Manufacture of Microparticles for use in
Electrophoretic Displays
by
Jonathan D. Albert
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Bachelor of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 1997
Massachusetts Institute of Technology 1997. All rights reserved.
Signature redacted
Author...........
Department of Mechanical Engineering
May 9, 1997
Signature redacted
C ertified by . .
..........................
Joseph Jacobson
Assistant Professor
Thesis Supervisor
Signature redacted
Accepted by......................
Professq/Peter Griffith
Chairman of the Undergraduate Thesis Committee
ARCHIVES
JUN 2 7 1997
Manufacture of Microparticles for use in Electrophoretic
Displays
by
Jonathan D. Albert
Submitted to the Department of Mechanical Engineering
on May 9, 1997, in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Mechanical Engineering
Abstract
Electrophoretic image displays (EPIDs) are a class of reflective display that was developed in the late 1970's. Recent work at the MIT Media Lab has revived interest in
this type of display. This thesis pertains to the production of the particles that give
the displays their color contrasting ability. These particles must be uniform in size,
composition, and color and they must have good electrophoretic mobilities. Various
production schemes were tried including spinning disk atomization, nozzle atomization, and ball milling. A fourth technique, jet milling was examined for suitability to
the task.
Thesis Supervisor: Joseph Jacobson
Title: Assistant Professor
2
Acknowledgments
Thanks to:
My advisor Joseph Jacobson for inspiring me to do work in this area, Barrett
Comisky for giving me a reason to do the work, and the rest of Team Micromedia for
general support.
My family.
Special thanks to my grandparents Abraham and Betty Eisen for working hard
all their lives so that I could come to MIT.
3
1
Contents
1
2
1.1
Background on Electrophoretic Displays
. . . . . . . . . . . . . . . .
8
1.2
Prior Art in Particle Fabrication . . . . . . . . . . . . . . . . . . . . .
10
1.3
Chemical processes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
1.4
Microencapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
1.5
Electrophoretic Suspension Fluid
. . . . . . . . . . . . . . . . . . . .
12
13
Particle Composition
2.1
3
8
Introduction
. . . . . . . . . . . . . . . . . . . . . . . . . .
13
. . . . . . . . . . . . . . . . . . . . . . . .
13
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
Polymer Requirements
2.1.1
Dielectric constant
2.1.2
Density
2.1.3
Charge controlling ability
. . . . . . . . . . . . . . . . . . . .
14
2.1.4
Pigment dispersion . . . . . . . . . . . . . . . . . . . . . . . .
14
2.1.5
Friability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
2.1.6
Hydrophobicity . . . . . . . . . . . . . . . . . . . . . . . . . .
16
2.2
Example polymers
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
2.3
Pigments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
2.4
Example pigments
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
2.5
Determining Loadings
. . . . . . . . . . . . . . . . . . . . . . . . . .
18
19
Atomization
3.1
Spinning Disk Atomization . . . . . . . . . . . . . . . . . . . . . . .
3.1.1
Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
19
19
3.2
4
3.1.2
Results . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .
20
3.1.3
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
. . . . . . . . . . . . . . . . . . . . .
22
3.2.1
Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
3.2.2
Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.2.3
Conclusions . . . . . . . .
Concentric Nozzle Atomization
. . . . . . . . . . . . . . ..
26
Milling
4.1
4.2
23
Ball Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
4.1.1
Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
4.1.2
Results. ..
28
4.1.3
Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
Jet Milling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
4.2.1
. . . . . . . . . . . . . . . . . . . . . . . ..
31
5 Conclusion
5
List of Figures
1-1
Electrophoretic Image Display . . . . . . . . . . . . . . . . . . . . . .
9
3-1
Diagram of spinning disk atomizer . . . . . . . . . . . . . . . . . . . .
20
3-2
Spinning disk atomized polyethylene and TiO2 . . . . . . . . . . . . .
21
3-3
Spinning disk atomized polyethylene and carbon black
. . . . . . . .
22
3-4
Diagram of concentric nozzle atomizer
. . . . . . . . . . . . . . . . .
