal to
about 50 m.
The layer of material can include an adhesive containing, for example, carbon
particles,
gold particles, aluminum particles, platinum particles, silver particles,
plated polymer
spheres, plated glass spheres, indium tin oxide particles, polyacetylene,
polyaniline,
polypyrrole, polyethylene dioxythiophene ("P-DOT"), andlor polythiophene.
The rear substrate can include at least one electrode, at least one
transistor, and/or
at least one diode. The transistor can be at least organic material or silicon-
based. The
rear substrate can include a polymeric material, a glass, or a metal.
In an embodiment, an encapsulated electrophoretic element includes a plurality
of
non-spherical capsules disposed substantially in a single layer on a
substrate, thereby to
form a film. Typically, the element of this embodiment contains substantially
no binder.
This aspect can have any of the features described above. Additionally, this
aspect can
have any of the following features. The capsules and layer of material can
form a
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stratum that has a substantially uniform thickness and/or that is
substantially free from
voids. Additionally, one or more of the elements of this aspect of the
invention can be
combined with other elements of this aspect or the other aspects of the
present invention
to form a display.
A process is disclosed for creating an encapsulated electrophoretic element
that is
capable of having a plurality of capsules disposed on a substrate in
substantially a single
layer includes the steps of (a) providing the capsules; (b) mixing at least
one of the
capsules with a binder to create a capsule/binder mixture; (c) coating the
capsule/binder
mixture onto an at least partially conductive substrate, thereby to create a
film; and (d)
curing the capsule/binder mixture.
This process can have any of the features listed above or any of the following
features. The binder can be selected from the group consisting of acrylic,
urethane, and
poly(vinyl alcohol). The binder can include a polymer latex. The binder can
have a
fraction that is capable of evaporating. The conductive substrate can include
an indium
tin oxide sputtered polyester film. At least one of the capsules can contain a
plurality of
particles (such as titanium dioxide particles) dispersed in a suspending
fluid. The
suspending fluid can include a halogenated hydrocarbon and/or an aliphatic
hydrocarbon.
The coating step can include applying pressurized gas to the capsule/binder
mixture, thereby to cause deposition of the capsule/binder mixture onto the
substrate
such that the capsules are disposed on the substrate in substantially a single
layer. The
coating step can further include heating, cooling, and/or adding a liquid to
the
pressurized gas prior to or during application of the pressurized gas to the
capsule/binder
mixture. The liquid can be in droplet form and/or can be an organic solvent.
The
organic solvent can include, for example, butyl acetate, methylene chloride,
and/or
chlorobenzene. The organic solvent can include an alcohol, for example,
isopropyl
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alcohol, methanol, and/or ethanol. The coating step can include applying the
pressurized
gas with an air knife at a distance of about 1 cm to about 15 cm from the
surface of the
capsule/binder mixture and/or at an angle of from about 0 degrees to about 90
degrees
from the surface of the conductive substrate. The pressurized gas can include
air.
The coating step can include coating at least some of the capsules onto a film
through a coating head, for example with a pump that, typically, provides
pumping
pressure with a low shear force. At least some of the capsules can be disposed
in and
form a single layer. The coating head can be a slot die coating head.
Typically, a width
of a slot of the slot die coating head is between about 1 and about 2.5 times
the mean
diameter of the capsules.
The process can further include laminating the film to a rear substrate. A
layer of
material can be disposed between the film and the rear substrate. The layer of
material
can be
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associated with the rear substrate and/or with the film prior to laminating.
Heating, pressurizing,
andlor evacuating of a gas can occur during the step of laminating. The layer
of material can be
insulating, conductive, or semiconductive. The layer of material can be tacky
or in a liquid state
during at least a portion of the laminating step. The layer of material can
include the binder.
The step of laminating can produce a stratum comprising the capsules, binder,
and layer of
material. The stratum can have at least one substantially planar side
proximate the rear substrate,
can be substantially free from voids, and/or have a substantially uniform
thickness. The layer of
material can substantially fill interstices within the film. The layer of
material can have a
thickness of less than or equal to about 50 m. The layer of material can
include an adhesive
l0 containing, for example, carbon particles, gold particles, aluminum
particles, platinum particles,
silver particles, plated polymer spheres, plated glass spheres, indium tin
oxide particles,
polyacetylene, polyaniline, polypyrrole, P-DOT, and/or polythiophene. The
binder can include a
binder solid and a ratio of the mass of the binder solid to the mass of the
capsules of at least a
portion of the element can be from about 1:2 to about 1:20.
The process can further include the step of removing water from association
with at least
some of the capsules. The step of removing water can include a process
selected from the group
consisting of centrifuging, absorbing, evaporating, mesh filtrating and
osmotic separating.
Brief Description of the Drawings
The invention, in accordance with preferred and exemplary embodiments,
together with
further advantages thereof, is more particularly described in the following
detailed description,
taken in conjunction with the accompanying drawings.
In the drawings, like reference characters generally refer to the same parts
throughout the
different views. Also, the drawings are not necessarily to scale, emphasis
instead generally
being placed upon illustrating principles of the invention.
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FIG. 1 A is a schematic representation of a cross-section of a monolayer of
capsules;
FIG. 1B is a schematic representation of a cross-section of a monolayer
ofdeformable,
non-spherical capsules;
FIG. 2 schematically illustrates a coating process using a slot die coater
according to one
embodiment of the invention;
FIG. 3 schematically illustrates a slot die coater depositing a monolayer of
capsules
according to one embodiment of the invention;
FIG. 4 schematically illustrates a coating process using an air knife coater
according to
one embodiment of the invention;
FIG. 5A is a schematic illustration of a film prior to contact with a layer of
material to fill
the interstices within the film;
FIG. 5B is a schematic illustration of the film of FIG. 5A after the layer of
material has
been applied and laminated to a rear substrate;
FIG. 6A is a schematic top view of a display illustrating calculation of
optically active
fraction;
FIG. 6B is a schematic side view of a display illustrating calculation of
optically active
fraction;
FIG. 7A is a schematic illustration of an apparatus for performing emulsion-
based
encapsulation;
FIG. 7B is a schematic illustration of an oil drop of suspending fluid having
white and
black particles dispersed within it;
FIG. 7C is a schematic illustration of an oil drop of darkly dyed suspending
fluid having
white microparticles and charge control agents dispersed within it;
FIG. 8 schematically depicts removing water from association with capsules;
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FIG. 9A is a schematic illustration of a film without binder prior to contact
with a layer
of material to fill the interstices within the film;
FIG. 9B is a schematic illustration of the film of FIG. 9A after the layer of
material has
been applied and after lamination to a rear substrate; and
FIG. 10 is a schematic illustration of capsules that are fluid-filled cavities
in a matrix.
Description
The present invention provides materials and methods that improve performance
of
encapsulated electrophoretic display devices. In the construction of
encapsulated electrophoretic
display devices, a closely packed structure of capsules (which typically
contain electrophoretic
particles) in a single layer is desirable. For example, capsules in a closely
packed structure
include those in a high-density, closely-spaced configuration. Additionally,
deformable capsules
that allow the walls of the capsules to fit together closely, with little
binding material between
them, are desirable. For example, these capsules may take on a non-spherical
shape.
Generally, an encapsulated electrophoretic display includes one or more
species of
particle that either absorb or scatter light. One example is a system in which
the capsules contain
one or more species of electrophoretically mobile particles dispersed in a
dyed suspending fluid.
Another example is a system in which the capsules contain two separate species
of particles
suspended in a clear suspending fluid, in which one species of particle
absorbs light (dark), while
the other species of particle scatters light (white). There are other
extensions (more than two
species of particles, with or without a dye, etc.). The particles are commonly
solid pigments,
dyed particles, or pigment/polymer composites.
In a closely packed state, typically, a single layer of capsules is desirable
as the optically
active portion of the device. The capsules typically contain an opaque
pigment, and transmit
little or no light in any state of the device. Thus, light impinging on the
first layer of close-
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packed capsules is either scattered or absorbed. Little light passes through
the capsules. If
individual capsules (or a second layer of capsules) are positioned underneath
the first layer of
capsules, little or no light reaches these capsules. As such, the second layer
does not contribute
significantly to the optical effect. Additionally, because additional layers
of capsules add
thickness to the film, the voltage required to operate the film is raised
without providing any
optical benefit.
Thus, construction of the encapsulated electrophoretic display device with
capsules in a
closely-packed monolayer is desirable. Referring to FIGS. lA and 1B,
typically, capsules 2 are
substantially uniform in size. Also, the capsules 2 can be deformable such
that a monolayer 4
can form a flat (or nearly flat) surface 6 as shown in FIG. 1 B. In one
instance, the flat surface 6
allows the capsules 2 to pack together more closely, thus allowing the
particles 8 within the
capsules 2 to more uniformly spread across the face of the display (compare
the distribution of
particles 8 in FIG. 1A with the distribution in FIG. 1B). Additionally, a flat
surface on the other
side of the capsules (not shown) allows for laminating a second substrate (or
second conductive
substrate) that makes good contact with the capsule layer. This flat top
surface can form
spontaneously, or can be formed by coating or laminating another material to
the capsules.
