U.S. patent application number 09/874391 was filed with the patent office on 2002-12-12 for composition and process for the sealing of microcups in roll-to-roll display manufacturing.
This patent application is currently assigned to SiPix Imaging, Inc.. Invention is credited to Liang, Rong-Chang, Wang, Xiaojia, Zang, HongMei.
Application Number | 20020188053 09/874391 |
Document ID | / |
Family ID | 25363637 |
Filed Date | 2002-12-12 |
United States Patent
Application |
20020188053 |
Kind Code |
A1 |
Zang, HongMei ; et
al. |
December 12, 2002 |
Composition and process for the sealing of microcups in
roll-to-roll display manufacturing
Abstract
The invention relates to a novel sealing composition for the
manufacture of an electrophoretic or liquid crystal display, and a
sealing process using the composition. The composition allows
electrophoretic or liquid crystal cells to be seamlessly sealed and
the sealing layer free of any defects.
Inventors: |
Zang, HongMei; (Sunnyvale,
CA) ; Wang, Xiaojia; (Fremont, CA) ; Liang,
Rong-Chang; (Sunnyvale, CA) |
Correspondence
Address: |
Heller Ehrman White & McAuliffe LLP
275 Middlefield Road
Menlo Park
CA
94025-3506
US
|
Assignee: |
SiPix Imaging, Inc.
|
Family ID: |
25363637 |
Appl. No.: |
09/874391 |
Filed: |
June 4, 2001 |
Current U.S.
Class: |
524/474 |
Current CPC
Class: |
G02F 1/1341 20130101;
G02F 1/13475 20130101; G02F 1/167 20130101; C09J 153/00 20130101;
G02F 1/133377 20130101; G02F 1/1334 20130101; G02F 1/1679 20190101;
G02F 1/1347 20130101 |
Class at
Publication: |
524/474 |
International
Class: |
C08K 005/01 |
Claims
What is claimed is:
1. A composition suitable for sealing electrophoretic cells, which
composition comprises: a) a solvent or solvent mixture which is
immiscible with the display fluid contained within the cells and
exhibits a specific gravity less than that of the display fluid,
and b) a thermoplastic elastomer.
2. The composition of claim 1 wherein said solvent or solvent
mixture has a surface tension of lower than 35 dyne/cm.
3. The composition of claim 2 wherein said solvent or solvent
mixture has a surface tension of lower than 30 dyne/cm.
4. The composition of claim I wherein said solvent or solvent
mixture is selected from a group consisting of alkanes, cyclic
alkanes, alkylbenzenes, alkyl esters and C.sub.3-5 alkyl
alcohols.
5. The composition of claim 4 wherein said solvent is heptane,
octane, nonane, cyclohexane, decalin, toluene, xylene, and their
isomers, and mixtures thereof.
6. The composition of claim 1 wherein said thermoplastic elastomer
is selected from a group consisting of ABA and (AB)n types of di
-block, tri-block and multi-block copolymers, in which: A is
styrene, .alpha.-methylstyrene, ethylene, propylene or norbonen e,
B is butadiene, isoprene, ethylene, propylene, butylene,
dimethylsiloxane or propylene sulfide, and A and B are not the
same, and n is.gtoreq.1.
7. The composition of claim 6 wherein n is 1-10.
8. The composition of claim 6 wherein said thermoplastic elastomer
is poly(styrene-b-butadiene), (poly(styrene-b-butadiene-b-styrene),
poly(styrene-b-isoprene-b-styrene),
poly(styrene-b-ethylene/butylenes-b-s- tyrene),
poly(styrene-b-dimethylsiloxane-b-styrene),
poly((.alpha.-methylstyrene-b-isoprene),
poly(.alpha.-methylstyrene-b-iso- prene-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-propylene
sulfide-b-.alpha.-methylstyrene),
poly((.alpha.-methylstyrene-b-dimethyls-
iloxane-b-.alpha.-methylstyrene), and their grafted co-polymers and
derivatives thereof.
9. The composition of claim 1 wherein the thermoplastic elastomer
is poly(ethylene-co-propylene-co-5-methylene-2-norbomene),
(ethylene-propylene-diene terpolymer), and their grafted
co-polymers and derivatives thereof.
10. The composition of claim 1, further comprising a thermoplastic
material that is compatible with one of the blocks of the
thermoplastic elastomer.
11. The composition of claim 10 wherein the thermoplastic material
is selected from the group consisting of polystyrene and poly
((.alpha.-methylstyrene).
12. The composition of claim 1, further comprising a wetting
agent.
13. The composition of claim 12 wherein said wetting agent is
selected from a group consisting of surfactants, ZONYL
fluorosurfactants, fluoroacrylates, fluoromethacrylates,
fluoro-substituted long chain alcohols, perfluoro-substituted long
chain carboxylic acids, SILWET silicone surfactants and their
derivatives.
14. The composition of claim 1, further comprising one or more of
the following agents: a crosslinking agent, a vulcanizer, a
multifunctional monomer or oligomer, a thermal initiator or a
photoinitiator.
15. The composition of claim 14 wherein said crosslinking agent is
a bisazide such as 4,4'-diazidodiphenylmethane, or
2,6-di-(4'-azidobenzal)-- 4-methylcyclohexanone), and said
vulcanizer is a disulfide such as 2-benzothiazolyl disulfide, or
tetramethylthiuram disulfide.
16. A sealing process for the preparation of electrophoretic
display, which process comprises: a) filling an array of microcups
with an electrophoretic fluid; b) overcoating the electrophoretic
fluid with a sealing composition comprising: a solvent or solvent
mixture which is immiscible with the display fluid contained within
the cells and exhibits a specific gravity less than that of the
display fluid, thermoplastic elastomer; and c) allowing the sealing
composition to dry to form a sealing layer.
17. The process of claim 16, further comprising exposing the
sealing layer to UV radiation or thermal baking.
18. The process of claim 16 wherein said solvent or solvent mixture
has a surface tension of lower than 35 dyne/cm.
19. The process of claim 18 wherein said solvent or solvent mixture
has a surface tension of lower than 30 dyne/cm.
20. The process of claim 16 wherein said solvent or solvent mixture
is selected from a group consisting of alkanes, cyclic alkanes,
alkylbenzenes, alkyl esters and C.sub.3-5 alkyl alcohols.
21. The process of claim 20 wherein said solvent is heptane,
octane, nonane, cyclohexane, decalin, toluene, xylene, and their
isomers, and mixtures thereof.
22. The process of claim 16 wherein said thermoplastic elastomer is
selected from a group consisting of ABA and (AB)n types of
di-block, tri-block and multi-block copolymers, in which: A is
styrene, .alpha.-methylstyrene, ethylene, propylene or norbonene, B
is butadiene, isoprene, ethylene, propylene, butylene,
dimethylsiloxane or propylene sulfide, and A and B are not the
same, and n is.gtoreq.1.
23. The process of claim 22 wherein n is 1-10.
24. The process of claim 16 wherein said thermoplastic elastomer is
poly(styrene-b-butadiene), (poly(styrene-b-butadiene-b-styrene),
poly(styrene-b-isoprene-b-styrene),
poly(styrene-b-ethylene/butylenes-b-s- tyrene),
poly(styrene-b-dimethylsiloxane-b-styrene),
poly(.alpha.-methylstyrene-b-isoprene),
poly(.alpha.-mthylstyrene-b-isopr- ene-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-propylene
sulfide-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-dimethylsi-
loxane-b-.alpha.-methylstyrene), and their grafted co-polymers and
derivatives thereof.
25. The process of claim 16 wherein the thermoplastic elastomer is
poly(ethylene-co-propylene-co-5-methylene-2-norbornene),
(ethylene-propylene-diene terpolymer), and their grafted
co-polymers and derivatives thereof.
26. The process of claim 16, further comprising a thermoplastic
material that is compatible with one of the blocks of the
thermoplastic elastomer.
27. The process of claim 26 wherein the thermoplastic material is
selected from the group consisting of polystyrene and poly
((.alpha.-methylstyrene- ).