23
3-5
Nozzle atomized polyethylene and TiO2 . . . . . . . . . . . . . . . . .
24
3-6
Nozzle atomized polyethylene and V-302, magnification 500x . . . . .
24
4-1
Diagram of a ball m ill
. . . . . . . . . . . . . . . . . . . . . . . . . .
27
4-2
Ball milled polyethylene and TiO2 . . . . . . . . . . . . . . . . . . . .
28
4-3
Diagram of a pancake jet mill . . . . . . . . . . . . . . . . . . . . . .
30
6
List of Tables
2.1
Triboelectric Series . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
15
Chapter 1
Introduction
Electrophoretic displays may now be entering a glorious renaissance thanks to recent
work done at the MIT Media Lab. These displays were first popularized in the late
1970's but interest since then has declined due to various problems. The major ninovation our lab has had is in microencapsulating the display, therefore overcoming a
number of inherent problems and opening up a new world of applications. Microencapsulated displays may obviate paper, providing the first truly flexible reflective
displays.
A key component of these displays are the microparticles that give them their
color switching capability.
My work has involved selecting the materials for these
particles and processing them into microparticles. This chapter reviews the history
of electrophoretic display particles.
Chapter two is about polymers and pigment
selection for displays. Chapter three pertains to the use of atomization in particle
production. Chapter four reports on milling for particle production. Finally, there is
a conclusion in chapter five.
1.1
Background on Electrophoretic Displays
Electrophoretic image displays (EPID) are a type of reflective display that was developed extensively in the late 1970's. The display works by moving charged opaquely
colored particles through a heavily dyed fluid. By placing this mixture between two
8
Figure 1-1: Electrophoretic Image Display
Glass
Particle
+
/0
+
+/
+
Colored fluid
--
/TO conductive coating
Spacer
sandwiched glass plates with clear Indium/Tin Oxide (ITO) electrodes, the particles
can be transported between the front and back of the sandwich.
A diagram of a
display is in Figure 1-1. When they are in front, the display appears as the color of
the particles. When moved to the back, the display looks like the color of the fluid.
A phenomenon called the zeta potential causes a charge layer to form on the surface
of the particle when submerged in a fluid. This is the charge that causes the particle
to move in a field. The magnitude and sign of this charge is high dependent on the
surface characteristics of the particle.
The advantages of an electrophoretic display as compared to other display technologies such as liquid crystal displays (LCDs) are numerous.
If the particles are
carefully density matched to the suspension fluid, the displays are bistable, meaning
that the display remains in its current state even after the power is removed. On the
contrary, LCDs require constant application of electricity to keep pixels in a certain
9
state. In addition, EPIDs draw little power when switching between states. The
other main advantage is the appearance of the display. It closely resembles that of a
standard printed sheet of paper, with high contrast and a wide viewing angle range.
Unfortunately, there are a number of problems associated with EPIDs. The first
is that these displays do not have a threshold that allows passive matrix addressing.
A threshold is a voltage level, below which the display will not move at all. This
is of vital importance in order to make simple, inexpensive matrix displays. Other
problems can be attributed to poor selection of contrasting particles.
1.2
Prior Art in Particle Fabrication
The first electrophoretic displays used raw pigment particles such as titanium dioxide
(TiO 2 ) and diarylide yellow (DAY).[3] The reasons why this was are obvious. Pigment
particles are small; many commercially available pigments consist of particles well
below 1 micron in size. Also, a raw pigment particle will give the greatest contrast
attainable from that pigment. It soon became clear however that there were problems
with using a raw pigment. The particles tended to agglomerate in the cell, forming
splotchy areas of high pigment concentration.
They also migrated throughout the
cell as it was cycled. Both of these problems hurt the contrast of the display, for
where there are no particles in the cell, there is no color change. Another problem
was that the pigment particles adhered to the exposed electrode, therefore requiring
high field strengths to break them free. Finally, the magnitude of the zeta potential
of the pigment particle was low.