Typically, capsules have a wall thickness of about 0.2 m to about 10 m, more
preferably about
1 m to about 5 m.
In fact, one way to measure the state of the display involves a variable
called the
"optically active fraction." This variable refers to an area of a display that
is capable of having
its appearance changed as compared with the total area of a display. The
variable can be
expressed as a ratio, namely, (changeable surface area of display) / (total
surface area of display).
When calculating total area, one can easily calculate a surface area of the
display using common
geometric formulae. However, due to the nature of capsules, a viewer sees
optically active areas
of capsules (i.e., visible portions of the capsule that change appearance)
that are not in the plane
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of the display, the plane upon which a total surface area usually is
calculated. Thus, the location
of optically active areas must be extrapolated to the plane upon which total
surface area is
calculated in order to estimate the optically active fraction.
FIGS. 6A and 6B illustrate extrapolation of the optically active areas for
estimating the
optically active fraction. FIG. 6A is a top view and FIG. 6B is a side view of
the same structure,
both views are aligned. Four capsules 100, 102, 104, 106 are shown in a
schematic plane 110 of
a display. This rectangular plane 110 represents the total surface area of the
display. Due to the
shape of the capsules 100, 102, 104, 106, the optically active area of the
capsules 100, 102, 104,
106 that is roughly coincident with the plane 110 (shown with solid lines in
FIG. 6A) is slightly
smaller than the total optically active area (shown with dashed lines in FIG.
6A). Thus,
according to the extrapolation technique, the total optically active area, as
represented by the
dashed lines, is superimposed on the plane 110. FIG. 6B shows how a portion of
the capsules
100, 102, 104, 106 is close to, but not coincident with, the plane 110 of the
display, explaining
why the solid lines and dashed lines in FIG. 6A are not coincident. In
practice, useful optically
active fractions are equal to or greater than about 70% and more preferably
equal to or greater
than about 90%.
Non-spherical microcapsules can be formed during the encapsulation phase, by,
for
example, using a non-uniform shear field or a compressive pressure. Such non-
spherical
capsules can also be formed during the processing of the display when the
binder is drying or
curing. In such a system, as the binder shrinks, it pulls capsules close to
one another and pulls
the capsules down toward the substrate on which they have been coated. For
example, an
aqueous evaporative binder, such as a waterbourne acrylic, urethane, or poly
(vinylalcohol),
tends to exhibit such shrinking properties. Typically, a fraction of the
binder, such as water,
evaporates. Other evaporative binders, emulsions, or solutions also are
suitable. The solvent
need not be water, but can be an organic liquid or a combination of liquids.
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Also, non-spherical capsules can be formed, for example, by applying a force
to the film
as it is drying or curing to permanently deform the capsules. Such a force can
be applied by a
pair of rollers, by a vacuum lamination press, by a mechanical press, or by
any other suitable
means. Such non-spherical capsules can also be formed by stretching the cured
film in one or
both of the planar axes of the film. After completion of the curing process,
the capsule can
protrude above the surface of the cured film, resulting in a lens effect that
enhances the optical
properties of the capsule. Finally, the capsule also can be formed of a
material which softens in
the binder, thus allowing the capsules to deform to form a flat surface when
the capsules and
binder are laid down and the binder is cured.
In another embodiment, a polymer-dispersed electrophoretic display is
constructed in a
manner similar to a polymer-dispersed liquid crystal display. A fluid is mixed
with a binder.
Typically, the fluid can be an oil. As the binder is dried or is cured, the
fluid is pulled into non-
spherical cavities. These fluid-containing cavities can be elastomeric
capsules. These cavities
typically lack capsule walls. For example, FIG. 10 shows a cavity 60 filled
with an oi164. The
cavity is situated in a matrix 62. The matrix 62 is adjacent a substrate 66.
Typically, the matrix
62 is formed from a polymer which can be a binder. In a preferred embodiment,
the aspect ratio
(i.e., ratio of width, w, to height, h) of these cavities is preferably
greater than about 1.2. The
aspect ratio is more preferably greater than about 1.5, and, in a particularly
preferred
embodiment, the aspect ratio is greater than about 1.75. In a preferred
embodiment, a display
having non-spherical capsules has a volume fraction (i.e., fraction of total
volume) of binder
between about 0 to about 0.9. More preferably, the volume fraction is between
about 0.05 and
about 0.2.
An electrophoretic display is constructed as either an encapsulated
electrophoretic
display or a polymer-dispersed electrophoretic display (similar in
construction to a polymer
dispersed liquid crystal display), and the non-spherical capsules or liquid
droplets are formed by
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flattening, by shrinkage of the binder, or by mechanical force. In each case,
the capsules should
be capable of deforming, or they may rupture. In the case of a polymer-
dispersed electrophoretic
display, the encapsulated phases change shape as the polymer shrinks. In
addition, the
encapsulated phases may be deformed asymmetrically by stretching the
substrate. Another
technique which may be employed is to first dry the binder in such a way that
a tough top skin is
formed. The rest of the binder may then be dried slowly with no fear of the
top surface breaking
or becoming too uneven.
Taking a step back from the specifics of monolayers and methods of forming
monolayers
according to the invention, Section I generally describes some of the
components of
electrophoretic displays according to the invention. More detail is provided
in United States
Application Serial No. 09/141,105 filed August 27, 1998, the entirety of which
is incorporated
herein by reference. Section II describes components of displays and processes
for constructing
displays in a monolayer.
I. Electrophoretic Display Components
A. Particles
There is much flexibility in the choice of particles for use in
electrophoretic displays, as
described above. For purposes of this invention, a particle is any component
that is charged or
capable of acquiring a charge (i.e., has or is capable of acquiring
electrophoretic mobility), and,
in some cases, this mobility may be zero or close to zero (i.e., the particles
will not move). The
particles may be neat pigments, dyed (laked) pigments or pigment/polymer
composites, or any
other component that is charged or capable of acquiring a charge. Typical
considerations for the
electrophoretic particle are its optical properties, electricai properties,
and surface chemistry.
The particles may be organic or inorganic compounds, and they may either
absorb light or scatter
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light. The particles for use in the invention may further include scattering
pigments, absorbing
pigments and luminescent particles. The particles may be retroreflective, such
as corner cubes,
or they may be electroluminescent, such as zinc sulfide particles, which emit
light when excited
by an AC field, or they may be photoluminescent. Finally, the particles may be
surface treated
so as to improve charging or interaction with a charging agent, or to improve
dispersibility.
One particle for use in electrophoretic displays of the invention is titania.
The titania
particles may be coated with a metal oxide, such as aluminum oxide or silicon
oxide, for
example. The titania particles may have one, two, or more layers of metal-
oxide coating. For
example, a titania particle for use in electrophoretic displays of the
invention may have a coating
of aluminum oxide and a coating of silicon oxide. The coatings may be added to
the particle in
any order.
The electrophoretic particle is usually a pigment, a polymer, a laked pigment,
or some
combination of the above. A neat pigment can be any pigment, and, usually for
a light colored
particle, pigments such as rutile (titania), anatase (titania), barium
sulfate, kaolin, or zinc oxide
are useful. Some typical particles have high refractive indices, high
scattering coefficients, and
low absorption coefficients. Other particles are absorptive, such as carbon
black or colored
pigments used in paints and inks. The pigment should also be insoluble in the
suspending fluid.
Yellow pigments such as diarylide yellow, hansa yellow, and benzidin yellow
have also found
use in similar displays. Any other reflective material can be employed for a
light colored
particle, including non-pigment materials, such as metallic particles.
Useful neat pigments include, but are not limited to, PbCrO4, Cyan blue GT 55-
3295
(American Cyanamid Company, Wayne, NJ), Cibacron Black BG (Ciba Company, Inc.,
Newport, DE), Cibacron Turquoise Blue G (Ciba), Cibalon Black BGL (Ciba),
Orasol Black
BRG (Ciba), Orasol Black RBL (Ciba), Acetamine Blac, CBS (E. I. du Pont de
Nemours and
Company, Inc., Wilmington, DE), Crocein Scarlet N Ex (du Pont) (27290), Fiber
Black VF
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(duPont) (30235), Luxol Fast Black L (duPont) (Solv. Black 17), Nirosine Base
No. 424
(duPont) (50415 B), Oil Black BG (duPont) (Solv. Black 16), Rotalin Black RM
(duPont),
Sevron Brilliant Red 3 B (duPont); Basic Black DSC (Dye Specialties, Inc.),
Hectolene Black
(Dye Specialties, Inc.), Azosol Brilliant Blue B(GAF, Dyestuff and Chemical
Division, Wayne,
NJ) (Solv. Blue 9), Azosol Brilliant Green BA (GAF) (Solv. Green 2), Azosol
Fast Brilliant Red
B (GAF), Azosol Fast Orange RA Cone. (GAF) (Solv. Orange 20), Azosol Fast
Yellow GRA
Conc. (GAF) (13900 A), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF)
(35435),
Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black 9), Celliton Fast Blue AF Ex
Conc (GAF)
(Disp. Blue 9), Cyper Black IA (GAF) (Basic Blk. 3), Diamine Black CAP Ex Conc
(GAF)
(30235), Diamond Black EAN Hi Con. CF (GAF) (15710), Diamond Black PBBA Ex
(GAF)
(16505); Direct Deep Black EA Ex CF (GAF) (30235), Hansa Yellow G(GAF)
(11680);
Indanthrene Black BBK Powd. (GAF) (59850), Indocarbon CLGS Conc. CF (GAF)
(53295),
Katigen Deep Black NND Hi Conc. CF (GAF) (15711), Rapidogen Black 3 G(GAF)
(Azoic
Blk. 4); Sulphone Cyanine Black BA-CF (GAF) (26370), Zambezi Black VD Ex Conc.
