U.S. patent application number 10/905746 was filed with the patent office on 2005-07-21 for preparation of capsules.
This patent application is currently assigned to E INK CORPORATION. Invention is credited to Chebiyam, Rajesh, Manning, Jeremy J., Steiner, Michael L., Valianatos, Peter J., Walls, Michael D., Whitesides, Thomas H..
Application Number | 20050156340 10/905746 |
Document ID | / |
Family ID | 34830318 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050156340 |
Kind Code |
A1 |
Valianatos, Peter J. ; et
al. |
July 21, 2005 |
PREPARATION OF CAPSULES
Abstract
Prior art processes for producing protein-based capsules (for
example, capsules for use in electrophoretic media) tend to be
wasteful because they produce many capsules outside the desired
size range, which is typically about 20 to 50 .mu.m. Capsule size
distribution and yields can be improved by either (a) emulsifying a
water-immiscible phase in a preformed coacervate of the protein; or
(b) using a limited coalescence process with colloidal alumina as
the surface-active particulate material.
Inventors: |
Valianatos, Peter J.;
(Boston, MA) ; Chebiyam, Rajesh; (Nashua, NH)
; Manning, Jeremy J.; (Douglas, MA) ; Steiner,
Michael L.; (New Richmond, WI) ; Whitesides, Thomas
H.; (Somerville, MA) ; Walls, Michael D.;
(Boston, MA) |
Correspondence
Address: |
DAVID J COLE
E INK CORPORATION
733 CONCORD AVE
CAMBRIDGE
MA
02138-1002
US
|
Assignee: |
E INK CORPORATION
733 Concord Avenue
Cambridge
MA
|
Family ID: |
34830318 |
Appl. No.: |
10/905746 |
Filed: |
January 19, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60481920 |
Jan 20, 2004 |
|
|
|
60521010 |
Feb 5, 2004 |
|
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Current U.S.
Class: |
264/4.1 |
Current CPC
Class: |
Y10T 428/2984 20150115;
Y10T 428/2998 20150115; Y10T 428/2982 20150115; Y10T 428/2991
20150115; B01J 13/08 20130101; G02F 1/167 20130101 |
Class at
Publication: |
264/004.1 |
International
Class: |
B65B 001/00 |
Claims
What is claimed is:
1. A process for encapsulating a water-immiscible phase in a
protein coacervate, the process comprising: forming a coacervate of
the protein and a coacervating agent in an aqueous medium; and
emulsifying the water-immiscible phase in the aqueous medium
comprising the coacervate under conditions effective to cause
deposition of the coacervate around the water-immiscible phase,
thereby forming capsules of the water-immiscible phase surrounded
by capsule walls of the coacervate.
2. A process according to claim 1 wherein the protein is
gelatin.
3. A process according to claim 1 wherein the coacervating agent is
acacia.
4. A process according to claim 1 wherein the water-immiscible
phase comprises an aliphatic hydrocarbon.
5. A process according to claim 4 wherein the water-immiscible
phase further comprises solid particles suspended in the
hydrocarbon.
6. A process according to claim 5 wherein the solid particles are
electrically charged.
7. A process according to claim 6 wherein the water-immiscible
phase comprises two different types of solid particles bearing
charges of opposite polarity.
8. A process according to claim 1 wherein at least part of the
emulsification/capsule formation step is conducted at a temperature
below about 35.degree. C.
9. A process according to claim 1 wherein the capsules formed are
thereafter treated with a cross-linking agent effective to cause
cross-linking of the protein.
10. A process according to claim 9 wherein the cross-linking agent
is an aldehydes.
11. A process according to claim 1 wherein the capsules formed are
thereafter mixed with a polymeric binder and the capsules/binder
mixture coated on to a substrate and dried to form a coherent layer
of capsules on the substrate.
12. A process for encapsulating a water-immiscible phase in a
protein coacervate, the process comprising: forming an aqueous
phase comprising a colloidal alumina suspension and a promoter;
emulsifying the water-immiscible phase in the aqueous phase under
conditions effective to cause the formation of an unstable emulsion
comprising small droplets of the water-immiscible phase in the
aqueous phase; and admixing the emulsion with the protein and a
coacervating agent under conditions permitting coalescence of the
emulsion and the formation of capsules of the aqueous phase
surrounded by capsule walls of the coacervate.
13. A process according to claim 12 wherein the alumina comprises
from about 0.1 to about 1.0 percent by weight of the aqueous
phase.
14. A process according to claim 12 wherein the promoter comprises
a polyacid.
15. A process according to claim 14 wherein the promoter comprises
a copolymer of a carboxylic acid and an olefin.
16. A process according to claim 12 wherein the protein comprises
gelatin.
17. A process according to claim 12 wherein the coacervating agent
comprises an anionic polymer.
18. A process according to claim 17 wherein the coacervating agent
comprises a polyanionic polymer having a vinyl main chain and a
plurality of anionic groups bonded to the main chain.
19. A process according to claim 18 wherein the polyanionic polymer
comprises any one or more of poly(acrylic acid); poly(methacrylic
acid); copolymers of poly(acrylic acid) and/or poly(methacrylic
acid) with esters of the same acids; styrene sulfonate copolymers
with styrene; methyl vinyl ether or vinyl acetate copolymers with
(meth)acrylic acid; copolymers of alkyl-substituted olefins, methyl
vinyl ether and vinyl carboxylate with maleic acid, maleic esters,
and maleic half ester, half acids.
20. A process according to claim 12 wherein the wherein the
water-immiscible phase comprises an aliphatic hydrocarbon.
21. A process according to claim 20 wherein the water-immiscible
phase further comprises solid particles suspended in the
hydrocarbon.
22. A process according to claim 21 wherein the solid particles are
electrically charged.
23. A process according to claim 22 wherein the water-immiscible
phase comprises two different types of solid particles bearing
charges of opposite polarity.
24. A process according to claim 21 wherein the hydrocarbon has
dispersed therein a di-block copolymer or an aromatic-substituted
alkene and an alkene.
25. A process according to claim 12 wherein the capsules formed are
thereafter treated with a cross-linking agent effective to cause
cross-linking of the protein.
26. A process according to claim 25 wherein the cross-linking agent
is an aldehyde.
27. A process according to claim 12 wherein the capsules formed are
thereafter mixed with a polymeric binder and the capsules/binder
mixture coated on to a substrate and dried to form a coherent layer
of capsules on the substrate.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of Provisional Application
Ser. No. 60/481,920, filed Jan. 20, 2004, and Provisional
Application Ser. No. 60/521,010, filed Feb. 5, 2004.