In time, many of these problems were fixed. The problem of migration was solved
by placing a grid structure within the macroscopic cell, thereby segregating the suspension into discrete regions that could not migrate.[5] Agglomeration was solved by
using various charge controlling agents that affixed themselves to the surface of the
pigment particle. These charged ion group caused the pigment particles to have a
mild repulsion for one another and therefore cease to agglomerate.[2]
One way put forth of overcoming the problems associated with raw pigments was
10
to embed the pigment in phenol resin and then grind the resin into a fine powder. [4]
This technique yielded a phenol composite particle that was negatively charged. Phenol resin was chosen for its ease in grinding and for the fact that the resin and pigment
could be well mixed before hardening it into a solid. Other experiments used a similar
procedure with polyethylene. It is from these earlier research efforts that my research
has proceeded.
1.3
Chemical processes
A number of techniques that synthesize particles have been developed. One system
polymerizes a latex coating around the pigment particle.[6] Recent work by Copytele
Corporation have involved a sphere that is polymerized in solution.[9] Another technique coated a hollow glass sphere with the pigment particle to get a low density,
highly colored sphere.[1] My area of research was in mechanical means of production for the simple reason that it is easier to scale such a process to a commercial
production level than it is to scale a chemical process.
1.4
Microencapsulation
The problem of agglomeration and migration was solved in our lab in a very different
way, using microencapsulation. Microencapsulation is a technique that places a fluid
or solid inside a spherical shell of another material. In the case of microencapsulated
electrophoretic displays, the same fluid and particle mixture that was put between
pieces of glass is microencapsulated into a clear, strong shell of plastic. This microcapsule can then be dispersed in a binder and coated onto a substrate. There is still
a need for the electrodes, but those too can be printed. In general, the microcapsules
are between 25 and 100 microns in diameter. This mandates that the particles be
below around 5 microns in size.
11
1.5
Electrophoretic Suspension Fluid
Halogenated hydrocarbons are used as the suspension fluid inside the cell. These
fluids have very low dielectric constants, high densities, low viscosities, and are nontoxic. However, because of the effects they have on the ozone layer, these chemicals
have become undesirable to use. Paraffanic waxes as used in the liquid toner industry
have been used to some extent, but because of their low densities they are less useful
with pigment particles that are dense and are prone to settling out of the suspension.
12
Chapter 2
Particle Composition
The path by which I selected the materials for particles was an interesting one. Initially, my experiments involved re-creating an apparatus to produce bichromal spheres
for use in twisting ball displays. It was at that time that work in our lab began on electrophoretic displays. The yields from my failed initial experiments were used to make
the first electrophoretic displays that our lab built. In time our focus shifted away
from these bichromal spheres and a more formal search for polymers was initiated.
2.1
Polymer Requirements
This section is a list of requirements that can be used to determine if a polymer is
a good candidate for use as an electrophoretic particle modifier. It is a fairly constraining list, therefore making polymer selection somewhat troublesome. However,
given the remarkable variety of polymers available, finding polymers that possess
these properties is not impossible.
2.1.1
Dielectric constant
The first is that the polymer be as non-conductive as possible. A particle that conducts will have a low electrophoretic mobility. Most polymers are very good insulators. The dielectric constant of the polymer should be as low as possible.
13
2.1.2
Density
The density of the polymer should be as low as possible. This is generally not a
problem, as most polymers have a density around 1.0 g/cm3 . The cause for low
density comes from the suspending fluid. Generally, the fluid has at most a density
of 1.6 g/cm 3 if a halogenated hydrocarbon is used. Due to the high densities of most
pigments, it is therefore desirable to bring the density of the final mixture as low as
possible. This means that the polymer modifier should have a low density.
2.1.3
Charge controlling ability
The charge controlling ability of the polymer is one of the most significant attributes
that affects the usefulness of a given polymer. The atoms that make up the monomers
of the polymer have an ability to form either a positive or negative potential on the
surface of the particle. The sign and magnitude of this charge is highly dependent
on the particular polymer used. With a single particle system as is described in
the introduction, the sign of the charge is less critical than with the dual particle
system that our lab advocates. In this system, it is vital that the two plastics exhibit
opposite charge characteristics.