(GAF)
(30015); Rubanox Red CP-1495 (The Sherwin-Williams Company, Cleveland, OH)
(15630);
Raven 11 (Columbian Carbon Company, Atlanta, GA), (carbon black aggregates
with a particle
size of about 25 m), Statex B-12 (Columbian Carbon Co.) (a furnace black of
33 m average
particle size), and chrome green.
Particles may also include laked, or dyed, pigments. Laked pigments are
particles that
have a dye precipitated on them or which are stained. Lakes are metal salts of
readily soluble
anionic dyes. These are dyes of azo, triphenylmethane or anthraquinone
structure containing one
or more sulphonic or carboxylic acid groupings. They are usually precipitated
by a calcium,
barium or aluminum salt onto a substrate. Typical examples are peacock blue
lake (CI Pigment
Blue 24) and Persian orange (lake of CI Acid Orange 7), Black M Toner (GAF) (a
mixture of
carbon black and black dye precipitated on a lake).
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A dark particle of the dyed type may be constructed from any light absorbing
material,
such as carbon black, or inorganic black materials. The dark material may also
be selectively
absorbing. For example, a dark green pigment may be used. Black particles may
also be formed
by staining latices with metal oxides, such latex copolymers consisting of any
of butadiene,
styrene, isoprene, methacrylic acid, methyl methacrylate, acrylonitrile, vinyl
chloride, acrylic
acid, sodium styrene sulfonate, vinyl acetate, chlorostyrene,
dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate and N-
(isobutoxymethacrylamide), and optionally including conjugated diene compounds
such as
diacrylate, triacrylate, dimethylacrylate and trimethacrylate. Black particles
may also be formed
by a dispersion polymerization technique.
In the systems containing pigments and polymers, the pigments and polymers may
form
multiple domains within the electrophoretic particle, or be aggregates of
smaller
pigment/polymer combined particles. Alternatively, a central pigment core may
be surrounded
by a polymer shell. The pigment, polymer, or both can contain a dye. The
optical purpose of the
particle may be to scatter light, absorb light, or both. Useful sizes may
range from I nm up to
about 100 m, as long as the particles are smaller than the bounding capsule.
The density of the
electrophoretic particle may be substantially matched to that of the
suspending (i.e.,
electrophoretic) fluid. As defined herein, a suspending fluid has a density
that is "substantially
matched" to the density of the particle if the difference in their respective
densities is between
about zero and about two grams/milliliter ("g/ml"). This difference is
preferably between about
zero and about 0.5 g/ml.
Useful polymers for the particles include, but are not limited to:
polystyrene,
polyethylene, polypropylene, phenolic resins, Du Pont Elvax resins (ethylene-
vinyl acetate
copolymers), polyesters, polyacrylates, polymethacrylates, ethylene acrylic
acid or methacrylic
acid copolymers (Nucrel Resins - Dupont, Primacor Resins- Dow Chemical),
acrylic copolymers
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and terpolymers (Elvacite Resins, DuPont) and PMMA. Useful materials for
homopolymer /
pigment phase separation in high shear melt include, but are not limited to,
polyethylene,
polypropylene, polymethylmethacrylate, polyisobutylmethacrylate, polystyrene,
polybutadiene,
polyisoprene, polyisobutylene, polylauryl methacrylate, polystearyl
methacrylate, polyisobornyl
methacrylate, poly-t-butyl methacrylate, polyethyl methacrylate, polymethyl
acrylate, polyethyl
acrylate, polyacrylonitrile, and copolymers of two or more of these materials.
Some useful
pigment/polymer complexes that are commercially available include, but are not
limited to,
Process Magenta PM 1776 (Magruder Color Company, Inc., Elizabeth, NJ), Methyl
Violet PMA
VM6223 (Magruder Color Company, Inc., Elizabeth, NJ), and Naphthol FGR RF6257
(Magruder Color Company, Inc., Elizabeth, NJ).
The pigment-polymer composite may be formed by a physical process, (e.g.,
attrition or
ball milling), a chemical process (e.g., microencapsulation or dispersion
polymerization), or any
other process known in the art of particle production. For example, the
processes and materials
for both the fabrication of liquid toner particles and the charging of those
particles may be
relevant.
New and useful electrophoretic particles may still be discovered, but a number
of
particles already known to those skilled in the art of electrophoretic
displays and liquid toners
can also prove useful. In general, the polymer requirements for liquid toners
and encapsulated
electrophoretic inks are similar, in that the pigment or dye must be easily
incorporated therein,
either by a physical, chemical, or physicochemical process, may aid in the
colloidal stability, and
may contain charging sites or may be able to incorporate materials which
contain charging sites.
One general requirement from the liquid toner industry that is not shared by
encapsulated
electrophoretic inks is that the toner must be capable of "fixing" the image,
i.e., heat fusing
together to create a uniform film after the deposition of the toner particles.
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Typical manufacturing techniques for particles may be drawn from the liquid
toner and
other arts and include ball milling, attrition, jet milling, etc. The process
will be illustrated for
the case of a pigmented polymeric particle. In such a case the pigment is
compounded in the
polymer, usually in some kind of high shear mechanism such as a screw
extruder. The
composite material is then (wet or dry) ground to a starting size of around 10
m. It is then
dispersed in a carrier liquid, for example ISOPARO (Exxon, Houston, TX),
optionally with some
charge control agent(s), and milled under high shear for several hours down to
a final particle
size and/or size distribution.
Another manufacturing technique for particles is to add the polymer, pigment,
and
suspending fluid to a media mill. The mill is started and simultaneously
heated to temperature at
which the polymer swells substantially with the solvent. This temperature is
typically near
100 C. In this state, the pigment is easily encapsulated into the swollen
polymer. After a
suitable time, typically a few hours, the mill is gradually cooled back to
ambient temperature
while stirring. The milling may be continued for some time to achieve a small
enough particle
size, typically a few microns in diameter. The charging agents may be added at
this time.
Optionally, more suspending fluid may be added.
Chemical processes such as dispersion polymerization, mini- or micro-emulsion
polymerization, suspension polymerization precipitation, phase separation,
solvent evaporation,
in situ polymerization, seeded emulsion polymerization, or any process which
falls under the
general category of microencapsulation may be used. A typical process of this
type is a phase
separation process wherein a dissolved polymeric material is precipitated out
of solution onto a
dispersed pigment surface through solvent dilution, evaporation, or a thermal
change. Other
processes include chemical means for staining polymeric latices, for example
with metal oxides
or dyes.
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B. Suspending Fluid
The suspending fluid containing the particles can be chosen based on
properties such as
density, refractive index, and solubility. A preferred suspending fluid has a
low dielectric
constant (about 2), high volume resistivity (about 1015 ohm-cm), low viscosity
(less than 5
centistokes ("cst")), low toxicity and environmental impact, low water
solubility (less than 10
parts per million ("ppm")), high specific gravity (greater than 1.5), a high
boiling point (greater
than 90 C), and a low refractive index (less than 1.2).
The choice of suspending fluid may be based on concerns of chemical inertness,
density
matching to the electrophoretic particle, or chemical compatibility with both
the electrophoretic
particle and bounding capsule. The viscosity of the fluid should be low when
movement of the
particles is desired. The refractive index of the suspending fluid may also be
substantially
matched to that of the particles. As used herein, the refractive index of a
suspending fluid "is
substantially matched" to that of a particle if the difference between their
respective refractive
indices is between about zero and about 0.3, and is preferably between about
0.05 and about 0.2.
Additionally, the fluid may be chosen to be a poor solvent for some polymers,
which is
advantageous for use in the fabrication of microparticles, because it
increases the range of
polymeric materials useful in fabricating particles of polymers and pigments.
Organic solvents,
such as halogenated organic solvents, saturated linear or branched
hydrocarbons, silicone oils,
and low molecular weight halogen-containing polymers are some useful
suspending fluids. The
suspending fluid may comprise a single fluid. The fluid will, however, often
be a blend of more
than one fluid in order to tune its chemical and physical properties.
Furthermore, the fluid may
contain surface modifiers to modify the surface energy or charge of the
electrophoretic particle
or bounding capsule. Reactants or solvents for the microencapsulation process
(oil soluble
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monomers, for example) can also be contained in the suspending fluid. Charge
control agents
can also be added to the suspending fluid.