[0002] The entire disclosures of these two applications, and of all
U.S. patents and published and copending applications referred to
below are also herein incorporated by reference.
BACKGROUND OF INVENTION
[0003] This invention relates to the preparation of capsules,
especially capsules intended for use in forming electrophoretic
media.
[0004] Particle-based electrophoretic displays, in which a
plurality of charged particles move through a suspending fluid
under the influence of an electric field, have been the subject of
intense research and development for a number of years. Such
displays can have attributes of good brightness and contrast, wide
viewing angles, state bistability, and low power consumption when
compared with liquid crystal displays.
[0005] (The terms "bistable" and "bistability" are used herein in
their conventional meaning in the art to refer to displays
comprising display elements having first and second display states
differing in at least one optical property, and such that after any
given element has been driven, by means of an addressing pulse of
finite duration, to assume either its first or second display
state, after the addressing pulse has terminated, that state will
persist for at least several times, for example at least four
times, the minimum duration of the addressing pulse required to
change the state of the display element. It is shown in published
U.S. patent application No. 2002/0180687 that some particle-based
electrophoretic displays capable of gray scale are stable not only
in their extreme black and white states but also in their
intermediate gray states, and the same is true of some other types
of electro-optic displays. This type of display is properly called
"multi-stable" rather than bistable, although for convenience the
term "bistable" may be used herein to cover both bistable and
multi-stable displays.)
[0006] Nevertheless, problems with the long-term image quality of
electrophoretic displays have prevented their widespread usage. For
example, particles that make up electrophoretic displays tend to
settle, resulting in inadequate service-life for these
displays.
[0007] Numerous patents and applications assigned to or in the
names of the Massachusetts Institute of Technology (MIT) and E Ink
Corporation have recently been published describing encapsulated
electrophoretic media. Such encapsulated media comprise numerous
small capsules, each of which itself comprises an internal phase
containing electrophoretically-mobile particles suspended in a
liquid suspending medium, and a capsule wall surrounding the
internal phase. Typically, the capsules are themselves held within
a polymeric binder to form a coherent layer positioned between two
electrodes. Encapsulated media of this type are described, for
example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584;
6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773;
6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,721; 6,252,564;
6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989;
6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790;
6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182;
6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949;
6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545;
6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333;
6,704,133; 6,710,540; 6,721,083; 6,727,881; 6,738,050; 6,750,473;
6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829;
6,825,970; 6,831,769; and 6,839,158; and U.S. patent applications
Publication Nos. 2002/0060321; 2002/0060321; 2002/0063661;
2002/0090980; 2002/0113770; 2002/0130832; 2002/0131147;
2002/0171910; 2002/0180687; 2002/0180688; 2003/0011560;
2003/0020844; 2003/0025855; 2003/0102858; 2003/0132908;
2003/0137521; 2003/0151702; 2003/0214695; 2003/0214697;
2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265;
2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036;
2004/0112750; 2004/0119681; and 2004/0196215; and International
Applications Publication Nos. WO 99/67678; WO 00/05704; WO
00/38000; WO 00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO
01/07961; WO 01/08241; WO 03/107,315; WO 2004/023195; WO
2004/049045; WO 2004/059378; WO 2004/088002; WO 2004/088395; WO
2004/090857; and WO 2004/099862.
[0008] Known electrophoretic media, both encapsulated and
unencapsulated, can be divided into two main types, referred to
hereinafter for convenience as "single particle" and "dual
particle" respectively. A single particle medium has only a single
type of electrophoretic particle suspended in a suspending medium,
at least one optical characteristic of which differs from that of
the particles. (In referring to a single type of particle, we do
not imply that all particles of the type are absolutely identical.
For example, provided that all particles of the type possess
substantially the same optical characteristic and a charge of the
same polarity, considerable variation in parameters such as
particle size and electrophoretic mobility can be tolerated without
affecting the utility of the medium.) When such a medium is placed
between a pair of electrodes, at least one of which is transparent,
depending upon the relative potentials of the two electrodes, the
medium can display the optical characteristic of the particles
(when the particles are adjacent the electrode closer to the
observer, hereinafter called the "front" electrode) or the optical
characteristic of the suspending medium (when the particles are
adjacent the electrode remote from the observer, hereinafter called
the "rear" electrode (so that the particles are hidden by the
suspending medium).
[0009] A dual particle medium has two different types of particles
differing in at least one optical characteristic and a suspending
fluid which may be uncolored or colored, but which is typically
uncolored. The two types of particles differ in electrophoretic
mobility; this difference in mobility may be in polarity (this type
may hereinafter be referred to as an "opposite charge dual
particle" medium) and/or magnitude. When such a dual particle
medium is placed between the aforementioned pair of electrodes,
depending upon the relative potentials of the two electrodes, the
medium can display the optical characteristic of either set of
particles, although the exact manner in which this is achieved
differs depending upon whether the difference in mobility is in
polarity or only in magnitude. For ease of illustration, consider
an electrophoretic medium in which one type of particles is black
and the other type white. If the two types of particles differ in
polarity (if, for example, the black particles are positively
charged and the white particles negatively charged), the particles
will be attracted to the two different electrodes, so that if, for
example, the front electrode is negative relative to the rear
electrode, the black particles will be attracted to the front
electrode and the white particles to the rear electrode, so that
the medium will appear black to the observer. Conversely, if the
front electrode is positive relative to the rear electrode, the
white particles will be attracted to the front electrode and the
black particles to the rear electrode, so that the medium will
appear white to the observer.
[0010] If the two types of particles have charges of the same
polarity, but differ in electrophoretic mobility (this type of
medium may hereinafter to referred to as a "same polarity dual
particle" medium), both types of particles will be attracted to the
same electrode, but one type will reach the electrode before the
other, so that the type facing the observer differs depending upon
the electrode to which the particles are attracted. For example
suppose the previous illustration is modified so that both the
black and white particles are positively charged, but the black
particles have the higher electrophoretic mobility. If now the
front electrode is negative relative to the rear electrode, both
the black and white particles will be attracted to the front
electrode, but the black particles, because of their higher
mobility will reach it first, so that a layer of black particles
will coat the front electrode and the medium will appear black to
the observer. Conversely, if the front electrode is positive
relative to the rear electrode, both the black and white particles
will be attracted to the rear electrode, but the black particles,
because of their higher mobility will reach it first, so that a
layer of black particles will coat the rear electrode, leaving a
layer of white particles remote from the rear electrode and facing
the observer, so that the medium will appear white to the observer:
note that this type of dual particle medium requires that the
suspending fluid be sufficiently transparent to allow the layer of
white particles remote from the rear electrode to be readily
visible to the observer. Typically, the suspending fluid in such a
display is not colored at all, but some color may be incorporated
for the purpose of correcting any undesirable tint in the white
particles seen therethrough.