The triboelectric series, Table 2.1 [8], and simple
atomic composition analysis can yield insight into the magnitude and charge of the
polymer, but experiments yield the most accurate results.
2.1.4
Pigment dispersion
Pigment dispersion refers to the ability of a pigment to be homogeneously dispersed in
the polymer. This is especially important when the mixture is going to be ground to
a size not much larger than the pigment particle. Dispersing pigments into polymers
depends on whether the polymer is a thermoplastic or a thermosetting plastic. In
the case of a thermoplastic, the plastic must be melted and the pigment must be
thoroughly mixed into the melt. A polymer with a melt flow index (MFI) above 70
should provide better dispersion characteristics.
If a thermosetting plastic is used, it should be one that has a liquid resin system.
14
Table 2.1: Triboelectric Series
Positive
Silicone elastomer with silica filler
Borosilicate glass, fire polished
Window glass
Aniline-formol resin, acid catalyzed
Polyformaldehyde
Polymethylmethacrylate
Ethylcellulose
Polyamide
Melamine formol
Wool, knitted
Silica, fire polished
Silk, woven
Polyethylene glycol adipate
Polydiallyl phthalate
Cellulose sponge
Cotton, woven
Polyurethane elastomer
Styrene-acrylonitrile copolymer
Styrene-butadiene copolymer
Polystyrene
Polyisobutylene
Polyurethane sponge
Borosilicate glass, ground state
Polyethylene glycol terephthalate
Polyvinyl butyral
Formo-phenolique, hardened
Epoxide resin
Polychlorobutadiene
Butadiene-acrylonitrile copolymer
Natural rubber
Polyacrylonitrile
Polyethylene
Polydiphenylol propane carbonate
Chlorinated polyether
Polyvinylchloride with 25% DOP
Polyvinylchloride without plasticizer
Polytrifluorochloroethylene
Polytetrafluoroethylene
Negative
15
This provides an opportunity to mix the pigment into the resin just as the pigment
is mixed into the molten thermoplastic. Later, the resin can be hardened.
Mixing can be accomplished through a variety of means. In the case of a liquid
resin or a very low viscosity thermoplastic melt, a standard kitchen blender or homogenizer can be used. If the thermoplastic comes in a micronized form, the pigment
and the thermoplastic can be ball milled together. This has the additional advantage of assuring that the pigment particles are fully broken up. Finally, it may be
advantageous to use an ultrasonic homogenizer to break up the pigment particles and
disperse them into a resin or melt.
2.1.5
Friability
Friability refers to ease with which a compound can be fractured.
This is most
important in terms of particle production if a grinding process is to be used in the
final production of a powder. Fracture is not normally a desired property in polymers
when used as a structural material, so this attribute does limit the scope of polymers
that are useful. However, there are industries such as toner manufacturing and powder
coating that required ground polymers.
2.1.6
Hydrophobicity
Microencapsulation places additional requirements on polymers that can be used. In
some types of interfacial polymerizations, the electrophoretic mixture is emulsified in
water. This necessitates that the particles be hydrophobic so as not to be embedded
in the walls of the microcapsules. As an experiment, drops of water were placed on
two types of plastic, polyethylene and polyester. The water beaded up on both, but
the drop on the polyethylene was more spherical indicating greater hydrophobicity.
16
2.2
Example polymers
Most of the particles we use are made of a low molecular weight, high density polyethylene that is manufactured by Allied Signal under the name ACcumist B-18. This polymer is used as a lubricant and paper coating. It was designed to be easily ground. It
is in a powder form when received, with an average particle size of 18 microns and a
density of 0.96 g/cm 3 . After hardening, the polymer pigment mixture is very brittle.
In general, polyethylenes are very hydrophobic. Experimentally, it has been found
to have a negative charge controlling ability. Its dielectric constant is 1.3. It works
well in electrophoretic cells. Another polymer we have used is polyester, specifically
the resin system common for use with fiberglass because it is extremely brittle when
cured. It has a positive charge controlling ability. Unfortunately, it may not be as
hydrophobic as necessary. Additional tests are needed to verify its abilities.