Useful organic solvents include, but are not limited to, epoxides, such as
decane epoxide
and dodecane epoxide; vinyl ethers, such as cyclohexyl vinyl ether and Decave
(International
Flavors & Fragrances, Inc., New York, NY); and aromatic hydrocarbons, such as
toluene and
naphthalene. Useful halogenated organic solvents include, but are not limited
to,
tetrafluorodibromoethylene, tetrachloroethylene, trifluorochloroethylene,
1,2,4-trichlorobenzene,
carbon tetrachloride. These materials have high densities. Useful hydrocarbons
include, but are
not limited to, dodecane, tetradecane, the aliphatic hydrocarbons in the
Isopar series (Exxon,
Houston, TX), Norpar ( series of normal paraffinic liquids), Shell-Sol
(Shell, Houston, TX),
and Sol-Trol (Shell), naphtha, and other petroleum solvents. These materials
usually have low
densities. Useful examples of silicone oils include, but are not limited to,
octamethyl
cyclosiloxane and higher molecular weight cyclic siloxanes, poly (methyl
phenyl siloxane),
hexamethyldisiloxane, and polydimethylsiloxane. These materials usually have
low densities.
Useful low molecular weight halogen-containing polymers include, but are not
limited to,
poly(chlorotrifluoroethylene) polymer (Halogenated hydrocarbon Inc., River
Edge, NJ), Galdent
(a perfluorinated ether from Ausimont, Morristown, NJ), or Krytox from Dupont
(Wilmington,
DE). In a preferred embodiment, the suspending fluid is a
poly(chlorotrifluoroethylene)
polymer. In a particularly preferred embodiment, this polymer has a degree of
polymerization
from about 2 to about 10. Many of the above materials are available in a range
of viscosities,
densities, and boiling points.
The fluid must be capable of being formed into small droplets prior to a
capsule being
formed. Processes for forming small droplets include flow-through jets,
membranes, nozzles, or
orifices, as well as shear-based emulsifying schemes. The formation of small
drops may be
assisted by electrical or sonic fields. Surfactants and polymers can be used
to aid in the
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stabilization and emulsification of the droplets in the case of an emulsion
type encapsulation.
One surfactant for use in displays of the invention is sodium dodecylsulfate.
It can be advantageous in some displays for the suspending fluid to contain an
optically
absorbing dye. This dye must be soluble in the fluid, but will generally be
insoluble in the other
components of the capsule. There is much flexibility in the choice of dye
material. The dye can
be a pure compound, or blends of dyes to achieve a particular color, including
black. The dyes
can be fluorescent, which would produce a display in which the fluorescence
properties depend
on the position of the particles. The dyes can be photoactive, changing to
another color or
becoming colorless upon irradiation with either visible or ultraviolet light,
providing another
means for obtaining an optical response. Dyes could also be polymerizable,
forming a solid
absorbing polymer inside the bounding shell.
There are many dyes that can be chosen for use in encapsulated electrophoretic
display.
Properties important here include light fastness, solubility in the suspending
liquid, color, and
cost. These are generally from the class of azo, anthraquinone, and
triphenylmethane type dyes
and may be chemically modified so as to increase the solubility in the oil
phase and reduce the
adsorption by the particle surface.
A number of dyes already known to those skilled in the art of electrophoretic
displays
will prove useful. Useful azo dyes include, but are not limited to: the Oil
Red dyes, and the
Sudan Red and Sudan Black series of dyes. Useful anthraquinone dyes include,
but are not
limited to: the Oil Blue dyes, and the Macrolex Blue series of dyes. Useful
triphenylmethane
dyes include, but are not limited to, Michler's hydrol, Malachite Green,
Crystal Violet, and
Auramine O.
C. Charge Control Agents and Particle Stabilizers
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Charge control agents are used to provide good electrophoretic mobility to the
electrophoretic particles. Stabilizers are used to prevent agglomeration of
the electrophoretic
particles, as well as prevent the electrophoretic particles from irreversibly
depositing onto the
capsule wall. Either component can be constructed from materials across a wide
range of
molecular weights (low molecular weight, oligomeric, or polymeric), and may be
pure or a
mixture. The charge control agent used to modify and/or stabilize the particle
surface charge is
applied as generally known in the arts of liquid toners, electrophoretic
displays, non-aqueous
paint dispersions, and engine-oil additives. In all of these arts, charging
species may be added to
non-aqueous media in order to increase electrophoretic mobility or increase
electrostatic
stabilization. The materials can improve steric stabilization as well.
Different theories of
charging are postulated, including selective ion adsorption, proton transfer,
and contact
electrification.
An optional charge control agent or charge director may be used. These
constituents
typically consist of low molecular weight surfactants, polymeric agents, or
blends of one or more
components and serve to stabilize or otherwise modify the sign and/or
magnitude of the charge
on the electrophoretic particles. The charging properties of the pigment
itself may be accounted
for by taking into account the acidic or basic surface properties of the
pigment, or the charging
sites may take place on the carrier resin surface (if present), or a
combination of the two.
Additional pigment properties which may be relevant are the particle size
distribution, the
chemical composition, and the lightfastness. The charge control agent used to
modify and/or
stabilize the particle surface charge may be applied as generally known in the
arts of liquid
toners, electrophoretic displays, non-aqueous paint dispersions, and engine-
oil additives. In all
of these arts, charging species may be added to non-aqueous media in order to
increase
electrophoretic mobility or increase electrostatic stabilization. The
materials can improve steric
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stabilization as well. Different theories of charging are postulated,
including selective ion
adsorption, proton transfer, and contact electrification.
Charge adjuvents may also be added. These materials increase the effectiveness
of the
charge control agents or charge directors. The charge adjuvent may be a
polyhydroxy compound
or an aminoalcohol compound, which are preferably soluble in the suspending
fluid in an amount
of at least 2% by weight. Examples of polyhydroxy compounds which contain at
least two
hydroxyl groups include, but are not limited to, ethylene glycol, 2,4,7,9-
tetramethyl-decyn-4,7-
diol, poly (propylene glycol), pentaethylene glycol, tripropylene glycol,
triethylene glycol,
glycerol, pentaerythritol, glycerol-tri-12 hydroxystearate, propylene glycerol
monohydroxystearate, and ethylene glycol monohydroxystrearate. Examples of
aminoalcohol
compounds which contain at least one alcohol function and one amine function
in the same
- molecule include, but are not limited to, triisopropanolamine,
triethanolamine, ethanolamine, 3-
amino-1 propanol, o-aminophenol, 5-amino-1-pentanol, and tetra(2-
hydroxyethyl)ethylene-
diamine. The charge adjuvent is preferably present in the suspending fluid in
an amount of about
1 to about 100 milligrams per gram ("mg/g") of the particle mass, and more
preferably about 50
to about 200 mg/g.
The surface of the particle may also be chemically modified to aid dispersion,
to improve
surface charge, and to improve the stability of the dispersion, for example.
Surface modifiers
include organic siloxanes, organohalogen silanes and other functional silane
coupling agents
(Dow Corning Z-6070, Z-6124, and 3 additive, Midland, MI); organic titanates
and zirconates
(Tyzor TOT, TBT, and TE Series, Dupont, Wilmington, DE); hydrophobing agents,
such as
long chain (C12 to C50) alkyl and alkyl benzene sulphonic acids, fatty amines
or diamines and
their salts or quatemary derivatives; and amphipathic polymers which can be
covalently bonded
to the particle surface.
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in general, it is believed that charging results as an acid-base reaction
between some
moiety present in the continuous phase and the particle surface. Thus useful
materials are those
which are capable of participating in such a reaction, or any other charging
reaction as known in
the art.
Different non-limiting classes of charge control agents which are useful
include organic
sulfates or sulfonates, metal soaps, block or comb copolymers, organic amides,
organic
zwitterions, and organic phosphates and phosphonates. Useful organic sulfates
and sulfonates
include, but are not limited to, bis(2-ethyl hexyl) sodium sulfosuccinate,
calcium dodecyl
benzene sulfonate, calcium petroleum sulfonate, neutral or basic barium
dinonylnaphthalene
sulfonate, neutral or basic calcium dinonylnaphthalene sulfonate,
dodecylbenzenesulfonic acid
sodium salt, and ammonium lauryl sulphate. Useful metal soaps include, but are
not limited to,
basic or neutral barium petronate, calcium petronate, Co-, Ca-, Cu-, Mn-, Ni-,
Zn-, and Fe- salts
of naphthenic acid, Ba-, Al-, Zn-, Cu-, Pb-, and Fe- salts of stearic acid,
divalent and trivalent
metal carboxylates, such as aluminum tristearate, aluminum octoate, lithium
heptanoate, iron
stearate, iron distearate, barium stearate, chromium stearate, magnesium
octoate, calcium
stearate, iron naphthenate, and zinc naphthenate, Mn- and Zn- heptanoate, and
Ba-, Al-, Co-,
Mn-, and Zn- Octoate. Useful block or comb copolymers include, but are not
limited to, AB
diblock copolymers of (A) polymers of 2-(N,N) dimethylaminoethyl methacrylate
quatemized
with methyl-p-toluenesulfonate and (B) poly-2-ethylhexyl methacrylate, and
comb graft
copolymers with oil soluble tails of poly (12-hydroxystearic acid) and having
a molecular weight
of about 1800, pendant on an oil-soluble anchor group of poly (methyl
methacrylate-methacrylic
acid). Useful organic amides include, but are not limited to, polyisobutylene
succinimides such
as OLOA 1200, and N-vinyl pyrrolidone polymers. Useful organic zwitterions
include, but are
not limited to, lecithin. Useful organic phosphates and phosphonates include,
but are not limited
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to, the sodium salts of phosphated mono- and di-glycerides with saturated and
unsaturated acid
substituents.