[0011] Both single and dual particle electrophoretic displays may
be capable of intermediate gray states having optical
characteristics intermediate the two extreme optical states already
described.
[0012] Some of the aforementioned patents and published
applications disclose encapsulated electrophoretic media having
three or more different types of particles within each capsule. For
purposes of the present application, such multi-particle media are
regarded as sub-species of dual particle media.
[0013] Many of the aforementioned patents and applications
recognize that the walls surrounding the discrete microcapsules in
an encapsulated electrophoretic medium could be replaced by a
continuous phase, thus producing a so-called polymer-dispersed
electrophoretic display, in which the electrophoretic medium
comprises a plurality of discrete droplets of an electrophoretic
fluid and a continuous phase of a polymeric material, and that the
discrete droplets of electrophoretic fluid within such a
polymer-dispersed electrophoretic display may be regarded as
capsules or microcapsules even though no discrete capsule membrane
is associated with each individual droplet; see for example, the
aforementioned 2002/0131147. Accordingly, for purposes of the
present application, such polymer-dispersed electrophoretic media
are regarded as sub-species of encapsulated electrophoretic
media.
[0014] Although electrophoretic media are often opaque (since, for
example, in many electrophoretic media, the particles substantially
block transmission of visible light through the display) and
operate in a reflective mode, many electrophoretic displays can be
made to operate in a so-called "shutter mode" in which one display
state is substantially opaque and one is light-transmissive. See,
for example, the aforementioned U.S. Pat. Nos. 6,130,774 and
6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823;
6,225,971; and 6,184,856. Dielectrophoretic displays, which are
similar to electrophoretic displays but rely upon variations in
electric field strength, can operate in a similar mode; see U.S.
Pat. No. 4,418,346. Other types of electro-optic displays may also
be capable of operating in shutter mode.
[0015] An encapsulated or microcell electrophoretic display
typically does not suffer from the clustering and settling failure
mode of traditional electrophoretic devices and provides further
advantages, such as the ability to print or coat the display on a
wide variety of flexible and rigid substrates. (Use of the word
"printing" is intended to include all forms of printing and
coating, including, but without limitation: pre-metered coatings
such as patch die coating, slot or extrusion coating, slide or
cascade coating, curtain coating; roll coating such as knife over
roll coating, forward and reverse roll coating; gravure coating;
dip coating; spray coating; meniscus coating; spin coating; brush
coating; air knife coating; silk screen printing processes;
electrostatic printing processes; thermal printing processes; ink
jet printing processes; electrophoretic deposition; and other
similar techniques.) Thus, the resulting display can be flexible.
Further, because the display medium can be printed (using a variety
of methods), the display itself can be made inexpensively.
[0016] The preferred process for preparing electrophoretic capsules
described in the aforementioned E Ink and MIT patents and
applications uses a gelatin/acacia coacervate as the encapsulation
material, and the process for forming such gelatin/acacia capsules
may be summarized as follows; see, for example, the aforementioned
2002/0180687, Paragraphs [0069] to [0074]. An internal phase is
prepared containing one or more types of electrophoretic particles
in a suspending fluid; typically, the internal phase comprises
titania and carbon black particles in an uncolored hydrocarbon
suspending fluid. The internal phase is thoroughly stirred to
ensure that it is homogeneous. Gelatin is dissolved in deionized
water at a temperature of 40.degree. C., and vigorously stirred.
The internal phase, heated to the same temperature, is added
dropwise to the stirred gelatin solution through a tube the outlet
of which is below the surface of the stirred solution. The
resultant mixture is held at 40.degree. C. with continued vigorous
stirring to produce a dispersion of droplets of the internal phase
in a continuous gelatin-containing aqueous phase.
[0017] A solution of acacia in water at 40.degree. C. is then added
to the mixture, and the pH of the mixture is lowered to
approximately 4.9 to cause formation of the gelatin/acacia
coacervate, thereby forming capsules. The temperature of the
resultant mixture is then lowered to 10.degree. C. and an aqueous
solution of glutaraldehyde (an agent for cross-linking the capsule
walls) is added. The resultant mixture is then warmed to 25.degree.
C. and stirred vigorously for a further 12 hours.
[0018] The capsules produced are separated from the liquid and
washed by redispersion in water. The capsules are then separated by
size by sieving or otherwise. For reasons explained in several of
the aforementioned E Ink and MIT patents, it is desirable that an
encapsulated electrophoretic medium comprises a single,
substantially close-packed layer of capsules. Also, when such an
electrophoretic medium is produced by coating capsules on to a
substrate, it is desirable that the exposed surface of the capsule
layer be reasonably flat, since otherwise difficulties may be
encountered in laminating the capsule layer to other layers in the
final display. Production of such a substantially close-packed
layer with a reasonably flat exposed surface is best achieved by
coating capsules which are of substantially the same size.
Typically, the desired range of capsule diameters will be 30-50
.mu.m, with an average of 40 .mu.m.
[0019] Unfortunately, it has been found that the prior art
encapsulation process described above does not produce a high yield
of capsules with the desired range of diameters. Typically, the
yield of the desired capsules (measured as volume percent of the
total capsules produced) ranges from 12 to 34 percent, with an
average of about 27 percent. Although the average capsule diameter
can be varied over a considerable range by controlling parameters
such as gelatin concentration, stirring rate, etc., it has not
proved possible to increase yields above this range, the major
problem being the high proportion of "fines" or capsules having
diameters below the acceptable range; for example, as shown in the
Example below, in a typical optimized prior art process designed to
produce 30-50 .mu.m capsules, the volume percent capsules actually
peaks at about 10-12 .mu.m. Obviously such low yields of useful
capsules are a major problem, since they greatly increase the cost
of the electrophoretic medium and generate substantial costs for
disposing of the unacceptable capsules. The prior art process is
also not easily scalable, requiring re-optimization each time the
size and/or shape of the reactor is changed.
[0020] The present invention provides two different approaches to
overcoming the aforementioned disadvantages of the prior art
encapsulation process, and thus substantially increasing the yield
of useful capsules. The first approach involves a relatively
straightforward modification of the prior art process in which the
coacervate is first formed and thereafter the material to be
encapsulated is emulsified in this coacervate phase. The second
approach represents a more fundamental change in the process using
a limited coalescence ("LC") process.
[0021] Techniques for capsule formation other than stirring of one
phase into another are known. One group of such techniques, which
have been developed primarily for the preparation of droplets of
monomer for use in suspension polymerization processes, are limited
coalescence processes; in such processes, the increased particle
size uniformity and suspension volume fraction provided by limited
coalescence processes allow more efficient use of equipment. There
is an extensive patent literature regarding such processes; see,
for example, U.S. Pat. Nos. 4,965,131 and 2,932,629.
[0022] Limited coalescence processes involves the use of so-called
Pickering emulsions for the stabilization of droplets at the
critical stage of the process. Pickering emulsions are emulsions
(either oil-in-water or water-in-oil) stabilized against
coalescence by surface-active particulate materials (also referred
to as "particulate colloids" or "PC's"), rather than, as in the
case of most conventional emulsions, small surfactant molecules or
surface-active soluble polymers. Typical surface-active particulate
materials include clay particles, colloidal silica dispersions, and
lightly crosslinked latex particles, though many other materials
have also been used. Since most of these materials are not
intrinsically surface active (that is, they will not partition
specifically to an oil-water interface), a second component, called
a promoter, is required to achieve stable Pickering emulsions.
Promoters are typically themselves somewhat surface active;
examples include amphiphilic species of many types, including
typical surfactants (soaps, alkyl sulfonates, alkyl ammonium salts,
and the like), as well as soluble polymers (gelatin,
poly(vinylpyrrolidone), poly(ethylene oxide), poly(vinyl alcohol),
etc.) and oligomers (e.g., the condensation product of
alkylaminoethanol and adipic acid and similar species as described
in the aforementioned patents). The promoter is believed to
function by adsorbing on the surface-active particulate material,
thus rendering it surface active at the oil-water interface. The
amount of promoter is therefore very important, since good
promoters are generally materials that will destabilize the PC. If
too little promoter is present, the surface-active particulate
material will not adsorb at the oil-water interface, and the oil
droplets will not acquire stability; however, if too much promoter
is present, the surface-active particulate material will either
coagulate and separate from the suspension, or partition into the
oil phase. A large excess of promoter is, in many cases, capable of
stabilizing a conventional emulsion at the expense of a Pickering
emulsion. In each of these cases, the result is failure of the
limited coalescence process, which requires the formation of a
stable Pickering emulsion.
[0023] In a typical limited coalescence process, an oil phase is
suspended in an aqueous suspension comprising water, surface-active
particulate material, and promoter. The resulting crude emulsion is
then homogenized under conditions expected to yield a very small
particle-sized oil-in-water emulsion. This emulsion is unstable,
because the small size of the particles makes the oil-water
interfacial area very much larger than that which can be covered by
the limited amount of surface-active particulate material present.
Coalescence therefore occurs, with a concomitant reduction in
interfacial area. As this area decreases, the fraction of the
interfacial area covered by adsorbed particulate material
increases, and as this fraction approaches 1 (complete coverage)
coalescence stops (hence the term "limited coalescence" for the
process as a whole). The process has been studied theoretically and
mechanistically (see, for example, Whitesides and Ross, J.
Interface Colloid Sci. 196, 48-59(1995)). Monte Carlo simulations
have shown that, with reasonable assumptions concerning the
probability that collision between droplets will result in
coalescence (in particular that the probability of coalescence is a
decreasing function of the fractional coverage of the interface by
the particulate material), the final droplet particle size
distribution will be quite narrow. It is found that the volume of
the largest droplets will be approximately twice the volume of the
smallest, so that the particle diameters vary by only about the
cube root of 2 (approximately 1.26). The best limited coalescence
processes match this expectation well.
[0024] As already mentioned, limited coalescence processes have
primarily been developed for the preparation of droplets of monomer
for use in suspension polymerization processes, and there are a
number of problems with applying such processes in other contexts.
Firstly, a particulate colloidal stabilizer must be found that is
compatible with the particular system in which it is to be used.
Secondly, the oil-in-water emulsion formed during homogenization
must be unstable with respect to coalescence, and this requires
that any surfactants already present in the system must be rendered
ineffective.
[0025] It has now been found that these problems can be overcome
and limited coalescence processes used successfully for the
production of capsules for use in electrophoretic displays with
substantially improved yields as compared with the prior art
capsule production processes described above.
SUMMARY OF INVENTION
[0026] Accordingly, in one aspect this invention provides a process
for encapsulating a water-immiscible phase in a protein coacervate,
the process comprising:
[0027] forming a coacervate of the protein and a coacervating agent
in an aqueous medium; and
[0028] emulsifying the water-immiscible phase in the aqueous medium
comprising the coacervate under conditions effective to cause
deposition of the coacervate around the water-immiscible phase,
thereby forming capsules of the water-immiscible phase surrounded
by capsule walls of the coacervate.
[0029] This process may hereinafter be called the "preformed
coacervate" process of the present invention. In this process, the
protein may be gelatin and the coacervating agent may be acacia.
The water-immiscible phase may comprise an aliphatic
hydrocarbon.
[0030] As already indicated, the preformed coacervate process of
the invention is especially, though not exclusively, intended for
use in producing capsules for electrophoretic media. In this
application, the water-immiscible phase used in the process
normally comprises the complete internal phase (electrophoretic
particles and suspending fluid, plus any additives desired in or on
either the fluid or the particles) of the electrophoretic medium.
Such an internal phase may be of any of the types known to be
useful in electrophoretic media. Thus, in the preformed coacervate
process of the invention the hydrocarbon may have solid particles
suspended therein, and these solid particles may be electrically
charged. In a preferred form of the process, intended for the
production of an opposite charge dual particle electrophoretic
medium as described above, the water-immiscible phase comprises two
different types of solid particles bearing charges of opposite
polarity.
[0031] For reasons explained in detail below, it is desirable that
at least part of the emulsification/capsule formation step take
place at a temperature close to the gelation temperature of the
aqueous medium; thus, typically at least part of the
emulsification/capsule formation step should take place below about
35.degree. C.
[0032] Once the capsules have been formed in the preformed
coacervate process have been formed, they may be treated with a
cross-linking agent effective to cause cross-linking of the
protein. This cross-linking agent may be an aldehydes, for example
glutaraldehyde. Also, as described in several of the aforementioned
E Ink and MIT patents and applications, the capsules formed may
thereafter be mixed with a polymeric binder and the capsules/binder
mixture coated on to a substrate and dried to form a coherent layer
of capsules on the substrate.