2.3
Pigments
Fortunately, pigment selection appears to be somewhat less critical than polymer
selection. The only major rule seems to be to make sure the pigment has low conductivity, or that it can be surface modified to have low conductivity. It should also
be hydrophobic so that it disperses in polymers. Again, the pigment can be modified
to permit this. Ideally, the density should be as low as possible. When selecting a
white pigment, a pigment with a high index of refraction should be used. This will
permit the greatest level of scattering possible.
2.4
Example pigments
For white, Ti-Pure R-104 from Dupont works well in polyethylene. It has an average
particle size of 180 nm which is an ideal size for scattering light. It is organically
surface modified so that it disperses in polymers and its density is between 3.8 and
4.2. The index of refraction is 2.7.
For black, inorganic pigments from the Ferro Corporation such as V-302-2 (formula
17
CuCr 2 0 4 :MnO:MoO 3 , density 5.6 g/cm 3 ) and a blacker pigment F-6331 (formula
(Fe,Mn)(Fe,Mn) 2 0 4 :CuO, density 6.0 g/cm 3 ), work well. Carbon black should be
avoided due to its conductivity.
There are almost no limit to what colors can be used. Inorganic pigments come
in an amazing variety of colors. They all are suitable for use as coloring agents.
2.5
Determining Loadings
Loading ratios are driven by two factors. The first is appearance, both in terms of
opacity and overall color. In order to give such a small particle a rich color, loadings
need to be at least 10% pigment in a typical 20 micron particle by volume. Opacity is
achieved through the pigment also because most raw polymers are clear or translucent.
The ratio of pigment to polymer for a desired final density is
=
m2
d2
-d+
(2.1)
where m is mass and d is density. Subscripts 1 and 2 refer to the two species that
are being combined, normally a polymer and a pigment. The desired density is df.
As an example, to achieve a final density of 1.5, by mass there should be 11 parts
.
polyethylene to 10 parts TiO 2
18
Chapter 3
Atomization
Atomization refers to the process of breaking a fluid up into small droplets using
external energy. Two techniques, spinning disk atomization and concentric nozzle
atomization were used.
3.1
Spinning Disk Atomization
Spinning disk atomization is a technique for producing small spheres from a liquid.
In this type of atomization, a liquid is flowed onto a high speed spinning disk. The
fluid accelerates due to the centripetal force, moves to the outside of the disk, and
breaks off into ligaments from the edge of the disk. From there the ligaments form
droplets. The major advantage of spinning disk atomization is that the particles can
be of very uniform size. However, there is often secondary breakup in air that causes
satellite droplets to form.
3.1.1
Apparatus
An apparatus for spinning disk atomization was build. Figure 3-1 shows a diagram
of the apparatus. Polyethylene and pigments were melted together, mixed well, and
cast into a sheet. This sheet was granulated in a kitchen blender. The granules were
placed into the galvanized funnel where they were fed down by the auger bit powered
19
Figure 3-1: Diagram of spinning disk atomizer
DC Gearhead Motor
Galvanized 8" diameter funnel
1/4" shaft
3/4" .D. aluminum tube
314" auger drillbit
Heaters
Aluminum nozzle
Soft copper tubing
DC motor
Heated aluminum block
Disk
*
e
Coiiection funnel
0W6
by the gearhead motor. The walls of the aluminum tube and nozzle block were kept
at 185 degrees C. This melted the plastic as it flowed through the tube. This molten
plastic was then forced through the soft copper tube, which was also heated, to the
heated aluminum block mounted above and concentric to the disk. There the plastic
flowed out of an opening onto the surface of a disk spinning at 7500 RPM. The plastic
moved to the outside of the disk, eventually breaking up off the edge and forming
droplets. These droplets cooled as they fell and were collected in a 3 feet diameter
funnel.