Particle dispersion stabilizers may be added to prevent particle flocculation
or attachment
to the capsule walls. For the typical high resistivity liquids used as
suspending fluids in
electrophoretic displays, nonaqueous surfactants may be used. These include,
but are not limited
to, glycol ethers, acetylenic glycols, alkanolamides, sorbitol derivatives,
alkyl amines, quaternary
amines, imidazolines, dialkyl oxides, and sulfosuccinates.
D. Encapsulation
Encapsulation of the internal phase may be accomplished in a number of
different ways.
Numerous suitable procedures for microencapsulation are detailed in both
Microencapsulation,
Processes and Applications, (I. E. Vandegaer, ed.), Plenuin Press, New York,
NY (1974) and
Gutcho, Microcapsules and Microencapsulation Techniques, Nuyes Data Corp.,
Park Ridge, N.J.
(1976). The processes fall into several general categories, all of which can
be applied to the
present invention: interfacial polymerization, in situ polymerization,
physical processes, such as
coextrusion and other phase separation processes, in-liquid curing, and
simple/complex
coacervation.
Numerous materials and processes should prove useful in formulating displays
of the
present invention. Useful materials for simple coacervation processes to form
the capsule
include, but are not limited to, gelatin, polyvinyl alcohol, polyvinyl
acetate, and cellulosic
derivatives, such as, for example, carboxymethylcellulose. Useful materials
for complex
coacervation processes include, but are not limited to, gelatin, acacia,
carageenan,
carboxymethylcellulose, hydrolyzed styrene anhydride copolymers, agar,
alginate, casein,
albumin, methyl vinyl ether co-maleic anhydride, and cellulose phthalate.
Useful materials for
phase separation processes include, but are not limited to, polystyrene, PMMA,
polyethyl
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methacrylate, polybutyl methacrylate, ethyl cellulose, polyvinyl pyridine, and
poly acrylonitrile.
Useful materials for in situ polymerization processes include, but are not
limited to,
polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde; water-
soluble
oligomers of the condensate of melamine, or urea and formaldehyde; and vinyl
monomers, such
as, for example, styrene, MMA and acrylonitrile. Finally, useful materials for
interfacial
polymerization processes include, but are not limited to, diacyl chlorides,
such as, for example,
sebacoyl, adipoyl, and di- or poly- amines or alcohols, and isocyanates.
Useful emulsion
polymerization materials may include, but are not limited to, styrene, vinyl
acetate, acrylic acid,
butyl acrylate, t-butyl acrylate, methyl methacrylate, and butyl methacrylate.
Capsules produced may be dispersed into a curable carrier, resulting in an ink
which may
be printed or coated on large and arbitrarily shaped or curved surfaces using
conventional
printing and coating techniques.
In the context of the present invention, one skilled in the art will select an
encapsulation
procedure and wall material based on the desired capsule properties. These
properties include
the distribution of capsule radii; electrical, mechanical, diffusion, and
optical properties of the
capsule wall; and chemical compatibility with the internal phase of the
capsule.
The capsule wall generally has a high electrical resistivity. Although it is
possible to use
walls with relatively low resistivities, this may limit performance in
requiring relatively higher
addressing voltages. The capsule wall should also be mechanically strong
(although if the
finished capsule powder is to be dispersed in a curable polymeric binder for
coating, mechanical
strength is not as critical). The capsule wall should generally not be porous.
If, however, it is
desired to use an encapsulation procedure that produces porous capsules, these
can be overcoated
in a post-processing step (t. e., a second encapsulation). Moreover, if the
capsules are to be
dispersed in a curable binder, the binder will serve to close the pores. The
capsule walls should
be optically clear. The wall material may, however, be chosen to match the
refractive index of
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the internal phase of the capsule (i.e., the suspending fluid) or a binder in
which the capsules are
to be dispersed. For some applications (e.g., interposition between two fixed
electrodes),
monodispersed capsule radii are desirable.
An encapsulation technique that is suited to the present invention involves a
polymerization between urea and formaldehyde in an aqueous phase of an
oil/water emulsion in
the presence of a negatively charged, carboxyl-substituted, linear hydrocarbon
polyelectrolyte
material. The resulting capsule wall is a urea/formaldehyde copolymer, which
discretely
encloses the internal phase. The capsule is clear, mechanically strong, and
has good resistivity
properties.
The related technique of in situ polymerization utilizes an oil/water
emulsion, which is
formed by dispersing the electrophoretic composition (i.e., the dielectric
liquid containing a
suspension of the pigment particles) in an aqueous environment. The monomers
polymerize to
form a polymer with higher affinity for the internal phase than for the
aqueous phase, thus
condensing around the emulsified oily droplets. In one in situ polymerization
processes, urea
and formaldehyde condense in the presence of poly(acrylic acid) (See, e.g.,
U.S. Patent No.
4,001,140). In other processes, described in U.S. Patent No. 4,273,672, any of
a variety of cross-
linking agents borne in aqueous solution is deposited around microscopic oil
droplets. Such
cross-linking agents include aldehydes, especially formaldehyde, glyoxal, or
glutaraldehyde;
alum; zirconium salts; and poly isocyanates.
The coacervation approach also utilizes an oil/water emulsion. One or more
colloids are
coacervated (i.e., agglomerated) out of the aqueous phase and deposited as
shells around the oily
droplets through control of temperature, pH and/or relative concentrations,
thereby creating the
microcapsule. Materials suitable for coacervation include gelatins and gum
arabic. See, e.g.,
U.S. Patent No. 2,800,457.
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The interfacial polymerization approach relies on the presence of an oil-
soluble monomer
in the electrophoretic composition, which once again is present as an emulsion
in an aqueous
phase. The monomers in the minute hydrophobic droplets react with a monomer
introduced into
the aqueous phase, polymerizing at the interface between the droplets and the
surrounding
aqueous medium and forming shells around the droplets. Although the resulting
walls are
relatively thin and may be permeable, this process does not require the
elevated temperatures
characteristic of some other processes, and therefore affords greater
flexibility in terms of
choosing the dielectric liquid.
FIG. 7A illustrates an exemplary apparatus and environment for performing
emulsion-
based encapsulation. An oil/water emulsion, is prepared in a vessel 76
equipped with a device
78 for monitoring and a device 80 for controlling the temperature. A pH
monitor 82 may also be
included. An impeller 84 maintains agitation throughout the encapsulation
process, and in
combination with emulsifiers, can be used to control the size of the emulsion
droplets 86 that
will lead to the finished capsules. The aqueous continuous phase 88 may
contain, for example, a
prepolymer and various system modifiers.
FIG. 7B illustrates an oil drop 90 comprising a substantially transparent
suspending fluid
92, in which is dispersed white microparticles 94 and black particles 96.
Preferably, particles 94
and 96 have densities substantially matched to the density of suspending fluid
92. The liquid
phase may also contain some threshold/bistability modifiers, charge control
agents, and/or
hydrophobic monomers to effect an interfacial polymerization.
FIG. 7C illustrates a similar oil drop 98 comprising a darkly dyed suspending
fluid 100
containing a dispersion of white particles 94 and appropriate charge control
agents.
Coating aids can be used to improve the uniformity and quality of the coated
or printed
electrophoretic ink material. Wetting agents are typically added to adjust the
interfacial tension
at the coating/substrate interface and to adjust the liquid/air surface
tension. Wetting agents
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include, but are not limited to, anionic and cationic surfactants, and
nonionic species, such as
silicone or fluoropolymer based materials. Dispersing agents may be used to
modify the
interfacial tension between the capsules and binder, providing control over
flocculation and
particle settling.
Surface tension modifiers can be added to adjust the air/ink interfacial
tension.
Polysiloxanes are typically used in such an application to improve surface
leveling while
minimizing other defects within the coating. Surface tension modifiers
include, but are not
limited to, fluorinated surfactants, such as, for example, the Zonyl series
from DuPont
(Wilmington, DE), the Fluorod series from 3M (St. Paul, MN), and the
fluoroakyl series from
Autochem (Glen Rock, NJ); siloxanes, such as, for example, Silwet from Union
Carbide
(Danbury, CT); and polyethoxy and polypropoxy alcohols. Antifoams, such as
silicone and
silicone-free polymeric materials, may be added to enhance the movement of air
from within the
ink to the surface and to facilitate the rupture of bubbles at the coating
surface. Other useful
antifoams include, but are not limited to, glyceryl esters, polyhydric
alcohols, compounded
antifoams, such as oil solutions of alkyl benzenes, natural fats, fatty acids,
and metallic soaps,
and silicone antifoaming agents made from the combination of dimethyl siloxane
polymers and
silica. Stabilizers such as uv-absorbers and antioxidants may also be added to
improve the
lifetime of the ink.