28. The process of claim 16, further comprising a wetting
agent.
29. The process of claim 28 wherein said wetting agent is selected
from a group consisting of surfactants, ZONYL fluorosurfactants,
fluoroacrylates, fluoromethacrylates, fluoro-substituted long chain
alcohols, perfluoro-substituted long chain carboxylic acids, SILWET
silicone surfactants and their derivatives.
30. The process of claim 16, further comprising one or more of the
following agents: a crosslinking agent, a vulcanizer, a
multifunctional monomer or oligomer, a thermal initiator or a
photoinitiator.
31. The process of claim 30 wherein said crosslinking agent is a
bisazide such as 4,4'-diazidodiphenylmethane, or
2,6-di-(4'-azidobenzal)-4-methylc- yclohexanone), and said
vulcanizer is a disulfide such as 2-benzothiazolyl disulfide, or
tetramethylthiuram disulfide.
Description
BACKGROUND
[0001] The electrophoretic display (EPD) is a non-emissive device
based on the electrophoresis phenomenon influencing charged pigment
particles suspended in a solvent. This general type of display was
first proposed in 1969. An EPD typically comprises a pair of
opposed, spaced-apart plate-like electrodes, with spacers
predetermining a certain distance between the electrodes. One of
the electrodes is typically transparent. A suspension composed of a
colored solvent and suspended charged pigment particles is enclosed
between the two plates.
[0002] When a voltage difference is imposed between the two
electrodes, the pigment particles migrate by attraction to the
plate of polarity opposite that of the pigment particles. Thus the
color showing at the transparent plate may be determined by
selectively charging the plates to be either the color of the
solvent or the color of the pigment particles. Reversal of plate
polarity will cause the particles to migrate back to the opposite
plate, thereby reversing the color. Intermediate color density (or
shades of gray) due to intermediate pigment density at the
transparent plate may be obtained by controlling the voltage or
charging time.
[0003] Among the advantages of an electrophoretic display (EPD)
over other types of flat panel displays is the very low power
consumption. This salient advantage makes EPDs particularly
suitable for portable and battery powered devices such as laptops,
cell phones, personal digital assistants, portable electronic
medical and diagnostic devices, global positioning system devices,
and the like.
[0004] In order to prevent undesired movements of the particles
such as sedimentation, partitions were proposed between the two
electrodes for dividing the space into smaller cells. See, e.g., M.
A Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No.
8, pp 1148-1152 (1979). However, in the case of the partition-type
EPD, some difficulties are encountered in the formation of the
partitions and the process of enclosing the suspension.
Furthermore, it is also difficult to keep different colors of
suspensions separate from each other in the partition-type EPD.
[0005] Attempts have been made to enclose the suspension in
microcapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describe
microencapsulated EPDs. These displays have a substantially two
dimensional arrangement of microcapsules each containing an
electrophoretic composition comprising a dielectric fluid with
charged pigment particles suspended therein and the particles
visually contrast with the dielectric solvent. The microcapsules
can be formed by interfacial polymerization, in-situ polymerization
or other known methods such as in-liquid curing or simple/complex
coacervation. The microcapsules, after their formation, may be
injected into a cell housing two spaced-apart electrodes, or they
may be "printed" into or coated on a transparent conductor film.
The microcapsules may also be immobilized within a transparent
matrix or binder that is itself sandwiched between the two
electrodes.
[0006] The EPDs prepared by these prior art processes, in
particular the microencapsulation process, as disclosed in U.S.
Pat. Nos. 5,930,026, 5,961,804, and 6,017,584, have several
shortcomings. For example, the EPDs manufactured by the
microencapsulation process suffer from sensitivity to environmental
changes (in particular sensitivity to moisture and temperature) due
to the wall chemistry of the microcapsules. Secondly the EPDs based
on the microcapsules have poor scratch resistance due to the thin
wall and large particle size of the microcapsules. To improve the
handleability of the display, microcapsules are embedded in a large
quantity of polymer matrix which results in a slow response time
due to the large distance between the two electrodes and a low
contrast ratio due to the low payload of pigment particles. It is
also difficult to increase the surface charge density on the
pigment particles because charge-controlling agents tend to diffuse
to the water/oil interface during the microencapsulation process.
The low charge density or zeta potential of the pigment particles
in the microcapsules also results in a slow response rate.
Furthermore, because of the large particle size and broad size
distribution of the microcapsules, the prior art EPD of this type
has poor resolution and addressability for color applications.
[0007] Recently an improved EPD technology was disclosed in
co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3,
2000 and U.S. Ser. No. 09/784,972, filed on Feb. 25, 2001. The
cells of the improved EPD are formed from a plurality of microcups
which are formed integrally with one another as portions of a
structured two-dimensional array assembly. Each microcup of the
array assembly is filled with a suspension or dispersion of charged
pigment particles in a dielectric solvent, and sealed to form an
electrophoretic cell.
[0008] The substrate web upon which the microcups are formed
includes a display addressing array comprising preformed conductor
film, such as ITO conductor lines. The conductor film (ITO lines)
is coated with a radiation curable polymer precursor layer. The
film and precursor layer are then exposed imagewise to radiation to
form the microcup wall structure. Following exposure, the precursor
material is removed from the unexposed areas, leaving the cured
microcup walls bonded to the conductor film/support web. The
imagewise exposure may be accomplished by UV or other forms of
radiation through a photomask to produce an image or predetermined
pattern of exposure of the radiation curable material coated on the
conductor film. Although it is generally not required, the mask may
be positioned and aligned with respect to the conductor film, i.e.,
ITO lines, so that the transparent mask portions align with the
spaces between ITO lines, and the opaque mask portions align with
the ITO material (intended for microcup cell floor areas).
[0009] Alternatively, the microcup array may be prepared by a
process including embossing a thermoplastic or thermoset precursor
layer coated on a conductor film with a pre-patterned male mold,
followed by releasing the mold. The precursor layer may be hardened
by radiation, cooling, solvent evaporation, or other means during
or after the embossing step. This novel micro-embossing method is
disclosed in the co-pending application, U.S. Ser. No. 09/518,488,
filed Mar. 3, 2000.
[0010] Solvent-resistant, thermomechanically stable microcups
having a wide range of size, shape, pattern and opening ratio can
be prepared by either one of the aforesaid methods.
[0011] The manufacture of a monochrome EPD from a microcup assembly
involves filling the microcups with a single pigment suspension
composition, sealing the microcups, and finally laminating the
sealed array of microcups with a second conductor film pre-coated
with an adhesive layer.
[0012] For a color EPD, its preparation from a microcup assembly
involves sequential selective opening and filling of predetermined
microcup subsets. The process includes laminating or coating the
preformed microcups with a layer of positively working photoresist,
selectively opening a certain number of the microcups by imagewise
exposing the positive photoresist, followed by developing the
resist, filling the opened cups with a colored electrophoretic
fluid, and sealing the filled microcups by a sealing process. These
steps may be repeated to create sealed microcups filled with
electrophoretic fluids of different colors. Thus, the array may be
filled with different colored compositions in predetermined areas
to form a color EPD. Various known pigments and dyes provide a wide
range of color options for both solvent phase and suspended
particles. Known fluid application and filling mechanisms may be
employed.
[0013] The sealing of the microcups after they are filled with a
dispersion of charged pigment particles in a dielectric fluid can
be accomplished by overcoating the electrophoretic fluid with a
solution containing a thermoplastic or thermoset precursor. To
reduce or eliminate the degree of intermixing during and after the
overcoating process, it is highly advantageous to use a sealing
composition that is immiscible with the electrophoretic fluid and
preferably has a specific gravity lower than the dielectric fluid.