3.1.2
Results
This technique was used to produce both black and white spheres. Photomicrographs
of these spheres are in Figure 3-2 and 3-3. The sphere size averaged around 40 microns
with many of the large spheres around 100 microns. It is believed the the smaller
20
Figure 3-2: Spinning disk atomized polyethylene and TiO 2
100 microns
satellite spheres were not collected very effectively due to their ability to float upwards
in the air heated by the apparatus.
3.1.3
Conclusions
Two drawbacks to spinning disk atomization were reveled by these experiments. The
first is that achieving a particle size below 15 microns was difficult. This limitation is
due to the viscosity of the molten polymer. The second is that the pigment particles
separated out of the suspending polymer when the disk was spun at very high speeds.
This problem was due to the heavy pigment particles settling out of the much lighter
plastic. A surfactant, OT-100 from the Cyanamid Corporation was used to overcome
this problem, but use of this additive was abandoned because the spheres ceased to
have any electrophoretic mobility. This surfactant probably caused the plastic to be
too conductive.
As a means of producing electrophoretic particles, spinning disk atomization
should be avoided. The spheres do not have a uniform density and are too large
in size to be useful in the current generation of microencapsulated displays.
21
Figure 3-3: Spinning disk atomized polyethylene and carbon black
200 microns
3.2
Concentric Nozzle Atomization
Another class of atomization is concentric nozzle atomization. This technique involves
flowing a liquid through a tube that is concentrically placed inside another tube. The
cavity between the two tubes is used for a high velocity air jet. As the liquid flows
out of the inner tube, it is brought into the turbulent flow of the exiting gas. The
impinging air breaks the droplet up into smaller volumes of liquid. Eventually, the
liquid droplets move away from the atomization nozzle and cool as spheres in free
fall. Unlike spinning disk atomization, this method inherently produces a large size
distribution.
3.2.1
Apparatus
Many different iterations were performed on the nozzle atomizer design. An earlier
version is shown in Figure 3-4. In this design, a reservoir of the plastic was kept
molten by heaters set at 180 degrees C. Gravity fed this liquid down through a small
diameter stainless steel tube. This tube ran into a channel that had high velocity
air at 140 degrees C running through it. At the mixing area the liquid broke up
22
Figure 3-4: Diagram of concentric nozzle atomizer
ResevoI r
Heaters
16 gaugo stainless steel tubing
Air inlet
Mixing area
Spray
into a spray of small droplets. Later designs employed electrical grounding between
the reservoir and atomization nozzle in order to allow charging of the polyethylene.
Although not extensively tested, the charge appeared to dissipate from the sphere
after a couple of days.
3.2.2
Results
This series of atomizers served their purpose well. Photomicrographs of the spheres
are in Figure 3-5 and 3-6.
Various polymer and pigment mixtures were passed
through the atomizer producing a good quality sphere that worked well in the electrophoretic displays.
The first microencapsulated displays were made using these
atomized spheres. The yield was about 5% of the particles below 25 microns in size.
3.2.3
Conclusions
Like spinning disk atomization, concentric nozzle atomization is not well suited for
use in electrophoretic particle production. While there is a usable portion of the
23
Figure 3-5: Nozzle atomized polyethylene and TiO 2
. . ........ .
50 microns
Figure 3-6: Nozzle atomized polyethylene and V-302, magnification 500x
20 microns
24
output that works in displays, the yield is not applicable to a manufacturing scenario.
Also, the density varies in the spheres too much to provide sufficient bistablity in the
displays.
25
Chapter 4
Milling
There are many milling techniques for use in the production of particles. Two techniques, ball milling and jet milling were examined for suitability to the task.
It is vital that an electrophoretic powder be free of contamination, especially conductive particles. This is a potential problem with hammer mills and ball mills. A
conductive contamination, such as steel particles can significantly reduce the electrophoretic mobility of the cell. It also will cause a greater power draw to switch the
cell.
4.1
Ball Milling
Ball milling is a technique widely used in industry for particle size reduction. It is
able to crush materials down to the micron level. This simple technique involves
placing the sample in a cylindrical drum along with a grinding medium. The drum is
then rotated at a specified speed. There are two modes of operation. If the drum is
turned fast enough, the grinding medium will climb up the wall of the cylinder and
fall down to the bottom. The critical speed in revolutions per second at which this
occurs is
n. =
(4.1)
26
Figure 4-1: Diagram of a ball mill
Container
Grinding medium
Particles
Idler
1W*
'4
Driven roller
Drive motor
(0
where the critical speed, nc, is related to the gravitational constant, g, and the
diameter of the tube, Dt.[7] In practice, the operating speed is
n = 0.68 - 0.75n,
(4.2)
At this speed, the grinding medium tumbles, but also rolls in the bottom of the
tube. This rolling action is the key to producing fine particulate sizes, while the
tumbling motion is necessary to initially break up the feed stock.
4.1.1
Apparatus
A commercially available ball mill was used to make electrophoretic display particles.
Figure 4-1 shows to basic setup.
The grinding medium was 1/2"xl/2" ceramic cylinders, chosen for their ability
to grind in a contamination free setting. Two mixtures were used in separate experiments. This first was a mixture of TiO 2 and polyethylene and the second was
27
Figure 4-2: Ball milled polyethylene and TiO 2
20 microns
396
polyester and an inorganic black pigment. They were both ball milled for two days.
4.1.2
Results
Both of the materials ground successfully in this ball mill. The yield was 10% of the
plastic below 20 microns in size. Both of these plastics are very brittle, which is vital
for the grinding process to work. Photomicrographs of the particles are in Figure 4-2.
4.1.3
Conclusion
Ball milling is a useful technique for the production of electrophoretic particles. Unfortunately, it is slow. This makes it undesirable in a laboratory setting where it is
advantageous to be able to try new polymers quickly.
4.2
Jet Milling
Jet mills, or air mills, are the most useful class of mills for producing very fine powders
with average sizes on the order of 1 micron. The size of the final particle is dependent
28
on the jet mill and the friability of the feed stock. A very brittle material like TiO 2
can be ground to below a micron in size, while a polymer may only be reduced to 5
microns in size. In a jet mill, the incoming powder is slowly fed into a stream of high
velocity air. The air is brought into a chamber that is shaped to cause a flow such
that the particles impact themselves. This stream is recirculated until classification
removes the particles below a desired size threshold.
Due to time constraints, no
experimental work has been performed on a jet mill.
4.2.1
Apparatus
A pancake jet mill, such as the one in Figure 4-3 should be used as a starting point.
The powder is fed through a hopper into a stream of high pressure air. This gas
particle mixture moves into the grinding chamber where is it quickly accelerated by
another stream of air that enters the chamber tangentially. The particles are swirled
around at high speed, as the flow is directed into itself by vanes. The particles thus
collide and break each other up into smaller particles. As the particle size decreases,
the smaller, lighter particles move towards the center of the grinding chamber where
they are eventually able to leave along with the exiting gas. The exiting gas is then
fed through a collection bag which filters out the particles that exit with it. This
type of mill can grind to a very small size by adjusting the feed rate and pressure of
the air. The only drawback to jet milling is that it is very inefficient and has large
energy requirements.
29
-
-
Figure 4-3: Diagram of a pancake jet mill
Feed
Fluid
Product
Vanes
30
Chapter 5
Conclusion
While all the techniques described in this thesis will work for manufacturing particles
for an electrophoretic display, it is believed that jet milling is the most ideal. This
is because of its ability to grind particles to a very small size with a very tight
distribution. The most obvious evidence that jet milling is the best technique is the
fact that toner manufacturers use it. Toner particles are cousins to the particles used
in electrophoretic displays.
A major difference between atomization and grinding is that atomizing produces
spheres where grinding produces jagged particles. This difference may be important
because of the increased surface area of the fractured particle.
Future work involves better techniques for incorporating pigments into polymers,
additional searches for better polymers, and experiments using jet mills.
31
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Electrophoretic composition and display devzce.
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[4] Isao Ota et al. Developments in electrophoretic displays. Proceeding of the S.I.D.,
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[5] M. A. Hopper and V. Novotny. An electrophoretic display, its properties, model
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