Other additives to control properties like coating viscosity and foaming can
also be used
in the coating fluid. Stabilizers (uv-absorbers, antioxidants) and other
additives which could
prove useful in practical materials.
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1. Example l
The following procedure describes gelatin/acacia microencapsulation for use in
electrophoretic displays of the present invention.
a. Preparation of Oil (Internal) Phase
To a 1 L flask is added 0.5 g of Oil Blue N (Aldrich, Milwaukee, WI), 0.5 g of
Sudan Red
7B (Aldrich), 417.25 g of Halogenated hydrocarbon C?i10.8 (Halogenated
hydrocarbon Products
Corp., River Edge, NJ), and 73.67 g of Isopar-G (Exxon, Houston, TX). The
mixture is stirred
at 60 C for six hours and is then cooled to room temperature. 50.13 g of the
resulting solution is
placed in a 50 mL polypropylene centrifuge tube, to which is added 1.8 g of
titanium dioxide
(TiO2) (DuPont, Wilmington, DE), 0.78 g of a 10% solution of OLOA 1200
(Chevron, Somerset,
NJ), in Halogenated hydrocarbon Oil 0.8, and 0.15 g of Span 85 (Aldrich). This
mixture is then
sonicated for five minutes at power 9 in an Aquasonic Model 75D sonicator
(VWR, Westchester,
PA) at 30 C.
b. Preparation of Aqueous Phase
10.0 g of acacia (Aldrich) is dissolved in 100.0 g of water with stirring at
room
temperature for 30 minutes. The resulting mixture is decanted into two 50 mL
polypropylene
centrifuge tubes and centrifuged at about 2000 rpm for 10 minutes to remove
insoluble material.
66 g of the purified solution is then decanted into a 500 mL non-baffled
jacketed reactor, and the
solution is then heated to 40 C. A six-blade (vertical geometry) paddle
agitator is then placed
just beneath the surface of the liquid. While agitating the solution at 200
rpm, 6 g of gelatin (300
bloom, type A, Aldrich) is carefully added over about 20 seconds in order to
avoid lumps.
Agitation is then reduced to 50 rpm to reduce foaming. The resulting solution
is then stirred for
minutes.
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c. Encapsulation
With agitation at 200 rpm, the oil phase, prepared as described above, is
slowly poured
over about 15 seconds into the aqueous phase, also prepared as described
above. The resulting
oil/water emulsion is allowed to emulsify for 20 minutes. To this emulsion is
slowly added over
about 20 seconds 200 g of water that has been preheated to 40 C. The pH is
then reduced to 4.4
over five minutes with a 10% acetic acid solution (acetic acid from Aldrich).
The pH is
monitored using a pH meter that was previously calibrated with pH 7.0 and pH
4.0 buffer
solutions. Stir for 40 minutes. 150 g of water that has been preheated to 40 C
is then added, and
the contents of the reactor are then cooled to 10 C. When the solution
temperature reaches
10 C, 3.0 mL of a 37% formalin solution (Aldrich) is added, and the solution
is further stirred
for another 60 minutes. 20 g of sodium carboxymethylcellulose (NaCMC) is
added, and the pH
is then raised to 10.0 by the addition of a 20 wt% solution of sodium
hydroxide (NaOH). The
thermostat bath is then set to 40 C and allowed to stir for another 70
minutes. The slurry is
allowed to cool to room temperature overnight with stirring. The resulting
capsule slurry is then
ready to be sieved.
d. Formation of Display
Two procedures for preparing an electrophoretic display are from the above
capsule
slurry are described below.
i. Procedure using a urethane binder
The resulting capsule slurry from above is mixed with the aqueous urethane
binder
NeoRez R-9320 (Zeneca Resins, Wilmington, MA) at a ratio of one part binder to
10 parts
capsules. The resulting mixture is then coated using a doctor blade onto about
a 100 m to about
a 125 m thick sheet of indium-tin oxide sputtered polyester film. The blade
gap of the doctor
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blade is controlled at 0.18 mm so as to lay down a single layer of capsules.
The coated film is
then dried in hot air (60 C) for 30 minutes. After drying, the dried film is
hot laminated at 60 C
to a backplane comprising about a 100 m to about a 125 m thick sheet of
polyester screen
printed with thick film silver and dielectric inks with a pressure of 15 psi
in a hot roll laminate
from Cheminstruments, Fairfield, OH. The backplane is connected to the film
using an
anisotropic tape. The conductive areas form addressable areas of the resulting
display.
ii. Procedure using a urethane/polyvinyl alcohol binder
The resulting capsule slurry from above is mixed with the aqueous binder
comprising a
mixture of NeoRez R-966 (Zeneca Resins) and a 20% solution of Airvol 203 (a
polyvinyl
alcohol, Airvol Industries, Allentown, PA) at a ratio of one part Airvol 203
solution to one part
NeoRez R-966 to five parts capsules. The resulting mixture is then coated
using a doctor blade
onto about a 100 m to about 125 m thick sheet of indium-tin oxide sputtered
polyester film.
The blade gap of the doctor blade is controlled to 0.18 mm so as to lay down
an single layer of
capsules. The coated film is then dried in hot air (60 C) for 30 minutes.
After drying, a thick
film silver ink is then printed directly onto the back of the dried film and
allowed to cure at
60 C. The conductive areas form the addressable areas of the display.
2. Example 2
The following is an example of the preparation of microcapsules by in situ
polymerization.
In a 500 mL non-baffled jacketed reactor is mixed 50 mL of a 10 wt% aqueous
solution
of ethylene co-maleic anhydride (Aldrich), 100 mL water, 0.5 g resorcinol
(Aldrich), and 5.0 g
urea (Aldrich). The mixture is stirred at 200 rpm and the pH adjusted to 3.5
with a 25 wt%
NaOH solution over a period of 1 minute. The pH is monitored using a pH meter
that was
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previously calibrated with pH 7.0 and pH 4.0 buffer solutions. To this is
slowly added the oil
phase, prepared as described above in Ex. 1, and agitation is increased to 450
rpm to reduce the
average particle size to less than 200 m. 12.5 g of a 37 wt% aqueous
formaldehyde solution is
then added and the temperature raised to 55 C. The solution is heated at 55 C
for two hours.
3. Example 3
The following is an example of the preparation of microcapsules by interfacial
polymerization.
To 44 g of the oil phase, prepared as described above in Ex. 1, is added 1.0 g
of sebacoyl
chloride (Aldrich). Three milliliters of the mixture is then dispersed in 200
mL of water with
stirring at 300 rpm at room temperature. To this dispersion is then added 2.5
mL of a 10 wt.%
aqueous solution of 1,6-diaminohexane. Capsules form after about one hour.
E. Binder Material
The binder typically is used as an adhesive medium that supports and protects
the
capsules, as well as binds the electrode materials to the capsule dispersion.
A binder can be non-
conducting, semiconductive, or conductive. Binders are available in many forms
and chemical
types. Among these are water-soluble polymers, water-borne polymers, oil-
soluble polymers,
thermoset and thermoplastic polymers, and radiation-cured polymers.
Among the water-soluble polymers are the various polysaccharides, the
polyvinyl
alcohols, N-methyl Pyrollidone, N-vinyl pyrollidone, the various Carbowax
species (Union
Carbide, Danbury, CT), and poly-2-hydroxyethylacrylate.
The water-dispersed or water-borne systems are generally latex compositions,
typified by
the Neorez and Neocryl resins (Zeneca Resins, Wilmington, MA), Acrysol
(Rohm and Haas,
Philadelphia, PA), Bayhydrol (Bayer, Pittsburgh, PA), and the Cytec
Industries (West Paterson,
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NJ) HP line. These are generally latices of polyurethanes, occasionally
compounded with one or
more of the acrylics, polyesters, polycarbonates or silicones, each lending
the final cured resin in
a specific set of properties defined by glass transition temperature, degree
of "tack," softness,
clarity, flexibility, water permeability and solvent resistance, elongation
modulus and tensile
strength, thermoplastic flow, and solids level. Some water-borne systems can
be mixed with
reactive monomers and catalyzed to form more complex resins. Some can be
further cross-
linked by the use of a crosslinking reagent, such as an aziridine, for
example, which reacts with
carboxyl groups.
A typical application of a water-borne resin and aqueous capsules follows. A
volume of
particles is centrifuged at low speed to separate excess water. After a given
centrifugation
process, for example 10 minutes at 60 x gravity ("G"), the capsules 180 are
found at the bottom
of the centrifuge tube 182, while the water portion 184 is at the top, as
shown in FIG. 8. The
water portion is carefully removed (by decanting or pipetting). The mass of
the remaining
capsules is measured, and a mass of resin is added such that the mass of resin
is, for example,
between one eighth and one tenth of the weight of the capsules. This mixture
is gently mixed on
an oscillating mixer for approximately one half hour. After about one half
hour, the mixture is
ready to be coated onto the appropriate substrate.
The thermoset systems are exemplified by the family of epoxies. These binary
systems
can vary greatly in viscosity, and the reactivity of the pair determines the
"pot life" of the
mixture. If the pot life is long enough to allow a coating operation, capsules
may be coated in an
ordered arrangement in a coating process prior to the resin curing and
hardening.