The sealing is then accomplished by hardening the precursor by
solvent evaporation, interfacial reaction, moisture, heat,
radiation, or a combination of curing mechanisms. Alternatively,
the sealing can be accomplished by dispersing a thermoplastic or
thermoset precursor in the electrophoretic fluid before the filling
step. The thermoplastic or thermoset precursor is immiscible with
the dielectric solvent and has a specific gravity lower than that
of the solvent and the pigment particles. After filling, the
thermoplastic or thermoset precursor phase separates from the
electrophoretic fluid and forms a supernatant layer at the top of
the fluid. The sealing of the microcups is then conveniently
accomplished by hardening the precursor layer by solvent
evaporation, interfacial reaction, moisture, heat, or radiation. UV
radiation is the preferred method to seal the microcups, although a
combination of two or more curing mechanisms as described above may
be used to increase the throughput of sealing.
[0014] The improved EPDs may also be manufactured by a synchronized
roll-to-roll photolithographic exposure process as described in the
co-pending application, U.S. Ser. No. 09/784,972, filed on Feb. 25,
2001. A photomask may be synchronized in motion with the support
web using mechanisms such as coupling or feedback circuitry or
common drives to maintain the coordinated motion (i.e., to move at
the same speed). Following exposure, the web moves into a
development area where the unexposed material is removed to form
the microcup wall structure. The microcups and ITO lines are
preferably of selected size and coordinately aligned with the
photomask, so that each completed display cell (i.e., filled and
sealed microcup) may be discretely addressed and controlled by the
display driver. The ITO lines may be pre-formed by either a wet or
a dry etching process on the substrate web.
[0015] For making color displays from the microcup array, the
synchronized roll-to-roll exposure photolithographic process also
enables continuous web processes of selective opening, filling and
sealing of pre-selected subsets of the microcup array.
[0016] The microcup array may be temporarily sealed by laminating
or coating with a positive-acting photoresist composition,
imagewise exposing through a corresponding photomask, and
developing the exposed area with a developer to selectively open a
desired subset of the microcups. Known laminating and coating
mechanisms may be employed. The term "developer" in this context
refers to a suitable known means for selectively removing the
exposed photoresist, while leaving the unexposed photoresist in
place.
[0017] Thus, the array may be sequentially filled with several
different color compositions (typically three primary colors) in a
pre-determined cell pattern. For example, the imagewise exposure
process may employ a positively working photoresist top laminate or
coating which initially seals the empty microcups. The microcups
are then exposed through a mask (e.g., a loop photomask in the
described roll-to-roll process) so that only a first selected
subset of microcups are exposed. Development with a developer
removes the exposed photoresist and thus opens the first microcup
subset to permit filling with a selected color pigment dispersion
composition, and sealing by one of the methods described herein.
The exposure and development process is repeated to expose and open
a second selected microcup subset, for filling with a second
pigment dispersion composition, with subsequent sealing. Finally,
the remaining photoresist is removed and the third subset of
microcups is filled and sealed.
[0018] Liquid crystal displays (LCDs) may also be prepared by the
method as described above when the electrophoretic fluid is
replaced by a suitable liquid crystal composition having the
ordinary refractive index matched to that of the isotropic cup
material. In the "on" state, the liquid crystal in the microcups is
aligned to the field direction and is transparent. In the "off"
state, the liquid crystal is not aligned and scatters light. To
maximize the light scattering effect of the LCDs, the diameter of
the microcups is typically in the range of 0.5-10 microns.
[0019] The roll-to-roll process may be employed to carry out a
sequence of processes on a single continuous web, by carrying and
guiding the web to a plurality of process stations in sequence. In
other words, the microcups may be formed, filled or coated,
developed, sealed, and laminated in a continuous sequence.
[0020] In addition to the manufacture of microcup displays, the
synchronized roll-to-roll process may be adapted to the preparation
of a wide range of structures or discrete patterns for electronic
devices formable upon a support web substrate, e.g., patterned
conductor films, flexible circuit boards and the like. As in the
process and apparatus for EPD microcups described herein, a
pre-patterned photomask is prepared which includes a plurality of
photomask portions corresponding to structural elements of the
subject device. Each such photomask portion may have a pre-selected
area of transparency or opacity to radiation so as to form an image
of such a structural element upon the correspondingly aligned
portion of the web during exposure. The method may be used for
selective curing of structural material, or may be used to expose
positively or negatively acting photoresist material during
manufacturing processes.
[0021] Because these multiple-step processes may be carried out
roll-to-roll continuously or semi-continuously, they are suitable
for high volume and low cost production. These processes are also
efficient and inexpensive as compared to other processes for
manufacturing display products. The improved EPD involving
microcups is not sensitive to environment, such as humidity and
temperature. The display is thin, flexible, durable,
easy-to-handle, and format-flexible. Since the EPD comprises cells
of favorable aspect ratio and well-defined shape and size, the
bi-stable reflective display has excellent color addressability,
high contrast ratio and color saturation, fast switching rate and
response time.
SUMMARY OF THE INVENTION
[0022] Sealing of the microcups by a continuous web process is one
of the most critical steps in the roll-to-roll manufacturing of the
improved EPDs. In order to prepare a high quality display, the
sealing layer must have at least the following characteristics: (1)
free of defects such as entrapped air bubble, pin holes, cracking
or leaking, etc; (2) good film integrity and barrier properties
against the display fluid such as dielectric fluids for EPDs; and
(3) good coating and adhesion properties. Since most of the
dielectric solvents used in EPDs are of low surface tension and low
viscosity, it has been a major challenge to achieve a seamless,
defect-free sealing with good adhesion properties for the
microcups.
[0023] It has now been found that microcups filled with a display
fluid such as an electrophoretic fluid can be sealed seamlessly and
free of defects by a continuous web process using a novel sealing
overcoat composition comprising the following ingredients:
[0024] (1) a solvent or solvent mixture which is immiscible with
the display fluid in the microcups and exhibits a specific gravity
less than that of the display fluid; and
[0025] (2) a thermoplastic elastomer.
[0026] Compositions containing at least a thermoplastic elastomer
having good compatibility with the microcups and good barrier
properties against the display fluid are particularly useful.
Examples of useful thermoplastic elastomers include di-block,
tri-block or multi-block copolymers represented by the formulas ABA
or (AB)n in which A is styrene, .alpha.-methylstyrene, ethylene,
propylene or norbonene; B is butadiene, isoprene, ethylene,
proplyene, butylene, dimethoylsiloxane or propylene sulfide; and A
and B cannot be the same in the formula. The number, n, is
.gtoreq.1, preferably 1-10. Representative copolymers include
poly(styrene-b-butadiene), poly(styrene-b-butadiene-b-styrene),
poly(styrene-b-isoprene-b-styrene),
poly(styrene-b-ethylene/butylene-b-st- yrene),
poly(styrene-b-dimethylsiloxane-b-styrene),
poly((.alpha.-methylstyrene-b-isoprene),
poly(.alpha.-methylstyrene-b-iso- prene-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-propylene
sulfide-b-.alpha.-methylstyrne), and
poly(.alpha.-methylstyrene-b-dimethy-
lsiloxane-b-.alpha.-methylstyrene). A review of the preparation of
the thermoplastic elastomers can be found in N. R. Legge, G.
Holden, and H. E. Schroeder ed., "Thermoplastic Elastomers", Hanser
Publisher (1987). Commercially available styrene block copolymers
such as Kraton D and G series from Shell Chemical Company are
particularly useful. Crystalline rubbers such as
poly(ethylene-co-propylene-co-5-methylene-2-norbomene) or EPDM
(ethylene-propylene-diene terpolymer) rubbers and their grafted
copolymers have also been found very useful. Not to be bound by the
theory, it is believed that the hard block of the thermoplastic
elastomers phase separates during or after the drying of the
sealing overcoat and serves as the physical crosslinker of the soft
continuous phase. The sealing composition of the present invention
significantly enhances the modulus and film integrity of the
sealing layer throughout the coating and drying processes.
Thermoplastic elastomers having low critical surface tension (lower
than 40 dyne/cm) and high modulus or Shore A hardness (higher than
60) have been found useful probably because of their favorable
wetting property and film integrity over the display fluid.