[0033] In another aspect, this invention provides a process for
encapsulating a water-immiscible phase in a protein coacervate, the
process comprising:
[0034] forming an aqueous phase comprising a colloidal alumina
suspension and a promoter;
[0035] emulsifying the water-immiscible phase in the aqueous phase
under conditions effective to cause the formation of an unstable
emulsion comprising small droplets of the water-immiscible phase in
the aqueous phase; and
[0036] admixing the emulsion with the protein and a coacervating
agent under conditions permitting coalescence of the emulsion and
the formation of capsules of the aqueous phase surrounded by
capsule walls of the coacervate.
[0037] This process may hereinafter be called the "limited
coalescence" or "LC" process of the present invention. In this
process, the alumina may comprise from about 0.1 to about 1.0
percent by weight of the aqueous phase. The promoter may comprise a
polyacid, for example a copolymer of a carboxylic acid and an
olefin. The protein may comprise gelatin, and the coacervating
agent may comprise an anionic polymer. Such an anionic polymer may
comprise a polyanionic polymer having a vinyl main chain and a
plurality of anionic groups bonded to the main chain; thus, for
example, the polyanionic polymer may comprise any one or more of
poly(acrylic acid); poly(methacrylic acid); copolymers of
poly(acrylic acid) and/or poly(methacrylic acid) with esters of the
same acids; styrene sulfonate copolymers with styrene; methyl vinyl
ether or vinyl acetate copolymers with (meth)acrylic acid;
copolymers of alkyl-substituted olefins, methyl vinyl ether and
vinyl carboxylate with maleic acid, maleic esters, and maleic half
ester, half acids.
[0038] As already indicated, the LC process of the invention is
especially, though not exclusively, intended for use in producing
capsules for electrophoretic media. In this application, the
water-immiscible phase used in the process normally comprises the
complete internal phase (electrophoretic particles and suspending
fluid, plus any additives desired in or on either the fluid or the
particles) of the electrophoretic medium. Such an internal phase
may be of any of the types known to be useful in electrophoretic
media. Thus, in the LC process of the invention the
water-immiscible phase may comprise a hydrocarbon, which may have
solid particles suspended therein, and these solid particles may be
electrically charged. In a preferred form of the process, intended
for the production of an opposite charge dual particle
electrophoretic medium as described above, the water-immiscible
phase comprises two different types of solid particles bearing
charges of opposite polarity. For reasons explained below, the
hydrocarbon may also have dispersed therein a di-block copolymer or
an aromatic-substituted alkene and an alkene.
[0039] As in the preformed coacervate process of the invention, in
the LC process the capsules formed may thereafter be treated with a
cross-linking agent effective to cause cross-linking of the
protein. This cross-linking agent maybe an aldehydes, for example
glutaraldehyde. The capsules formed may thereafter be mixed with a
polymeric binder and the capsules/binder mixture coated on to a
substrate and dried to form a coherent layer of capsules on the
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 of the accompanying drawings is a graph showing the
size distributions of capsules obtained from three preformed
coacervate processes of the present invention and a control process
in the experiments described in Example 1 below.
[0041] FIG. 2A is a photomicrograph of capsules obtained from one
of the preformed coacervate processes described in Example 1
below.
[0042] FIG. 2B is a photomicrograph, similar to that of FIG. 2A, of
capsules obtained from the control process described in Example 1
below.
[0043] FIG. 3A is a graph showing the number-weighted size
distribution of capsules obtained from an LC process of the present
invention and a control process in the experiments described in
Example 2 below.
[0044] FIG. 3B is a graph similar to that of FIG. 3A but showing
the volume-weighted size distribution of capsules obtained from the
experiments described in Example 2 below.
[0045] FIG. 4 is a graph showing the variation of capsule size
distribution against colloidal alumina content of the aqueous in
the experiments described in Example 2 below.
DETAILED DESCRIPTION OF THE INVENTION
[0046] As already mentioned, the present invention provides two
different processes intended to overcome the disadvantages of prior
art encapsulation processes described above, namely the preformed
coacervate and limited coalescence processes of the present
invention. Accordingly, after an introduction describing the common
features of the two processes, the details of each process will be
discussed separately.
[0047] As will readily be apparent to those skilled in
encapsulation technology, the present invention relates only to the
encapsulation step of what is normally a multi-step process for the
production of the final encapsulated material. Thus, except for
those aspects of material selection and processing specific to the
encapsulation processes of the invention, as discussed below, the
material, processes and techniques used in association with the
present processes may be any of those known in the art. For
example, as already indicated, the processes of the present
invention are especially but not exclusively intended for use in
the production of encapsulated electrophoretic media, and when the
present processes are being used for this purpose, the
electrophoretic particles, the suspending fluid and any additives
(for example, charge control agents)) included in the internal
(water-immiscible phase) may be any of those described in the
aforementioned E Ink and MIT patents and applications, provided of
course that the materials chosen are compatible with the specific
materials used in the encapsulation process. Similarly, the
"post-processing" steps required to convert the capsules formed by
the present processes to a finished electrophoretic medium may be
any of those described in the aforementioned E Ink and MIT patents
and applications, since the physical properties of capsules
produced by the present processes are generally similar to those
produced by the prior art processes.
[0048] The present processes are, not, however, confined to the
production of capsules for use in electrophoretic media, but may be
used for the encapsulation of other water-immiscible phases. For
example, the present processes may be useful for the encapsulation
of materials to be used in carbonless copying systems or
pharmaceuticals dissolved in organic solvents.
[0049] Preformed Coacervate Process
[0050] As already mentioned, the preformed coacervate process of
the present invention is a process for encapsulating a
water-immiscible phase in a protein coacervate. This process
comprises forming a coacervate of the protein and a coacervating
agent in an aqueous medium; and emulsifying the water-immiscible
phase in the aqueous medium comprising the coacervate under
conditions effective to cause deposition of the coacervate around
the water-immiscible phase, thereby forming capsules of the
water-immiscible phase surrounded by capsule walls of the
coacervate. Thus, in this process, a protein coacervate phase is
first formed, and thereafter a hydrocarbon internal phase (or other
water-immiscible phase which it is desired to encapsulate) is
emulsified in this coacervate phase to form capsules. It has been
found that emulsifying the internal phase in the coacervate phase,
rather than emulsifying the internal phase in an aqueous phase
containing protein but no coacervating agent and then adding the
coacervating agent to form the coacervate, substantially reduces
the proportion of fines in the final capsules, and produces a
sharper peak in the volume percent capsules against capsule
diameter curve. The combination of these two effects substantially
increases the yield of useful capsules; as shown in the Example
below, yields of capsules in the desired 30-50 .mu.m range
exceeding 50 percent have been achieved.
[0051] The reduction in the proportion of fines achieved by the
preformed coacervation process is surprising, since it is directly
contrary to what would be expected from commonly accepted fluid
mechanics theory. The preformed coacervate phase used in this
process has of course a substantially higher viscosity than the
protein-only aqueous phase used in the prior art processes
discussed above, and fluid mechanics theory predicts that this
increased viscosity will result in a more efficient transfer of
shear from the impeller used for the vigorous stirring of the
reaction mixture, thus resulting in an increased proportion of
fines at the same stirring rate, or production of the same sized
capsules at a lower stirring rate. However, empirically it has been
found that the exact opposite occurs; a substantially increased
stirring rate (about 30-35 percent greater) is needed in the
present process to produce the same mean capsule size, and
substantially fewer fines are produced.
[0052] It has also been found that, to secure capsules with optimum
mechanical properties, it is important to control the temperature
at which the preformed coacervate process is conducted. More
specifically, it has been found desirable to ensure that at least
the last part of the encapsulation process, prior to the normal
cross-linking of the capsules, is conducted at a temperature close
to but above the gelation temperature of the aqueous medium. It has
been found that use of such a temperature close to the gelation
temperature increases the wall thickness and the mechanical
robustness of the final capsules. In practice, when a gelatin
coacervate is being produced, the gelation temperature is typically
close to 30.degree. C., so at least part of the encapsulation
process should be conducted at a temperature below about 35.degree.
C.
[0053] At least when a gelatin/acacia coacervate is being used, it
has been found desirable to maintain the proportions of gelatin and
acacia in the preformed coacervate solution somewhat higher than in
the prior art processes discussed above. The preferred process
described in several of the aforementioned E Ink and MIT patents
and applications begins with a gelatin concentration of about 2.4
percent by weight; in contrast, it has been found that typically
the optimum concentration of gelatin and acacia in a preformed
coacervate solution is about 3.2 percent by weight of each
component. After the emulsification of the water-immiscible phase
in the preformed coacervate has been completed, it has been found
desirable to add additional water to reduce the gelatin and acacia
concentrations to about 2.4 percent by weight in the later stages
of the process.
[0054] The following Example is now given, though by way of
illustration only, to show the improved capsule yields which can be
achieved by the preformed coacervate process of the present
invention.
EXAMPLE 1
[0055] An internal phase was prepared substantially as described in
the aforementioned 2002/0180687, this internal phase comprising
polymer-coated titania and polymer-coated carbon black in a
hydrocarbon solvent. This internal phase was then encapsulated by
both a prior art ("PA") process and a preformed coacervate process
("PC") of the present invention. The material amounts given for the
PC process are for a scaled down volume of 75%, based on water and
oil amounts, of the original PA process. The material and amounts
used for the two encapsulations were are shown in Table 1
below:
1 TABLE 1 Material PA PC Deionized water 32776 18437 Gelatin 667
300 Makeup water 0 6145 Acacia 667 300 Internal Phase 10700 8025
Glutaraldehyde 167 87.7
[0056] The encapsulations were performed in a 50 L reactor provided
with a propeller type stirrer. The prior art process was carried
out substantially as described in the aforementioned 2002/0180687.
The gelatin was added to the deionized water in the reactor and the
mixture stirred at a slow speed and heated until the gelatin had
dissolved and the solution had reached 40.degree. C. Separately,
the internal phase was heated to 40.degree. C., and introduced
slowly below the surface of the gelatin solution, which was stirred
continuously. After the addition of the internal phase was
complete, the rate of stirring was increased and this stirring
continued for 60 minutes at 40.degree. C. in order to emulsify the
internal phase into droplets of a size appropriate for the
formation of capsules.
[0057] Separately, the acacia was dissolved in water and heated to
40.degree. C. The resultant acacia solution was then added rapidly
to the reactor, care being taken to avoid foaming. The pH of the
resultant mixture was lowered to 4.94-4.96 using 10 percent aqueous
acetic acid and the rapid stirring continued for a further 40
minutes at the same temperature. The temperature of the reaction
mixture was then lowered to 10.degree. C. over a period of several
hours, with continued rapid stirring, and the glutaraldehyde was
added. After this addition, the reaction mixture was gradually
warmed to 25.degree. C. and stirred vigorously for a further 12
hours. The liquid phase was then removed.
[0058] The preformed coacervation process was carried out as
follows. The 18437 g of deionized water was placed in the reactor,
and the gelatin dissolved therein under gentle stirring, then the
resultant solution was heated to 40.degree. C. The rate of stirring
was then increased, and the acacia was added in powder form
directly to the reactor. After the acacia had dissolved, the
mixture was stirred at the same rate for 60 minutes at 40.degree.
C. and then its pH was measured; this pH at this point in the
process may range from 5.05 to 5.50, depending on the quality of
the gelatin used. The internal phase, previously held at room
temperature, was added below the surface of the gelatin/acacia
solution over a period of about 15 minutes.
[0059] After addition of the internal phase had been completed, the
resultant mixture was stirred vigorously for 2.5 hours to form the
desired droplet size; the droplet size was checked, and if the
average size was found to be too large, the stirring rate was
increased. After the 2.5 hours of stirring, the makeup water,
previously heated to 40.degree. C., was added, and the pH of the
mixture measured; again, this pH may range from 5.05 to 5.50
depending on the quality of the gelatin used. The pH of the
reaction mixture was then adjusted to approximately 4.90 before
entering the cooling step of the process. The rate of stirring was
decreased slightly and, after 5 minutes, the reactor was gradually
cooled from 40.degree. C. to 10.degree. C. over a period of 5
hours; when the reactor temperature had reached 30.degree. C.
(close to the gelatin temperature of gelatin), the stirring was
interrupted for 30 minutes (to allow the capsule walls to thicken),
then resumed at the same rate as before. When the temperature of
the mixture reached 10.degree. C., the glutaraldehyde was added,
and after 5 minutes further stirring, the reactor was heated to
25.degree. C. and stirred at the same rate at this temperature for
8-12 hours. Finally, the capsules were separated from the liquid
phase.