Thermoplastic polymers, which are often polyesters, are molten at high
temperatures. A
typical application of this type of product is hot-melt glue. A dispersion of
heat-resistant
capsules could be coated in such a medium. The solidification process begins
during cooling,
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and the final hardness, clarity and flexibility are affected by the branching
and molecular weight
of the polymer.
Oil or solvent-soluble polymers are often similar in composition to the water-
borne
system, with the obvious exception of the water itself. The latitude in
formulation for solvent
systems is enormous, limited only by solvent choices and polymer solubility.
Of considerable
concern in solvent-based systems is the viability of the capsule itself - the
integrity of the capsule
wall cannot be compromised in any way by the solvent.
Radiation cure resins are generally found among the solvent-based systems.
Capsules
may be dispersed in such a medium and coated, and the resin may then be cured
by a timed
exposure to a threshold level of ultraviolet radiation, either long or short
wavelength. As in all
cases of curing polymer resins, final properties are determined by the
branching and molecular
weights of the monomers, oligomers and crosslinkers.
A number of "water-reducible" monomers and oligomers are, however, marketed.
In the
strictest sense, they are not water soluble, but water is an acceptable
diluent at low
concentrations and can be dispersed relatively easily in the mixture. Under
these circumstances,
water is used to reduce the viscosity (initially from thousands to hundreds of
thousands
centipoise). Water-based capsules, such as those made from a protein or
polysaccharide
material, for example, could be dispersed in such a medium and coated,
provided the viscosity
could be sufficiently lowered. Curing in such systems is generally by
ultraviolet radiation.
II. Components of a Monolayer and Processes for Display Construction in a
Monolayer
A. Coating the Capsules onto a Substrate in a Monolayer
Once capsules suitable for coating onto a substrate in a monolayer are
produced, the
present invention also provides methods for coating those capsules in a
monolayer. Generally,
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encapsulated electrophoretic displays include a dispersion of capsules in a
polymeric binder.
Alternatively, the dispersion can include capsules in a carrier (rather than
binder) or capsules
without a binder or carrier. The capsules contain an electrophoretically
active suspension. The
capsule dispersion (or "slurry") typically is coated onto a flexible polymeric
substrate that may
be associated with a front electrode at some point in time, achieving a
monolayer of capsules.
Having a binder with certain properties and/or altering a binder's physical
characteristics with,
for example, a change in pH or addition of a surfactant can be useful in
depositing a monolayer
of capsules. This resulting film is then laminated to a rear substrate. The
rear substrate can be
patterned in a single or multilayer electrode structure which may be printed
or formed by other
means on a polymeric (that can be flexible), a glass, and/or a metal
substrate. While the
invention is described in the context of a microencapsulated electrophoretic
display, it can be
relevant in the practice of any electronic display where the linking of a
electrode (i.e., a front
surface that applies voltage to the display) to a rear electrode substrate is
desired.
More particularly, after encapsulation, the capsule slurry is typically
dewatered to
achieve a target solids content. Dewatering, as described above, can be
achieved through
centrifugation, absorption, evaporation, mesh filtration, or osmotic
separation. This slurry is
then mixed with a binder, for example, a polymer latex (such as an aqueous
polyurethane
dispersion), and agitated to ensure uniform distribution of the binder
material. A binder can have
various fractions. A certain portion of the binder can be a solid ("binder
solid"), and a certain
portion of the binder can be a liquid, such as water, that is capable of
evaporating. Because a
binder can have more than one type of solid, the term "binder solid" can
include one or more
types of solid in a particular binder (i. e., the portion of solid(s) in the
binder relative to other
fractions in the binder). In one example, the binder typically is an aqueous
dispersion of latex
particles. The solid(s) can become integral with the film.
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To encourage close packing of the capsules in the monolayer film, the ratio of
binder
solid mass to capsule mass should be kept as low as possible. Minimizing the
amount of non-
optically active material (such as binder) allows for good packing and results
in good contrast
between the white and dark state of a display. See, e.g., FIGS. lA and IB.
However, the binder
is in the film to provide structural integrity, creating a tension between
desiring to reduce the
amount of binder (for optical properties) and desiring to increase the amount
to binder (for
structural reasons). In the film, a useful ratio of binder solid mass to
capsule mass is in the range
of about 1:2 to about 1:20, and preferably between about 1:4 and about 1:12,
and most preferably
between about 1:6 and about 1:10. These metrics also apply to the polymer
matrix content in a
polymer-dispersed EPID.
The slurry of capsules in aqueous binder, previously prepared, is coated on a
substrate as
a monolayer as described below. In one example, the slurry is coated onto a
polyester substrate
with indium tin oxide ("ITO") disposed on the substrate, which will ultimately
serve as the front
transparent electrode and substrate. The substrate can be about 25 m to about
500 gm thick.
This film typically is dried at about 60 C, evaporating the aqueous phase.
1. Slot Die Method
In one coating process, a slurry of capsules in aqueous binder, previously
prepared, is
coated in a monolayer. The coating process involves metering the
capsule/binder slurry through
a slot die coating head. Referring to FIGS. 2 and 3, a head 20 attached to a
pump 21 meters a
constant amount of capsule/binder slurry 22 through a tightly controlled gap
24. The gap 24
allows only a single layer of capsules 26 to pass through and out of the head
20. The flow rate of
the slurry can be set such that, as the head 20 moves past the receiving
substrate 28 that is placed
on a roller 29, a continuous monolayer 25 is formed. The substrate 28 and the
head 20 are
moved relative to each other. For example, the substrate is moved either
linearly (not shown) or
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on a rol129 (FIG. 2). The direction of movement of the ro1129 and the
substrate 28 is shown as
an arrowy. Alternatively, the head 20 can be moved, typically linearly (FIG.
3). The direction
of movement is shown as an arrow E. It may be a continuous or a batch process.
The
capsule/binder slurry typically is deposited at a rate of about 0.1 m/min to
about 100 m/min,
preferably at a rate of about 0.2 m/min to about 0.7 m/min. The fluid flow can
be actively
controlled, for example, to start and stop coating an area. The pump 21 used
to provide the
metering flow can be a low shear pump, for example a peristaltic pump. A low
shear pump can
prevent capsule breakage during coating.
The slurry of capsules can be deposited in a monolayer by controlling the gap
width to
mean sphere diameter ratio. The gap/mean sphere diameter ratio is the ratio of
the width of the
gap through which slurry moves to the mean of capsule diameters moving through
the gap. This
ratio can be based on the deformability and surface properties of the spheres
as well as the flow
properties of the coating fluid, but in one embodiment is between about 1 and
about 2.5, and
preferably between about 1.2 and about 1.6. Although capsules are generally
spherical during
coating, they can vary slightly due to deformation during processing such that
their diameter
varies slightly at any given time. Thus, calculations based on the size of a
capsule may vary
slightly at any given time. An equation that generally relates pump rate to
coating width, relative
die speed, and coating thickness is as follows.
Pump Rate = Coating Width X Die Speed X Coating Wet Thickness
The units for these variable are (length)3/(time) for Pump Rate; (length) for
Coating
Width; (length)/(time) for Die Speed; and (length) for Coating Wet Thickness,
where length and
time can be measured with any appropriate unit that a user chooses. The
Coating Wet Thickness
tends to deviate from a particular value in a monolayer formed with large,
easily deformable
particles because any deformation can change the thickness slightly.
Additionally, in certain
situations a film that is slightly thicker than or slightly thinner than the
diameter of one capsule
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may be desirable based on the capsule/binder ratio, how the capsules actually
deform into a film,
and/or the presence of any interstitial void, and the like. For example, the
more compliant and/or
resilient a capsule is, the thinner the film can be. The less compliant and/or
resilient a capsule is,
the thicker the film can be. However, once a film thickness is determined
(e.g., empirically), a
change in Die Speed or Coat Width will relate directly to the equation above.
2. Air Knife Method
In another embodiment of the present invention, an air knife coater forms an
encapsulated electrophoretic display having a monolayer of capsules. Air knife
coating has been
used in a variety of coating applications, including carbonless paper and
electroluminescence
coatings, which both contain encapsulated materials. The use of an air knife
in these
applications is, however, used for thickness control, and is not used for
forming a stable, durable
monolayer of capsules.
In methods of the present invention, a capsule slurry is applied to a
substrate, which is
either moving or still, by applying pressure so that the capsules in the
slurry form a monolayer
on the substrate. A substrate can be a conductive material such as ITO
sputtered polyester. The
pressure is applied to the mixture with a pressurized gas, typically air. An
air knife can be used
to apply the pressure so as to uniformly distribute the slurry. Referring to
FIG. 4, an air knife 30
is held at a distance of from about 1 cm to about 15 cm (distance oc) from the
surface of the
slurry 35. The slurry is provided on a surface of a substrate 31 and moves in
a direction S by a
coating roll 32. The air knife 30 is held at an angle of from about 00 to
about 90 (e.g., shown in
FIG. 4 are three positions of the air knife 30a, 30b, 30c, although many more
positions are
possible) from the surface of the slurry 35. A pressure source 34 provides
pressure to the air
knife 30. If conditions, such as gas pressure, distance from slurry, angle
relative to the slurry,
slurry viscosity, and relative speed between the air knife and the substrate
on which the slurry is
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deposited are optimized, a monolayer can be formed. For example, the air knife
can remove
excess slurry as well as create a monolayer of capsules. When an air knife is
used in conjunction
with controlled amounts of capsule slurry, waste of the slurry can be limited.