[0027] The thermoplastic elastomer is dissolved in a solvent or
solvent mixture which is immiscible with the display fluid in the
microcups and exhibits a specific gravity less than that of the
display fluid. Low surface tension solvents are preferred for the
overcoating composition because of their better wetting properties
over the microcup surface and the electrophoretic fluid. Solvents
or solvent mixtures having a surface tension lower than 35 dyne/cm
are preferred. A surface tension lower than 30 dyne/cm is more
preferred. Suitable solvents include alkanes (preferably C.sub.6-12
alkanes such as heptane, octane or Isopar solvents from Exxon
Chemical Company, nonane, decane and their isomers), cycloalkanes
(preferably C.sub.6-12 cycloalkanes such as cyclohexane, decalin
and the like), alkylbenzenes (preferably mono- or di-C.sub.1-6
alkyl benzenes such as toluene, xylene and the like), alkyl esters
(preferably C.sub.2-5 alkyl esters such as ethyl acetate, isobutyl
acetate and the like) and C.sub.3-5 alkyl alcohols (such as
isopropanol and the like and their isomers.
[0028] In addition to the fact that the electrophoretic cells
prepared from microcups may be sealed seamlessly and free of
defects by a continuous web process using this novel sealing
composition, the composition also has many other advantages. For
example, it also exhibits good wetting properties over the filled
microcups throughout the coating process and develops a good film
integrity over the display fluid even before the solvent evaporates
completely. As a result, the integrity of the coating is maintained
and no dewetting or beading on the electrophoretic fluid is
observed. In addition, the composition of the present invention
enables the continuous sealing of wider microcups, particularly
those having a width greater than 100 microns. Wider microcups are
preferred in some applications because of their higher microcup
opening-to-wall ratio and better display contrast ratio.
Furthermore, the sealing composition of the present invention
enables the formation of a sealing layer less than 3 microns thick
which is typically difficult to achieve by using traditional
sealing compositions. The thinner sealing layer shortens the
distance between the top and bottom electrodes and results in a
faster switching rate.
[0029] Co-solvents and wetting agents may also be included in the
composition to improve the adhesion of the sealant to the microcups
and provides a wider coating process latitude. Other ingredients
such as crosslinking agents, vulcanizers, multifunctional monomers
or oligomers, and high Tg polymers that are miscible with one of
the blocks of the thermoplastic elastomer are also highly useful to
enhance the physicomechanical properties of the sealing layer
during or after the overcoating process. The sealed microcups may
be post treated by UV radiation or thermal baking to further
improve the barrier properties. The adhesion of the sealing layer
to the microcups may also be improved by the post-curing reaction,
probably due to the formation of an interpenetration network at the
microcup-sealing sealing layer inter-phase.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic cross-section of an EPD, showing three
microcup cells in a neutral condition.
[0031] FIG. 2 is a schematic cross-section of the EPD of FIG. 1,
but with two of the cells charged, to cause the pigment to migrate
to one plate.
[0032] FIGS. 3A-3C shows the contours of an exemplary microcup
array, FIG. 3A showing a perspective view, FIG. 3B showing a plan
view, and FIG. 3C showing an elevation view, the vertical scale
being exaggerated for clarity.
[0033] FIGS. 4A and 4B show the basic processing steps for
preparing the microcups involving imagewise photolithographic
exposure through a photomask ("top exposure") of the conductor film
coated with a thermoset precursor, to UV radiation.
[0034] FIGS. 5A and 5B show alternative processing steps for
preparing the microcups involving imagewise photolithography
combining the top exposure and bottom exposure principles, whereby
the walls are cured in one lateral direction by top photomask
exposure and in the perpendicular lateral direction by bottom
exposure through the opaque base conductor film ("combined
exposure").
[0035] FIGS. 6A-6D are a sequence of cross sections of a microcup
array, illustrating the steps of assembling a monochrome
display.
DETAILED DESCRIPTION OF THE INVENTION
I. Definitions
[0036] Unless defined otherwise in this specification, all
technical terms are used herein according to their conventional
definitions as they are commonly used and understood by those of
ordinary skill in the art.
[0037] The term "microcup" refers to the cup-like indentations,
which may be created by methods such as micro-embossing or
imagewise exposure as described in the co-pending patent
applications identified above. Likewise, the plural form
"microcups" in a collective context may in general refer to the
microcup assembly comprising a plurality of such microcups
integrally formed or joined to make a structured two-dimensional
microcup array.
[0038] The term "cell", in the context of the present invention, is
intended to mean the single unit formed from a sealed microcup. The
cells are filled with charged pigment particles dispersed in a
solvent or solvent mixture.
[0039] The term "well-defined", when describing the microcups or
cells, is intended to indicate that the microcup or cell has a
definite shape, size, pattern and aspect ratio which are
predetermined according to the specific parameters of the
manufacturing process.
[0040] The term "aspect ratio" is a commonly known term in the art
and is the depth to width ratio or the depth to diameter ratio of
the microcup opening.
[0041] The term "imagewise exposure" means exposure of
radiation-curable material or photoresist composition to radiation,
such as UV, using one of the methods of the invention, whereby the
portions of the material so exposed are controlled to form a
pattern or "image" corresponding to the structure of the microcups,
e.g., the exposure is restricted to the portions of the material
corresponding to the microcup walls, leaving the microcup floor
portion unexposed. In the case of selectively opening photoresist
on predetermined portions of the microcup array, imagewise exposure
means exposure on the portions of material corresponding to the cup
opening, leaving the microcup walls unexposed. The pattern or image
may be formed by such methods as exposure through a photomask, or
alternatively by controlled particle beam exposure, and the
like.
II. The Microcup Array
[0042] FIGS. 1 and 2 are schematic cross-section views of an
exemplary microcup array assembly embodiment, simplified for
clarity, showing a microcup array assembly (10) of three microcup
cells (12a, b, and c).
[0043] As shown in FIG. 1, each cell (12) of array (10) comprises
two electrode plates (11, 13), at least one of which is transparent
(11), such as an ITO electrode, the electrodes (11) and (13)
bounding two opposite faces of the cell (12).
[0044] The microcup cell array assembly (10) comprises a plurality
of cells which are disposed adjacent to one another within a plane
to form a layer of cells (12) enclosed between the two electrodes
layers (11) and (13). Three exemplary cells (12a), (12b), and (12c)
are shown, bounded by their respective electrode plates (11a),
(11b), and (11c) (transparent) and (13a), (13b), and (13c) (back
plates), it being understood that a large number of such cells are
preferably arrayed two-dimensionally (to the right/left and in/out
of the plane in FIG. 1) to form a sheet-like display of any
selected area and two-dimensional shape. Likewise, several microcup
cells may be bounded by a single electrode plate (11) or (13),
although, for clarity, FIG. 1 shows an example in which each cell
(12) is bounded by separate electrode plates (11) and (13) having
the width of a single cell.
[0045] The cells are of well-defined shape and size and are filled
with a colored dielectric solvent (14) in which charged pigment
particles (15) are suspended and dispersed. The cells (12) may be
each filled with the same composition of pigment and solvent (e.g.,
in a monochrome display) or may be filled with different
compositions of pigment and solvent (e.g., in a color display).
FIG. 1 shows three different color combinations as indicated by the
different hatch pattern in each cell (12a), (12b), and (12c), the
solvents being designated (14a), (14b), and (14c) respectively, and
the pigment particles being designated (15a), (15b), and (15c)
respectively.
[0046] The microcup cells (12) each comprise enclosing walls (16)
bounding the cells on the sides (within the plane of array (10))
and floor (17) bounding the cell on one face, in this example the
face adjacent to electrode (13). On the opposite face (adjacent
electrode (11)) each cell comprises sealing cap portion (18). Where
the sealing cap portion is adjacent to the transparent electrode
(11) (as in FIG. 1), the sealing cap (18) comprises a transparent
composition. Although in the example of FIG. 1, the floor (17) and
the sealing cap (18) are shown as separate cell portions distinct
from adjacent electrodes (13) and (11) respectively, alternative
embodiments of the microcup array (10) of the invention may
comprise an integral floor/electrode structure or an integral
sealing cap/electrode structure.
[0047] FIG. 2 is a schematic cross-section of the EPD of FIG. 1,
but with two of the cells charged (12a and 12c), to cause the
pigment to migrate to one plate. When a voltage difference is
imposed between the two electrodes (11, 13), the charged particles
(15) migrate (i.e., toward electrode (11) or (13) depending on the
charge of the particle and electrode), such that either the color
of the pigment particle (15) or the color of the solvent (14) is
seen through the transparent conductor film (11). At least one of
the two conductors (11) or (13) is patterned (separately
addressable portions ) to permit a selective electric field to be
established with respect to either each cell or with respect to a
pre-defined group of cells (e.g., to form a pixel).
[0048] In the example of FIG. 2, two of the cells are shown charged
(12a and 12c), in which the pigment (15a and 15c) has migrated to
the respective transparent electrode plates (11a and 11c). The
remaining cell (12b) remains neutral and pigment (15b) is dispersed
throughout solvent (14b).
[0049] FIGS. 3A-3C shows the contours of an exemplary portion of a
microcup array, FIG. 3A showing a perspective view, FIG. 3B showing
a plan view, and FIG. 3C showing an elevation view, the vertical
scale being exaggerated for clarity. For reflective EPDs, the
opening area of each individual microcup may preferably be in the
range of about 10.sup.2 to about 5.times.10.sup.5 .mu.m.sup.2, more
preferably from about 10.sup.3 to about 5.times.10.sup.4
.mu.m.sup.2. The width w of the microcup (12) (distance between
adjacent walls (16)) may vary over a wide range, and is selectable
to suit the desired final display characteristics. The width w of
the microcup openings preferably is in the range of from about 15
to about 450 .mu.m, and more preferably from about 25 to about 300
.mu.m from edge to edge of the openings. Each microcup may form a
small segment of a pixel of the final display, or may be a full
pixel.
[0050] The wall thickness t relative to the cup width w may vary
over a large range, and is selectable to suit the desired final
display characteristics. The microcup wall thickness is typically
from about 0.01 to about 1 times the microcup width, and more
preferably about 0.05 to about 0.25 times the microcup width. The
opening-to-total area ratio is preferably in the range of about 0.1
to about 0.98, more preferably from about 0.3 to about 0.95.
[0051] The microcup wall height h (which defines the cup depth) is
shown exaggerated beyond its typical proportional dimensions for
clarity. For EPDs, the height of the microcups is typically in the
range of about 5 to about 100 microns (.mu.ms), preferably from
about 10 to about 50 microns. For LCDs, the height is typically in
the range of about 1 to 10 microns and more preferably from about 2
to 5 microns.
[0052] For simplicity and clarity, a square microcup arranged in a
linear two-dimensional array assembly is assumed in the description
herein of the microcup array assembly of the invention. However,
the microcup need not be square, it may be rectangular, circular,
or a more complex shape if desired. For example, the microcups may
be hexagonal and arranged in a hexagonal close-packed array, or
alternatively, triangular cups may be oriented to form hexagonal
sub-arrays, which in turn are arranged in a hexagonal close-packed
array.
[0053] In general, the microcups can be of any shape, and their
sizes, pattern and shapes may vary throughout the display. This may
be advantageous in the color EPD. In order to maximize the optical
effect, microcups having a mixture of different shapes and sizes
may be produced. For example, microcups filled with a dispersion of
the red color may have a different shape or size from the green
microcups or the blue microcups. Furthermore, a pixel may consist
of different numbers of microcups of different colors. For example,
a pixel may consist of a number of small green microcups, a number
of large red microcups, and a number of small blue microcups. It is
not necessary to have the same shape and number for the three
colors.
[0054] The openings of the microcups may be round, square,
rectangular, hexagonal, or any other shapes. The partition area
between the openings is preferably kept small in order to achieve a
high color saturation and contrast while maintaining desirable
mechanical properties. Consequently the honeycomb-shaped opening is
preferred over, for example, the circular opening.
III. Preparation Of The Microcup Array
[0055] The microcups may be prepared by microembossing or by
photolithography.
IIIa. Preparation of Microcups Array by Microembossing
Preparation of the Male Mold
[0056] The male mold may be prepared by any appropriate method,
such as a diamond turn process or a photoresist process followed by
either etching or electroplating. A master template for the male
mold may be manufactured by any appropriate method, such as
electroplating. With electroplating, a glass base is sputtered with
a thin layer (typically 3000 .ANG.) of a seed metal such as chrome
inconel. It is then coated with a layer of photoresist and exposed
to UV. A mask is placed between the UV and the layer of
photoresist. The exposed areas of the photoresist become hardened.
The unexposed areas are then removed by washing them with an
appropriate solvent. The remaining hardened photoresist is dried
and sputtered again with a thin layer of seed metal. The master is
then ready for electroforming. A typical material used for
electroforming is nickel cobalt. Alternatively, the master can be
made of nickel by electroforming or electroless nickel deposition
as described in "Continuous manufacturing of thin cover sheet
optical media", SPIE Proc. Vol. 1663, pp. 324 (1992). The floor of
the mold is typically between about 50 to 400 microns. The master
can also be made using other microengineering techniques including
e-beam writing, dry etching, chemical etching, laser writing or
laser interference as described in "Replication techniques for
micro-optics", SPIE Proc. Vol. 3099, pp. 76-82 (1997).
Alternatively, the mold can be made by photomachining using
plastics, ceramics or metals.
[0057] The male mold thus prepared typically has protrusions
between about 1 to 500 microns, preferably between about 2 to 100
microns, and most preferred about 4 to 50 microns. The male mold
may be in the form of a belt, a roller, or a sheet. For continuous
manufacturing, the belt type of mold is preferred.
Microcup Formation
[0058] Micro-cups may be formed either in a batchwise process or in
a continuous roll-to-roll process as disclosed in the co-pending
application, U.S. Ser. No. 09/784,972, filed on Feb. 25, 2001. The
latter offers a continuous, low cost, high throughput manufacturing
technology for production of compartments for use in
electrophoretic or LCDs. Prior to applying a UV curable resin
composition, the mold may be treated with a mold release to aid in
the demolding process. The UV curable resin may be degassed prior
to dispensing and may optionally contain a solvent. The solvent, if
present, readily evaporates. The UV curable resin is dispensed by
any appropriate means such as, coating, dipping, pouring and the
like, over the male mold. The dispenser may be moving or
stationary. A conductor film is overlaid the UV curable resin.
Examples of suitable conductor film include transparent conductor
ITO on plastic substrates such as polyethylene terephthalate,
polyethylene naphthate, polyaramid, polyimide, polycycloolefin,
polysulfone, epoxy and their composites. Pressure may be applied,
if necessary, to ensure proper bonding between the resin and the
plastic and to control the thickness of the floor of the
micro-cups. The pressure may be applied using a laminating roller,
vacuum molding, press device or any other like means. If the male
mold is metallic and opaque, the plastic substrate is typically
transparent to the actinic radiation used to cure the resin.
Conversely, the male mold can be transparent and the plastic
substrate can be opaque to the actinic radiation. To obtain good
transfer of the molded features onto the transfer sheet, the
conductor film needs to have good adhesion to the UV curable resin
which should have a good release property against the mold
surface.
IIIb. Preparation of Microcup Array by Photolithography
[0059] The photolithographic processes for preparation of the
microcup array are described in FIGS. 4 and 5.
Top Exposure
[0060] As shown in FIGS. 4A and 4B, the microcup array (40) may be
prepared by exposure of a radiation curable material (41a) coated
by known methods onto a conductor electrode film (42) to UV light
(or alternatively other forms of radiation, electron beams and the
like) through a mask (46) to form walls (41b) corresponding to the
image projected through the mask (46). The base conductor film (42)
is preferably mounted on a supportive substrate base web (43),
which may comprise a plastic material.
[0061] In the photomask (46) in FIG. 4A, the dark squares (44)
represent the opaque area and the space between the dark squares
represents the transparent area (45) of the mask (46). The UV
radiates through the transparent area (45) onto the radiation
curable material (41a). The exposure is preferably directly onto
the radiation curable material (41a), i.e., the UV does not pass
through the substrate (43) or base conductor (42) (top exposure).
For this reason, neither the substrate (43) nor the conductor (42)
needs to be transparent to the UV or other radiation wavelengths
employed.
[0062] As shown in FIG. 4B, the exposed areas (41b) become hardened
and the unexposed areas (protected by the opaque area (44) of the
mask (46) are then removed by an appropriate solvent or developer
to form the microcups (47). The solvent or developer is selected
from those commonly used for dissolving or reducing the viscosity
of radiation curable materials such as methylethylketone (MEK),
toluene, acetone, isopropanol or the like. The preparation of the
microcups may be similarly accomplished by placing a photomask
underneath the conductor film/substrate support web and in this
case the UV light radiates through the photomask from the bottom
and the substrate needs to be transparent to radiation.
Exposure Through Opaque Conductor Lines
[0063] Still another alternative method for the preparation of the
microcup array of the invention by imagewise exposure is
illustrated in FIGS. 5A and 5B. When opaque conductor lines are
used, the conductor lines can be used as the photomask for the
exposure from the bottom. Durable microcup walls are formed by
additional exposure from the top through a second photomask having
opaque lines perpendicular to the conductor lines.
[0064] FIG. 5A illustrates the use of both the top and bottom
exposure principals to produce the microcup array (50) of the
invention. The base conductor film (52) is opaque and
line-patterned. The radiation curable material (51a), which is
coated on the base conductor (52) and substrate (53), is exposed
from the bottom through the conductor line pattern (52) which
serves as the first photomask. A second exposure is performed from
the "top" side through the second photomask (56) having a line
pattern perpendicular to the conductor lines (52). The spaces (55)
between the lines (54) are substantially transparent to the UV
light. In this process, the wall material (51b) is cured from the
bottom up in one lateral orientation, and cured from the top down
in the perpendicular direction, joining to form an integral
microcup (57).
[0065] As shown in FIG. 5B, the unexposed area is then removed by a
solvent or developer as described above to reveal the microcups
(57).
IV. The Sealing Composition and Process of the Present
Invention
[0066] The novel sealing overcoat composition comprises the
following ingredients:
[0067] (1) a solvent or solvent mixture which is immiscible with
the display fluid in the microcups and exhibits a specific gravity
less than that of the display fluid; and
[0068] (2) a thermoplastic elastomer.
[0069] Compositions containing a thermoplastic elastomer having
good compatibility with the microcups and good barrier properties
against the display fluid are particularly useful. Examples of
useful thermoplastic elastomers include ABA, and (AB)n type of
di-block, tri-block, and multi-block copolymers wherein A is
styrene, .alpha.-methylstyrene, ethylene, propylene or norbonene; B
is butadiene, isoprene, ethylene, propylene, butylene,
dimethylsiloxane or propylene sulfide; and A and B cannot be the
same in the formula. The number, n, is .gtoreq.1, preferably 1-10.
Particularly useful are di-block or tri-block copolymers of styrene
or ox-methylstyrene such as SB (poly(styrene-b-butadiene)), SBS
(poly(styrene-b-butadiene-b-styrene)), SIS
(poly(styrene-b-isoprene-b- -styrene)), SEBS
(poly(styrene-b-ethylene/butylenes-b-stylene))
poly(styrene-b-dimethylsiloxane-b-styrene),
poly((.alpha.-methylstyrene-b- -isoprene),
poly(.alpha.-methylstyrene-b-isoprene-b-.alpha.-methylstyrene)- ,
poly(.alpha.-methylstyrene-b-propylene
sulfide-b-.alpha.-methylstyrene),
poly(.alpha.-methylstyrene-b-dimethylsiloxane-b-.alpha.-methylstyrene).
A review of the preparation of the thermoplastic elastomers can be
found in N. R. Legge, G. Holden, and H. E. Schroeder ed.,
"Thermoplastic Elastomers", Hanser Publisher (1987). Commercially
available styrene block copolymers such as Kraton D and G series
(from Kraton Polymer, Houston, Tex.) are particularly useful.
Crystalline rubbers such as
poly(ethylene-co-propylene-co-5-methylene-2-norbomene) or EPDM
(ethylene-propylene-diene terpolymer) rubbers such as Vistalon 6505
(from Exxon Mobil, Houston, Tex.) and their grafted copolymers have
also been found very useful.
[0070] Not to be bound by the theory, it is believed that the hard
block of the thermoplastic elastomers phase separates during or
after the drying of the sealing overcoat and serves as the physical
crosslinker of the soft continuous phase. The sealing composition
of the present invention significantly enhances the modulus and
film integrity of the sealing layer throughout the coating and
drying processes of the sealing layer. Thermoplastic elastomers
having low critical surface tension (lower than 40 dyne/cm) and
high modulus or Shore A hardness (higher than 60) have been found
useful probably because of their favorable wetting property and
film integrity over the display fluid.
[0071] The thermoplastic elastomer is dissolved in a solvent or
solvent mixture which is immiscible with the display fluid in the
microcups and exhibits a specific gravity less than that of the
display fluid. Low surface tension solvents are preferred for the
overcoating composition because of their better wetting properties
over the microcup walls and the electrophoretic fluid. Solvents or
solvent mixtures having a surface tension lower than 35 dyne/cm are
preferred. A surface tension of lower than 30 dyne/cm is more
preferred. Suitable solvents include alkanes (preferably C.sub.6-12
alkanes such as heptane, octane or Isopar solvents from Exxon
Chemical Company, nonane, decane and their isomers), cycloalkanes
(preferably C.sub.6-12 cycloalkanes such as cyclohexane and decalin
and the like), alkylbezenes (preferably mono- or
[0072] di-C.sub.1-6 alkyl benzenes such as toluene, xylene and the
like), alkyl esters (preferably C.sub.2-5 alkyl esters such as
ethyl acetate, isobutyl acetate and the like) and C.sub.3-5 alkyl
alcohols (such as isopropanol and the like and their isomers).
Mixtures of alkylbenzene and alkane are particularly useful.
[0073] Wetting agents (such as the FC surfactants from 3M Company,
Zonyl fluorosurfactants from DuPont, fluoroacrylates,
fluoromethacrylates, fluoro-substituted long chain alcohols,
perfluoro-substituted long chain carboxylic acids and their
derivatives, and Silwet silicone surfactants from OSi, Greenwich,
Conn.) may also be included in the composition to improve the
adhesion of the sealant to the microcups and provide a more
flexible coating process. Other ingredients including crosslinking
agents (e.g., bisazides such as 4,4'-diazidodiphenylmethane and
2,6-di-(4'-azidobenzal)-4-methylcyclohexanone), vulcanizers (e.g.,
2-benzothiazolyl disulfide and tetramethylthiuram disulfide),
multifunctional monomers or oligomers (e.g., hexanediol,
diacrylates, trimethylolpropane, triacrylate, divinylbenzene,
diallylphthalene), thermal initiators (e.g., dilauroryl peroxide,
benzoyl peroxide) and photoinitiators (e.g., isopropyl thioxanthone
(ITX), Irgacure 651 and Irgacure 369 from Ciba-Geigy) are also
highly useful to enhance the physicomechanical properties of the
sealing layer by crosslinking or polymerization reactions during or
after the overcoating process.
[0074] The sealing composition is typically overcoated onto
partially filled microcups and the overcoated microcups are dried
at room temperature. The sealed microcups optionally may be post
treated by UV radiation or thermal baking to further improve the
barrier properties. The adhesion of the sealing layer to the
microcups may also be improved by the post-curing reaction, likely
due to the formation of an interpenetration network at the
microcup-sealing layer inter-phase.
V. Preparation Of Electrophoretic Displays From The Microcup
Array
[0075] The preferred process of preparing the electrophoretic cells
is illustrated schematically in FIGS. 6A-6D.
[0076] As shown in FIG. 6A, the microcup array (60) may be prepared
by any of the alternative methods described in Section III above.
The unfilled microcup array made by the methods described herein
typically comprises a substrate web (63) upon which a base
electrode (62) is deposited. The microcup walls (61) extend upward
from the substrate (63) to form the open cups.
[0077] As shown in FIG. 6B, the microcups are filled with a
suspension of the charged pigment particles (65) in a colored
dielectric solvent composition (64). In the example shown, the
composition is the same in each cup, i.e., in a monochrome display.
In carrying out the sealing process of the present invention, the
microcups are preferably partially filled (to prevent overflow),
which can be achieved by diluting the electrophoretic fluid with a
volatile solvent (such as acetone, methyl ethyl ketone,
isopropanol, hexane, and perfluoro solvent FC-33 from 3M Co.,) and
allowing the volatile solvent to evaporate. When a high boiling
point perfluoro solvent such as HT-200 (from Ausimont Colo.,
Thorofare, N.J.) is used as the continuous phase of the display
fluid, a perfluoro volatile solvent such as FC-33 is particularly
useful to control the level of partial filling.
[0078] As shown in FIG. 6C, after filling, the microcups are sealed
with the sealing composition of the present invention to form a
sealing layer (66). The sealing composition is typically overcoated
onto the partially filled microcups and dried on the display fluid.
The sealed microcups optionally may be post treated by UV radiation
or thermal baking to further improve the barrier properties.
[0079] As shown in FIG. 6D, the sealed array of electrophoretic
microcup cells (60) is laminated with a second conductor film (67),
preferably by pre-coating the conductor (67) with an adhesive layer
(68) which may be a pressure sensitive adhesive, a hot melt
adhesive, or a heat, moisture, or radiation curable adhesive. The
laminate adhesive may be post-cured by radiation such as UV through
the top conductor film if the latter is transparent to the
radiation.
VI. Preparation Of The Pigment/Solvent Suspension Or Dispersion
Composition
[0080] As described herein with respect to the various embodiments
of the EPD of the invention, the microcups are preferably filled
with charged pigment particles dispersed in a dielectric solvent
(e.g., solvent (64) and pigment particles (65) in FIG. 6B.). The
dispersion may be prepared according to methods well known in the
art, such as U.S. Pat. Nos. 6,017,584, 5,914,806, 5,573,711,
5,403,518, 5,380,362, 4,680,103, 4,285,801, 4,093,534, 4,071,430,
and 3,668,106. See also IEEE Trans. Electron Devices, ED-24, 827
(1977), and J. Appl. Phys. 49(9), 4820 (1978).
[0081] The charged pigment particles visually contrast with the
medium in which the particles are suspended. The medium is a
dielectric solvent which preferably has a low viscosity and a
dielectric constant in the range of about 2 to about 30, preferably
about 2 to about 15 for high particle mobility. Examples of
suitable dielectric solvents include hydrocarbons such as
decahydronaphthalene (DECALIN), 5-ethylidene-2-norbomene, fatty
oils, paraffin oil, aromatic hydrocarbons such as toluene, xylene,
phenylxylylethane, dodecylbenzene and alkylnaphthalenes,
halogenated solvents such as, dichlorobenzotrifluoride- ,
3,4,5-trichlorobenzotrifluoride, chloropentafluoro-benzene,
dichlorononane, pentachlorobenzene, and perfluoro solvents such as
perfluorodecalin, perfluorotoluene, perfluoroxylene, FC-43, FC-70
and FC-5060 from 3M Company, St. Paul Minn., low molecular weight
halogen containing polymers such as poly(perfluoropropylene oxide)
from TCI America, Portland, Oregon, poly(chlorotrifluoroethylene)
such as Halocarbon Oils from Halocarbon Product Corp., River Edge,
N.J., perfluoropolyalkylether such as Galden, HT-200, and
Fluorolink from Ausimont (Thorofare, N.J.) or Krytox Oils and
Greases K-Fluid Series from DuPont, Del. In one preferred
embodiment, poly(chlorotrifluoroethylene) is used as the dielectric
solvent. In another preferred embodiment, poly(perfluoropropylene
oxide) is used as the dielectric solvent.
[0082] A non-migrating fluid colorant may be formed from dyes or
pigments. Nonionic azo and anthraquinone dyes are particularly
useful. Examples of useful dyes include, but are not limited to:
Oil Red EGN, Sudan Red, Sudan Blue, Oil Blue, Macrolex Blue,
Solvent Blue 35, Pylam Spirit Black and Fast Spirit Black from
Pylam Products Co., Arizona, Sudan Black B from Aldrich,
Thermoplastic Black X-70 from BASF, and anthraquinone blue,
anthraquinone yellow 114, anthraquinone red 111, 135, anthraquinone
green 28 from Aldrich. Fluorinated dyes are particularly useful
when perfluoro solvents are used. In the case of a pigment, the
non-migrating pigment particles for generating the color of the
medium may also be dispersed in the dielectric medium. These color
particles are preferably uncharged. If the non-migrating pigment
particles for generating color in the medium are charged, they
preferably carry a charge which is opposite from that of the
charged, migrating pigment particles. If both types of pigment
particles carry the same charge, then they should have different
charge density or different electrophoretic mobility. In any case,
the dye or pigment for generating the non-migrating fluid colorant
of the medium must be chemically stable and compatible with other
components in the suspension.
[0083] The charged, migrating pigment particles may be organic or
inorganic pigments, such as TiO.sub.2, phthalocyanine blue,
phthalocyanine green, diarylide yellow, diarylide AAOT Yellow, and
quinacridone, azo, rhodamine, perylene pigment series from Sun
Chemical, Hansa yellow G particles from Kanto Chemical, and Carbon
Lampblack from Fisher. Submicron particle size is preferred. These
particles should have acceptable optical characteristics, should
not be swollen or softened by the dielectric solvent, and should be
chemically stable. The resulting suspension must also be stable
against sedimentation, creaming or flocculation under normal
operating conditions.
[0084] The migrating pigment particles may exhibit a native charge,
or may be charged explicitly using a charge control agent, or may
acquire a charge when suspended in the dielectric solvent. Suitable
charge control agents are well known in the art; they may be
polymeric or non-polymeric in nature, and may also be ionic or
non-ionic, including ionic surfactants such as Aerosol OT, sodium
dodecylbenzenesulfonate, metal soaps, polybutene succinimide,
maleic anhydride copolymers, vinylpyridine copolymers,
vinylpyrrolidone copolymer (such as Ganex from International
Specialty Products), (meth)acrylic acid copolymers,
N,N-dimethylaminoethyl (meth)acrylate copolymers. Fluorosurfactants
are particularly useful as charge controlling agents in
perfluorocarbon solvents. These include FC fluorosurfactants such
as FC-170C, FC-171, FC-176, FC430, FC431 and FC-740 from 3M Company
and Zonyl fluorosurfactants such as Zonyl FSA, FSE, FSN, FSN-100,
FSO, FSO-100, FSD and UR from Dupont.
[0085] Suitable charged pigment dispersions may be manufactured by
any of the well-known methods including grinding, milling,
attriting, microfluidizing, and ultrasonic techniques. For example,
pigment particles in the form of a fine powder are added to the
suspending solvent and the resulting mixture is ball milled or
attrited for several hours to break up the highly agglomerated dry
pigment powder into primary particles. Although less preferred, a
dye or pigment for producing the non-migrating fluid colorant may
be added to the suspension during the ball milling process.
[0086] Sedimentation or creaming of the pigment particles may be
eliminated by microencapsulating the particles with suitable
polymers to match the specific gravity to that of the dielectric
solvent. Microencapsulation of the pigment particles may be
accomplished chemically or physically. Typical microencapsulation
processes include interfacial polymerization, in-situ
polymerization, phase separation, coacervation, electrostatic
coating, spray drying, fluidized bed coating and solvent
evaporation.
[0087] For a black/white EPD, the suspension comprises charged
white particles of titanium oxide (TiO.sub.2) dispersed in a black
dielectric solution containing a black dye or dispersed uncharged
black particles. A black dye or dye mixture such as Pylam Spirit
Black and Fast Spirit Black from Pylam Products Co. Arizona, Sudan
Black B from Aldrich, Thermoplastic Black X-70 from BASF, or an
insoluble black pigment such as carbon black may be used to
generate the black color of the solvent. For other colored
suspensions, there are many possibilities. For a subtractive color
system, the charged TiO.sub.2 particles may be suspended in a
dielectric fluid of cyan, yellow or magenta color. The cyan, yellow
or magenta color may be generated via the use of a dye or a
pigment. For an additive color system, the charged TiO.sub.2
particles may be suspended in a dielectric fluid of red, green or
blue color generated also via the use of a dye or a pigment. The
red, green, blue color system is preferred for most
applications.
EXAMPLES
Example 1
Microcup Formulation
[0088] 35 parts by weight of Ebecryl 600 (UCB), 40 parts of SR-399
(Sartomer), 10 parts of Ebecryl 4827 (UCB), 7 parts of Ebecryl 1360
(UCB), 8 parts of HDDA, (UCB), 0.05 parts of Irgacure 369 (Ciba
Specialty Chemicals) and 0.01 parts of isopropyl thioxanthone (ITX
from Aldrich) were mixed homogeneously and used for
micro-embossing.
Example 2
Preparation of Microcup Array
[0089] A primer solution comprising of 5 parts of Ebecryl 830, 2.6
parts of SR-399 (from Sartomer), 1.8 parts of Ebecry 1701, 1 part
of PMMA (Mw=350,000 from Aldrich), 0.5 parts of Irgacure 500, and
40 parts of methyl ethyl ketone (MEK) was coated onto a 2 mil 60
ohm/sq. ITO/PET film (from Sheldahl Inc., Minn.) using a #3 Myrad
bar, dried, and UV cured by using the Zeta 7410 (5 w/cm.sup.2, from
Loctite) exposure unit for 15 minutes in air. The microcup
formulation prepared in Example 1 was coated onto the treated
ITO/PET film with a targeted thickness of about 50 .mu.m, embossed
with a Ni--Co male mold having a 60 (length).times.60 (width) .mu.m
repetitive protrusion square pattern with 25-50 .mu.m protrusion
height and 10 .mu.m wide partition lines, UV cured from the PET
side for 20 seconds, removed from the mold with a 2" peeling bar at
a speed of about 4-5 ft/min. Well-defined micro-cups with depth
ranging from 25 to 50 .mu.m were prepared by using male molds
having corresponding protrusion heights. Microcup arrays of various
dimension such as 70 (length).times.70 (width).times.35
(depth).times.10 (partition), 100
(L).times.100(W).times.35(D).times.10(P), and 100
(L).times.100(W).times.30(D).times.10(P) .mu.m were also prepared
by the same procedure.
Example 3
Pigment Dispersion
[0090] 6.42 Grams of Ti Pure R706 were dispersed with a homogenizer
into a solution containing 1.94 grams of Fluorolink D from
Ausimont, 0.22 grams of Fluorolink 7004 also from Ausimont, 0.37
grams of a fluorinated copper phthalocyanine dye from 3M, 52.54
grams of perfluoro solvent HT-200 (Ausimont).
Example 4
Pigment Dispersion
[0091] The same as Example 3, except the Ti Pure R706 and
Fluorolink were replaced by a polymer coated TiO.sub.2 particles
PC-9003 from Elimentis (Highstown, N.J.) and Krytox (Du Pont)
respectively. Note: replacing 2 things, Ti & fluorolink, with 1
thing, TiO2 PC-90003 from 2 suppliers, elimentis & kryto
Example 5
Microcup Sealing
[0092] The electrophoretic fluid prepared in Example 3 was diluted
with a volatile perfluoro co-solvent FC-33 from 3M and coated onto
a 35 microns deep microcup array prepared in Example 2. The
volatile cosolvent was allowed to evaporate to expose a partially
filled microcup array. A 7.5% solution of polyisoprene (97% cis,
from Aldrich) in heptane was then overcoated onto the partially
filled cups by a Universal Blade Applicator with an opening of 3
mil. The overcoated microcups were then dried at room temperature.
A seamless sealing layer of about 7-8 .mu.m thickness (dry) with
acceptable adhesion and uniformity was formed on the microcup
array. No observable entrapped air bubble in the sealed microcups
was found under microscope. A second ITO/PET conductor precoated
with an adhesive layer was laminated onto the sealed microcups. The
electrophoretic cell showed satisfactory switching performance with
good flexure resistance. No observable weight loss was found after
being aged in a 66.degree. C. oven for 5 days.
Example 6
Microcup Sealing
[0093] The same as Example 5, except the thickness of the
polyisoprene layer was reduced to 4 microns by using a blade
applicator of 2 mil opening. Pinholes and broken sealing layer were
clearly observed under optical microscope.
Example 7-14
Microcup Sealing
[0094] The same as Example 5, except the sealing layer was replaced
by polystyrene (Mw=, polyvinylbutyral (Butvar 72, from Solutia
Inc., St. Louis, Mo.), and thermpoplastic elastomers such as SIS
(Kraton D1107, 15% styrene), SBS (Kraton D1101, 31% styrene) SEBS
(Kraton G1650 and FG1901, 30% styrene), and EPDM (Vistalon 6505,
57% ethylene). The results are summarized in Table 1. As it can be
seen from Table 1, thermoplastic elastomers enabled thinner and
higher quality sealing even on microcups of wide openings.
1TABLE 1 Sealing of microcups Estimated Example dry Cup dimension
Coating quality Coating quality No. Sealing Polymer Coating
solution thickness (L .times. W .times. D .times. P), um (visual)
(Microscopic) comparative Polyisoprene 7.5% in heptane 4-5 um 60
.times. 60 .times. 35 .times. 10 fair pinholes, broken 5 (97% cis)
layer comparative Polyisoprene 7.5% in heptane 7-8 um 60 .times. 60
.times. 35 .times. 10 good good 6 (97% cis) comparative Polystyrene
30% in toluene 7-8 um 60 .times. 60 .times. 35 .times. 10 very
poor, incomplete 7 severe dewetting sealing, defects comparative
Butvar 72 8.5% in 4-5 um 60 .times. 60 .times. 35 .times. 10 poor
fair 8 isopropanol reproducibility 9 SIS (Kratone 4% in Heptane 4-5
um 70 .times. 70 .times. 35 .times. 10 good good D1107); 15%
Styrene 10 SIS (Kratone 4% in Heptane 3-4 um 100 .times. 100
.times. 30 .times. 10 good good D1107); 15% Styrene 11 SBS (Kraton
10% in toluene/ 4-5 um 70 .times. 70 .times. 35 .times. 10 good
good D1101), 31% heptane (20/80) styrene 12 SEBS(Kraton FG 10% in
xylene/ 4-5 um 70 .times. 70 .times. 35 .times. 10 good good 1901,
30% Isopar E (5/95) styrene, 1.5% maleic anhrdride) 13 SEBS(Kraton
5% in toluene/ 4-5 um 70 .times. 70 .times. 35 .times. 10 good good
G1650, 30% heptane (5/95) styrene) 14 EPDM (Vistalon 10% in Isopar
E 4-5 um 70 .times. 70 .times. 35 .times. 10 good good 6505, 57%
ethylene)
[0095] While the present invention has been described with
reference to the specific embodiments thereof, it should be
understood by those skilled in the art that various changes may be
made and equivalents may be substituted without departing from the
true spirit and scope of the invention. In addition, many
modifications may be made to adapt a particular situation,
materials, compositions, processes, process step or steps, to the
objective, spirit and scope of the present invention. All such
modifications are intended to be within the scope of the claims
appended hereto.
[0096] For example, it should be noted that the method of the
invention for making microcups may also be used for manufacturing
microcup arrays for liquid crystal displays. Similarly, the
microcup selective filling, sealing and ITO laminating methods of
the invention may also be employed in the manufacture of liquid
crystal displays.
[0097] It is therefore wished that this invention to be defined by
the scope of the appended claims as broadly as the prior art will
permit, and in view of the specification if need be.
* * * * *