[0060] The results of these experiments are shown in FIG. 1 of the
accompanying drawings. From FIG. 1, it will be seen that the PA
(control) experiment (Curve A) showed a very broad distribution of
capsule sizes, peaking at about 10-12 .mu.m; integration of the
curve for the control experiments shows that the yield of capsules
in the desired 30-50 .mu.m range was only 23 percent. In contrast,
all three preformed coacervate processes of the present invention
showed a narrower distribution of capsule sizes, and with improved
yields, the yields of capsules in the desired 30-50 .mu.m range
being respectively 27, 36 and 57 percent for the PC processes
having impeller speeds of 550, 650 and 725 rpm (Curves B, C and D
respectively). As expected, the three PC processes showed reduced
capsule diameter with increasing impeller speed, with the 725 rpm
process (Curve D) producing a peak at about 40 .mu.m capsule
diameter.
[0061] The improvement in capsule uniformity produced by the
preformed coacervate process is further illustrated in FIGS. 2A and
2B, which are photomicrographs of capsules produced by,
respectively, the 725 rpm process of the present invention and the
prior art process. The reduced level of fines produced by the
present process is readily apparent by comparing these
photomicrographs.
[0062] From the foregoing, it will be seen that the preformed
coacervate process of the present invention provides a substantial
improvement in the yield of useful capsules, and thus a substantial
reduction in the cost of a given amount of electrophoretic medium
produced from such capsules. The present invention also
substantially reduces waste disposal costs associated with the
disposal of unusable capsules. These advantages can be achieved
without the need for new production apparatus, and thus without
capital costs. Furthermore, the preformed coacervate process does
not require any changes in the composition of either the internal
phase or the components used to form the capsule walls, since the
present invention requires only a change in the order in which the
various materials used to form the capsules are added, and changes
in certain process parameters, such as stirring rate, cooling
cycle, and mixing near the gelation temperature.
[0063] Limited Coalescence Process
[0064] As already mentioned, the LC process of the present
invention process is a process for encapsulating a water-immiscible
phase in a protein coacervate. This LC process comprises forming an
aqueous phase comprising a colloidal alumina suspension and a
promoter; emulsifying the water-immiscible phase in the aqueous
phase under conditions effective to cause the formation of an
unstable emulsion comprising small droplets of the water-immiscible
phase in the aqueous phase; and admixing the emulsion with the
protein and a coacervating agent under conditions permitting
coalescence of the emulsion and the formation of capsules of the
aqueous phase surrounded by capsule walls of the coacervate.
[0065] As already indicated, the major problems in applying limited
coalescence processes outside the context in which they were
developed are selecting a compatible particulate colloidal
stabilizer, and ensuring that any surfactants present not render
the fine oil-in-water emulsion initially formed stable against
coalescence. Producing capsules for use in electrophoretic displays
present particular challenges with respect to both these problems.
In practice, the internal phases (i.e., the electrophoretic
dispersions, that it to say the suspensions of one or more types of
electrophoretic particles in a suspending fluid) used in such
capsules require the presence of charging agents to develop and/or
maintain the electrical charges on the electrophoretic particles,
and these charging agents are typically surface active agents that
raise difficulties with respect to both the aforementioned
problems. For example, several of the aforementioned E Ink and MIT
patents and applications describe internal phases containing as a
charging agent Solsperse 17K, a commercial cationic surface active
oligomeric material. Because this material is cationic, if one
attempts to use an anionic particulate colloidal stabilizer in
conjunction therewith, the Solsperse 17K will interact strongly
with the anionic material in the aqueous phase or at the oil/water
interface, thus rendering the anionic material hydrophobic. The
stabilizer will then partition into the internal phase and become
ineffective in stabilizing the Pickering emulsion. Hence, a
cationic rather than anionic stabilizer must be used with an
internal phase containing a cationic stabilizer such as Solsperse
17K.
[0066] Also, like many other charging agents used in
electrophoretic internal phases, Solsperse 17K is a weakly surface
active material in its own right, and will stabilize the formation
of an oil-in-water emulsion of the internal phase. The resulting
emulsion is a charge-stabilized one, and can be destabilized by
addition of a neutral salt, for example potassium nitrate, to the
aqueous phase. However, the use of such salts in the aqueous phase
interferes with gelatin/acacia coacervation, so that either the
aqueous phase must be deionized prior to coacervation or an
alternate coacervation process is required.
[0067] Preferred forms of the LC process of the present invention
provide solutions to both these problems. It has been found that
colloidal alumina suspensions are cationic colloidal particulate
materials compatible with electrophoretic internal phases and other
water-immiscible phases. Such colloidal alumina suspensions are
readily available commercially, being manufactured for use in
silicon wafer polishing, and can be used without modification in at
least some LC processes of the present invention. Examples of such
colloidal alumina suspensions which have been found useful in the
present LC process are Ultra-sol 201A/60, Ultra-sol 201A/140, and
Ultra-sol 201A/280, all available from Eminess Technologies, Inc.,
Monroe, N.C.
[0068] The problems associated with salt addition can be avoided by
using as the coacervating agent an anionic polymer, desirably one
having a higher charge density and greater degree of hydrophobicity
than acacia. The use of such anionic polymers as coacervating
agents is discussed in detail in the aforementioned 2004/0012839.
Typical examples of such anionic polymers are poly(acrylic acid)
and poly(olefin-co-maleic acid), where the olefin component is
ethylene, isobutylene, vinyl ether, octene, or a mixed primary
olefin. Several of these materials are commercially available from
a variety of sources.
[0069] There is one further problem which may be encountered in
certain LC processes of the invention used for production of
electrophoretic media (and possibly other materials), namely
degradation of polymer present in the suspending fluid. The
aforementioned 2002/0180687 describes how the addition of
polyisobutylene and certain other polymers to the suspending fluid
of an electrophoretic medium can greatly increase the bistability
of the medium without excessively decreasing the switching speed
thereof. The vigorous homogenization conditions required in a
limited coalescence process to form the initial small particle
oil-in-water emulsion are sufficient to degrade polyisobutylene and
other high molecular weight polymers in the internal phase to such
an extent that the polymer is no longer effective at providing
bistability in the resultant electrophoretic medium. If high
bistability is necessary, such bistability may be achieved either
in the manner described in copending application Ser. No.
10/711,829, filed Oct. 7, 2004 (see also the corresponding
International Application PCT/US2004/033188); this application
describes manipulation of a polymeric shell around the
electrophoretic particles to increase bistability) or an image
stabilizing additive that is not degraded under shear may be used.
Examples of such non-shear-degraded additives include aggregating
di-block copolymers made from styrene and poly(ethylene-propylene),
such as Kraton (Registered Trade Mark) G1701, G1702 or G1730, all
available from Kraton Polymers, Inc, Belpre, Ohio).
[0070] The following Example is now given, though by way of
illustration only, to show the improved capsule yields which can be
achieved by the limited coalescence process of the present
invention.
EXAMPLE 2
[0071] Two identical internal phases were prepared each comprising
42.5 g of a 60 weight percent suspension of a polymer-coated copper
chromite pigment in Isopar E (a hydrocarbon solvent available from
Exxon Corporation, Houston Tx.; ISOPAR is a Registered Trade Mark),
85 g of a 60 weight percent suspension of a polymer-coated titania
pigment in Isopar E, 10.71 g of a solution of Solsperse 17K in
Isopar G (a hydrocarbon solvent from the same manufacturer as
Isopar E), 0.77 g of Span 80 (a surfactant available from Aldrich
Chemical Corporation), and 31.03 g of additional Isopar E.
[0072] One of internal phases was used in a control experiment
using a prior art encapsulation process substantially as described
in the aforementioned 2002/0180687. The emulsification step was
carried out by addition of the 140 mL internal phase to a solution
of 10 g of gelatin in 240 g water at 42.5.degree. C., and agitating
the resulting mixture with a 2 inch (51 mm) prop impeller at 650
rpm for one hour. By this time, the particle size distribution of
the emulsion was essentially fixed, as indicated by analysis using
a Coulter Counter. Water (153 mL) was then added to the emulsion,
followed by a solution of 10 g of acacia in 98 mL of water. At this
point, particle size analysis using the Coulter Counter showed the
distribution indicated by the curves denoted "Control" in FIGS. 3A
and 3B of the accompanying drawings. After cooling the resultant
mixture to 10.degree. C., the gelatin/acacia coacervate was
crosslinked by addition of about 2 g of a 50 percent solution of
glutaraldehyde in water. The capsules were isolated by
sedimentation and sieving, using 25 and 38 .mu.m sieves. Capsules
remaining on the 25 .mu.m sieve were retained.
[0073] The other internal phase was encapsulated by a limited
coalescence process of the present invention, as follows. Ultra-sol
201A140 (3.252 g, 19.8 percent solids) was weighed into a 1 L
screw-cap jar, and diluted with 300 g of water. The internal phase
(70 g) was added, and the mixture shaken by hand to produce a
suspension. A promoter (0.644 g of a 1 percent solution of
poly(maleic acid-alt-olefin) (molecular weight approximately 12000,
from Aldrich Chemical Corporation, Milwaukee, Wis.) in water) was
weighed into a small glass vial, and transferred to the suspension
with the aid of a few milliliters of water, and the suspension was
shaken again. Finally, 50 mL of 0.1 N potassium nitrate solution in
water was added. After shaking one more time, the suspension was
passed once through a Microfluidizer (Registered Trade Mark)
homogenizer (from Microfluidics, Inc.). The homogenized material
was poured into a solution of 4 g of gelatin in 140 g of water held
at 40.degree. C., and the mixture stirred with an impeller at about
500-600 rpm. To the resultant suspension was added 5 g of a
solution of 5% poly(vinyl ether-alt-maleic acid) (prepared by
hydrolysis of poly(vinyl ether-alt-maleic anhydride), molecular
weight 41000, from Polysciences, Inc., Warren, Pa.). The pH of the
stirred suspension was adjusted to 6.1 by addition of a small
amount of 2 N sodium hydroxide, and the suspension was then cooled
to 10.degree. C. At this point, particle size analysis using the
Coulter Counter showed the distribution indicated by the curves
marked "Invention" in FIGS. 3A and 3B. A solution of glutaraldehyde
(approximately 2 g of a 50 percent solution in water) was then
added. After cross linking, the capsules were isolated by sieving
as described above.
[0074] From FIGS. 3A and 3B, it will be seen that the LC process of
the present invention gave a much tighter particle size
distribution than that of the control, with a better yield of
particles in the desired size range (25 to 45 .mu.m in this
instance). In addition, the process of the present invention
resulted in a much smaller population of fine particles as shown by
the quantitative data in Table 2 below:
2 TABLE 2 Number Number Volume Number percent percent percent
percent capsules 20-45 capsules less capsules 20-45 capsules less
.mu.m than 20 .mu.m .mu.m than 20 .mu.m Control 35 45 53 7
Invention 78 11 64 0.8
[0075] From this Table, it will be seen that the yield of the
desired 20-45 .mu.m capsules produced by the LC process of the
present invention was at least 20 percent greater than that of the
prior art process, and that the invention reduced the number of
fine capsules having diameters less than 20 .mu.m by a factor of at
least 4.
[0076] Further experiments were conducted to demonstrate the
scalability of the LC process of the present invention. The LC
process described above was repeated, except that the ratio of the
alumina stabilizer to the internal phase was varied. The results
are shown in FIG. 4 of the accompanying drawings, from which it
will be seen that the modal capsule size produced by the process
was a simple function of the ratio of stabilizer to internal phase,
as theory predicts for an LC process.
[0077] It was also found experimentally that the LC process of the
present invention was insensitive to wide variations in the
homogenization method used, as long as the method was sufficient to
reduce the droplets of internal phase to a substantially smaller
(about ten times smaller) size than that of the capsules eventually
desired. In contrast, the development of a desirable particle size
distribution by conventional emulsification processes is highly
dependent on scale and reactor geometry, and the process thus
requires substantial development work for optimization during
scale-up.
[0078] From the foregoing, it will be seen that the LC process of
the present invention provides a substantial improvement in the
yield of useful capsules, and a more favorable particle size
distribution and should be able to improve process cycle time;
thus, the LC process achieves a substantial reduction in the cost
of a given amount of electrophoretic medium produced from such
capsules. The LC process of the present invention also
substantially reduces waste disposal costs associated with the
disposal of unusable capsules. These advantages can be achieved
without incurring large capital costs. Furthermore, the LC process
does not require any changes in the composition of the internal
phase. In addition, the LC process of the present invention is more
readily scalable than are prior art processes.
[0079] Numerous changes and modifications can be made in the
preferred embodiments of the present invention already described
without departing from the scope of the invention. Accordingly, the
whole of the foregoing description is to be construed in an
illustrative and not in a limitative sense.
* * * * *