Also, because the
air knife does not touch the slurry, the probability of scratching the
substrate or breaking the
capsules is reduced.
The gas being blown over the coating material can be other than air at ambient
temperature. The gas may be heated or cooled, changing the coatability and
viscosity properties
of the capsule slurry. The gas may be blown on wet (e.g., a liquid in
droplets) or dry, controlling
evaporation of solvents in the capsule slurry. Temperature or addition of
liquids can be
controlled, for example, with a heater, refrigeration unit, liquid pump,
and/or other devices
known to those skilled in the art, as represented by a controller 33 in FIG.
4. The gas may be
mixed with solvents that help cure the capsule slurry and/or make the slurry
easier to coat onto
the substrate. The solvent is chosen to be compatible with the substrate
layer. For example, if
the substrate is water-based, the solvent can be water or an alcohol. Useful
alcohols include
isopropyl alcohol, methanol, and ethanol. If the substrate is organic, a
compatible organic
solvent can be mixed with the gas. For example, a butyl acetate substrate is
coated with "acetate
humidified" air. Other useful organic solvents for use in methods of the
invention include
methylene chloride and chlorobenzene. In certain embodiments, the suspending
fluid inside the
capsule is a halogenated hydrocarbon, such as tetrachloroethylene or
poly(chlorotrifluoroethylene). Also, the evaporative binder in certain
embodiments can be, for
example, a polymer latex, an acrylic, urethane, poly(vinyl alcohol), or water-
based binder.
3. Coating With Substantially No Binder
In certain situations, it may be desirable to coat capsules onto a substrate
substantially in
the absence of a binder. For example, and referring to FIG. 9A, a capsule 50
has a capsule wall
52 that is constructed from a polymer. For example, the capsule can be formed
from
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gelatin/acacia, as described above. The capsule wall 52 is heavily swelled
with water and/or a
polar solvent. For example, about 1% to about 90%, preferably about 5% to
about 20%, of the
capsule wall 52 can be formed from the polymer while about 10% to about 99%,
preferably
about 80% to about 95% can be formed from the water and/or polar solvent.
These capsules 50
are coated onto a substrate 54, as described above for the slurry. The
capsules 52 can be
suspended in a carrier such as water or not suspended in any carrier. As the
capsules 50 are
dried, for example, at 60 C, the water and/or polar solvent evaporates from
the capsule walls 52
of the capsules 50 in the coated monolayer. As the water and/or polar solvent
evaporates, the
capsule walls 52 intermingle with, associate with, and/or adhere to each other
and/or the
substrate 54. A film is formed. Without a binder in the film, capsules can
deform to a greater
degree than with binder in the film, in some situations.
B. Laminating the Rear Substrate
Once a capsule slurry is coated onto a front electrode on a flexible or rigid
substrate (e.g.,
a polymeric material or glass), achieving a monolayer of microcapsules, this
coated film is then
laminated to a rear substrate (alternatively know as a"backplane"). The rear
substrate can be, for
example, a polymeric material (that can be flexible), glass, or metal. The
rear substrate can be
patterned in a single or multi-layer electrode structure which may be printed
or formed by other
means on a second flexible polymeric substrate. While the present invention is
described in the
context of an encapsulated display, it can be relevant in the practice of any
electronic display for
linking a front active surface to a rear electrode substrate (either with or
without additional layers
between the front surface and rear substrate).
Lamination, typically, occurs under vacuum conditions and involves the
application of
heat and/or pressure. For example, temperatures from about 40 C to about 150 C
and more
preferably from about 50 C to about 120 C can be used, depending upon the
lamination
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procedure and/or laminate material that is used. A typical rear substrate is
constructed from a
base substrate either with or without an electrode layered on the base
substrate. One example of
a rear substrate is a polyester or polyimide base film and a set of patterned
electrodes. Generally,
these electrodes are a single- or multi-layer conductor/insulator stack that
can printed on the
substrate. The substrates can be a polymeric material (which can be flexible),
glass, or a metal.
Typically, the conductors are conductive particles (e.g., carbon, silver,
palladium, tin oxide,
doped tin oxide, copper), doped polyesters, and vinyls which are screen
printed and heat cured.
Furthermore, a rear substrate can have transistors (organic and/or silicon-
based), diodes, and/or
electrodes disposed on the substrate.
The film can be combined with the rear substrate in a variety of ways.
Typically, the film
40, after being coated as a monolayer, contains voids and irregularities in
the binder 44 between
the capsules 42, irregular surfaces on the capsules 42, and/or irregular
surfaces at the edge of the
binder 44, as shown in FIG. 5A. As discussed above, the binder level in the
slurry is kept as low
as possible to maximize optically active area, leaving little binder to flow
and absorb the stresses
of lamination. The capsule wall bears the majority of these stresses, which
can result in capsule
breakage. Additionally, with no material filling the voids between capsules as
well as voids in
other areas such as the surface of the capsules or edge of the binder, the
laminated product may
have trapped air, or trapped vacuum packets, which can both change the
mechanical stresses
experienced by the film and affect the electrical characteristics (which may
result in non-uniform
switching).
In one embodiment, an additional layer of material can be included between the
film and
the rear substrate to address the problem above. This layer of material can be
an adhesive which
can flow at the lamination temperature. The layer also may be tacky. For
example, it may be a
polymeric material identical or similar to that of the binder material coated
onto the front
substrate previously, or it may be a hotmelt adhesive sheet, which could be
thermoplastic or
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thermoset. Alternatively, it may be a material which is initially in liquid
form at room
temperature but forms a solid matrix after curing or crosslinking. These
solutions provide a
flowable material which can fill in any voids between capsules and smooth out
the roughness of
the rear capsule surface during the lamination process without putting undue
stresses on the
capsule walls themselves. Such a final result can be seen in FIG. 5B. The
additional layer of
material 46 has filled the interstices (including, but without limitation, the
voids between the
capsules 42, the irregular surfaces of the binder 44 and the irregular
surfaces of capsules 42 ) and
adheres to a rear substrate 48. The additional layer of material 46 can be
initially coated onto the
film 40 (i.e., the rear of the capsules) or onto the rear substrate 48, before
the lamination
procedure, or the layer 46 can be a hotmelt adhesive sheet (which can be
thermoplastic or
thermoset) introduced between the film and rear substrate during the
lamination procedure. The
more uniformly sized the capsules are and/or the more monodisperse (i.e., the
more like a perfect
monolayer) the capsules are, the thinner the additional layer of material can
be because such
conditions produce a more uniform film that includes fewer interstices and the
like. Uniform
capsule sizes and distributions can be obtained as described above. The
additional layer of
material can be about 50 m or less in thickness. Typically, this layer and/or
the capsules and/or
the substrate forms a uniformly thick stratum. The stratum can be between
about 10 m thick
and about 500 m thick, preferably about 50 m thick to about 300 m thick.
Alternatively,
certain capsules can be used without a binder, as described above. In that
case, and referring to
2o FIG. 9B, a final structure can be formed in which the interstices and
irregular surfaces of
capsules 50 on a substrate 54 are filled with a layer of material 56 that is
adjacent a rear
substrate 58.
If the layer of material is initially coated on to the front film, it can
substantially planarize
the capsule film facing the rear substrate and/or provide a tacky surface to
enable lamination to
the rear substrate. Thus, the capsule film is planar prior to lamination, and
tacky front and rear
CA 02345619 2001-03-27
WO 00/20922 PCT/US99/23313
- 44 -
substrates do not need to be stored separately, because only the front
substrate is tacky.
Additionally, a front film need not have a rear substrate. For example, a film
that has been
planarized with the layer of material can be operated with a stylus. This can
occur with or
without a rear substrate, and the layer of material need not be tacky if no
rear substrate is used.
A semiconducting or anisotropically conducting adhesive can be used as the
additional
layer. This material will conduct an electric field from the backplane to the
capsules with little
loss of field strength. An adhesive containing carbon particles, gold
particles, aluminum
particles, platinum particles, silver particles, plated polymer spheres,
plated glass spheres, or ITO
particles may be used. Also, conductive polymers such as polyacetylene,
polyaniline,
polypyrrole, P-DOT, or polythiophene can be used to dope the additional layer
of material,
causing it to conduct well in the Z-axis but not in the plane of the adhesive.
Thus, the electric
field is rriore efficiently transmitted to the capsules. In order to make
these films, the adhesive
sheet can be cast and then stretched in one or both axes. The resistivity of
the layer of material
can be about 105 to about 1015 ohm-cm, more preferably about 108 to about 1013
ohm-cm.
Additionally, an insulating layer of material can be used.
Variations, modifications, and other implementations of what is described
herein will
occur to those of ordinary skill in the art without departing from the spirit
and the scope of the
invention as claimed. Accordingly, the invention is to be defined not by the
preceding
illustrative description but instead by the spirit and scope of the following
claims.
What is claimed is:
