Electrophoretic displays attracted a lot of attention recently due to its low cost, low weight, low power consumption and reliability. Based on these characteristics, they are going to replace the conventional paper. Electrophoretic displays are called non- emitting displays based on light particle suspensions. They behave as motion of charged particles in a dielectric fluid towards the electrodes with the opposite charge. Electrical and optical properties of electrophoretic displays suspension composition are dependent on the electronic ink that is called E- ink. Thus, key factors in determining image quality depended on electrophoretic particle properties. Enhancing the great image quality for accurate image control and faster response to the voltage applied is dependent on the very small particle size, narrow size distribution and high surface charge of particles. So a lot of research has been done on the modification of particles, surface morphology, surface charge and their stability in media. Therefore, this article reviews the studies of these topics

Khatibzadeh, M.

Kholghi Eshkalak, S

English

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 3 No 2, 02NAESF06(5pp) (2014) 
 
 
2304-1862/2014/3(2)02NAESF06(5) 02NAESF06-1  2014 Sumy State University 
A Review In Preparation of Electronic Ink for Electrophoretic Displays 
 
S. Kholghi Eshkalak*, M. Khatibzadeh† 
 
Department of Polymer Engineering and Color Technology, Amirkabir University of Technology P.O. Box 15875  
Hafez Street, Tehran, Iran 
 
(Received 18 May 2014; published online 29 August 2014) 
 
Electrophoretic displays attracted a lot of attention recently due to its low cost, low weight, low power 
consumption and reliability. Based on these characteristics, they are going to replace the conventional paper. 
Electrophoretic displays are called non- emitting displays based on light particle suspensions. They behave as 
motion of charged particles in a dielectric fluid towards the electrodes with the opposite charge. Electrical 
and optical properties of electrophoretic displays suspension composition are dependent on the electronic ink 
that is called E- ink. Thus, key factors in determining image quality depended on electrophoretic particle 
properties. Enhancing the great image quality for accurate image control and faster response to the voltage 
applied is dependent on the very small particle size, narrow size distribution and high surface charge of parti-
cles. So a lot of research has been done on the modification of particles, surface morphology, surface charge 
and their stability in media. Therefore, this article reviews the studies of these topics. 
 
Keywords: Electrophoretic display, Microcapsules, E-ink, Electric response. 
 
 PACS numbers: 85.60.Pg, 82.35.Np 
 
 
                                                          
* S.kholghi@aut.ac.ir 
† Khatib@aut.ac.ir 
1. INTRODUCTION 
 
Electrophoretic displays have been one of the great-
est interest to researchers in the past decade due to its 
low power, paper-like, flexibility, response to the driv-
ing applied voltage and image memory [1-3]. The elec-
trophoretic displays potential applications are in differ-
ent devices for example E-reader, E-book, wearable 
computer screens, electronic signs and smart identity 
cards (Fig. 1) [4, 5]. 
 
 
 
Fig. 1 – Commercially available electrophoretic displays and 
some prototypes [6] 
 
The image quality of the electrophoretic displays is 
highly dependent on the quality of suspension parti-
cles, which are called electronic ink [E-ink]. The elec-
trical and optical properties of the suspended particles 
are very important in a dielectric fluid. There are a few 
important parameters which can be affected in E-ink 
as particle size, size distribution, surface charge of par-
ticles and response to the applied electric field depend-
ent [7-9]. 
 
2. EXPERIMENTAL 
 
2.1 Electrophoretic Displays 
 
The term electrophoresis is a composition of electro 
and phoresis, two words that are derived from the 
Greek words for charge and the act of carrying. In that 
way, the name electrophoretic display (ED) already 
gives a hint about its basic working principle. As shown 
on the picture below an ED is made of an ink layer, 
sandwiched between two layers that can be plastic, 
glass or even paper. The total thickness of the layer 
structure is between 0.5 mm on glass and 0.1 mm on 
plastic which is in the order of a sheet of paper. 
In the simplest case of a black and white display, the 
top substrate is covered with a single transparent elec-
trode, while the bottom substrate contains a complex pat-
tern of line-electrodes. Using active matrix driving, a sin-
gle pixel can be addressed, meaning that the bottom elec-
trode can be made either positive or negative compared to 
the top-electrode. The electrophoretic ink between these 
electrodes is a mixture of transparent liquid and micro-
scopic charged pigment particles. The usual choice is neg-
atively charged black particles [carbon black] and posi-
tively charged white ones (TiO2). When a voltage is ap-
plied over the top- and bottom of the electrode, the 
charged pigments will move due to an electrostatic force 
to the attracting electrodes. For instance, when the bot-
tom electrode is positive, it will attract black particles and 
repel white ones. These white particles gather at the top-
electrode, where they reflect incident light in all direc-
tions. This is the white state. In the opposite case, a nega-
tive bottom electrode pushes the black particles to the 
surface, where they absorb the light. This is the black 
state (Fig. 2). This basic principle is different than most 
displays by the fact that it is reflective. So, an ED is a type 
of display that reflects or absorbs ambient light in con-
trast to transmissive displays such as the CRT or LCD. 
 S. KHOLGHI ESHKALAK, M. KHATIBZADEH PROC. NAP 3, 02NAESF06 (2014) 
 
 
02NAESF06-2 
 
 
Fig. 2 – Schematic illustration of microcapsule-type electro-
phoretic display 
 
2.2 Advantages of Electrophoretic Displays 
 
The major advantages of the electrophoretic tech-
nology are: 
 High reflectivity of the white state and high con-
trast: 
 High resolution: 
 Very low energy consumption 
 Possibility of thin, mobile and flexible displays 
 
2.3 E-ink Microcapsules 
 
Microcapsules process is said to be very fine parti-
cles or droplets which are surrounded by a capsule of 
small useful features. The tiny sphere microcapsules 
with uniform wall have to be put simply around its 
core. The microcapsules are formed of the core, internal 
phase or filled, and the walls of the shell, coating, or 
membrane [10]. 
Several different technologies are used in the elec-
tronic display such as a microencapsulated electropho-
retic display (MC-EPD), a twisting ball display, an 
electrochromic display and an electrowetting display. 
Among them, MC-EPD is a reflective device based on 
the electrostatic migration of suspended particles in a 
dielectric fluid [11]. The disadvantages of electrophoret-
ic displays is the lack of stability of the electrophoretic 
particles that obtained by agglomerate, clustering and 
delayed migration in a long time thus microcapsules 
are the solution to fix this problem [10, 12]. In 1997, 
Chomsky et al., produced the first MC-EPD by using 
electrophoretic particles [13]. 
Various materials have been used as the shell ma-
terials of microcapsules such as melamine-
formaldehyde [10, 14, 15], urea–formaldehyde [16-18], 
gelatin [19], polyurethane [20] and wax. For electro-
phoretic displays, the wall of the microcapsules re-
quires not only the properties of a solid to retain its 
interior materials but also transparency such that the 
core can be observed during the migration of particles 
and be relatively inexpensive to produce. The UM/F 
resin was chosen as a wall material, which was pre-
pared by an in situ polymerization [21], occurring at 
the interface formed by dispersed core materials and a 
continuous phase [10]. 
On the other hand, wall materials of the microcap-
sule not only should be strong enough to keep its inter-
nal phase but also be transparent enough to have an 
observable change through the migration of internal 
particles. Melamine-formaldehyde (MF) resin has been 
regarded to be useful as wall materials [16,17]. The 
choice of emulsifier, suitable pH after dissolving and 
good adsorption into the wall materials is very im-
portant parameters to gain the stability during micro-
capsulation process. 
 
 
 
Fig. 3 – Mechanism for polymerization of MF resin [14] 
 
Thus several microencapsulation techniques have 
been developed including the chemical methodin situ 
and interfacial polymerization, the physical-chemical 
method coacervation and the mechanical-physical 
method spray encapsulation. Among these, in situ and 
interfacial polymerization methods have become the 
most commonly used methods for the preparation of 
microcapsules [22]. 
 
3. CHEMICAL APPROACH 
 
In 1997 a hardware researcher at the MIT Media lab 
tech began to produce the electronic paper display (EPD) 
as a result of E-books which was called the Amazon Kin-
dle. Joseph Jacobson and Barrett Comiskey were listed 
as inventors on the original patent filed in 1996 [23]. 
 
 
 
Fig. 4 – An example of Amazons Kindle 
 
In a study conducted by Comiskey et al., E-ink mi-
crocapsule with white particles dispersing in the blue 
fluid was reported by providing in situ urea/ formalde-
hyde polymerization. Titanium dioxide with a density 
of 2.4 was used because of high color purity reflection 
as white particles. Polyethylene was used as a coating 
on titanium dioxide to reduce the density and surface 
modification of charged particles and the response to 
 A REVIEW IN PREPARATION OF ELECTRONIC INK… PROC. NAP 3, 02NAESF06 (2014) 
 
 
02NAESF06-3 
applied electric field. In this study, 0.1 sec was reported 
as the response time [13]. 
In 2002, for the first time the E-Ink Corporation, 
Boston (Massachusetts) showed a prototype of a full 
color electronic ink display at the "Symposium, Semi-
nar and Exhibition of the Society for Information Dis-
play (SID)" [24]. 
Particles of titanium dioxide (TiO2) was used as a 
white particle electrophoretic ink [25-27]. In order 
toreduce thesurfacedensity and particles chargeability, 
surfactants and other modifiers was applied as modifi-
cation or other treatments [28, 29]. In a study, the in-
ternal phase, TiO2 nanoparticles was coated with 
poly(methyl methacrylate)(PMMA) to enhance the sur-
face modification and dispersion stability via dispersion 
polymerization [30, 31]. The investigation which was 
conducted by B.J. Park et al., dodecylbenzensulfonic 
acid (DBSA) was used for pretreatmentto change the 
surface characteristics of TiO2. Moreover, microencap-
sulation of the modified TiO2 nanoparticles suspended 
in the medium was carried out with amino preconden-
sates as a wall material and poly(ammonium sty-
renesulfonic acid) (PASA) asan emulsifier. As the re-
sultsare showed in Figure 5, it can be noticed that the 
PMMA and TiO2 are combined with each other [29]. 
 
 
 
Fig. 5 – FT-IR spectra of PMMA, TiO2, and TiO2-coated 
PMMA nanoparticles [29] 
 
H.L. Guo and his colleagues in 2004 reported an en-
capsulated electrophoretic ink. In this work Scarlet pig-
ment powder was used because of brightness, small size 
and low density. They prepared modified pigment sus-
pension in tetrachloro ethylene. E-ink Microcapsules 
was provided by insitu polymerization method using 
urea/formaldehyde shell materials. It was shown that 
the presence of polyethylene surface modification leaded 
to increased spread and stability of pigment particles.It 
also caused the particles tended to be negative elec-
trode.The response time of this study was 3.2 seconds 
(Fig. 6), when the microcapsules were placed under zero 
electric field thepigments were placed randomly inside 
the capsule (Fig. 6a). However, the electric field immedi-
ately responded directly E  120 V/mm (Fig. 6b) and was 
oriented towards the negative pole, and vice versa in the 
reverse electric field, are pushing back [32]. 
J.P. Wang et al., produced blue E-Ink particles from 
phthalocyanine blue BGS (PB15 : 3) due to the high 
brightness and low prices.Actually the urea and formal-
dehyde polymer was used as the core and shell materi-
als, respectively. Modified particles in tetrachloroeth-
ylene (TCE) were prepared by insitu polymerization. In 
this work, the effects of the various factors such as the 
type of modifier, the reaction conditions and the concen-
tration of surfactant in TCE were experimentally inves-
tigated to study the dispersibility of PB15 : 3 particles in 
TCE, the capsule morphology and the adsorption of 
PB15 : 3 particles on internal surface of capsule wall. 
Table 1 shows the fundamental characteristics of pre-
pared electronic ink microcapsules at different experi-
mental conditions. 
 
 
 
Fig. 6 – Microcapsules in the electric field ((a) E  0, (b) 
E  + 120 V/mm, (c) E  – 120 V/mm) [32] 
 
It was shown that using octadecylamine (ODA) to 
modify PB15 : 3 particles resulted in a significant in-
crease of the dispersing extent (D.E) [about four times 
more than that of unmodified]. The response time of par-
ticles to 0.1 V/m DC electric field improved from 2.6 s to 
0.6 s. Figure 8 showsthe relation between concentration 
of Span80 and the interfacial tension. The concentration 
of Span80 in TCE was not less than 0.062 mM, the ad-
sorption of PB15 : 3 particles on internal surface of wall 
were restrained [33]. 
In a study conducted by HouXin-Yan et al., in 2012, 
the conventional organic pigment PR2, Permanent Red 
F2R, was used as colored electrophoretic particles. Sur-
face charge and surface modification was carried out in 
an organic media to increase the stability of the disper-
sions. So the spherical shape of particles as well as in-
creasing the electrical charge of surface leads to increas-
ing of dispersity in media. According to the results 
 S. KHOLGHI ESHKALAK, M. KHATIBZADEH PROC. NAP 3, 02NAESF06 (2014) 
 
 
02NAESF06-4 
 
 
Fig. 7 – The micrography of prepared blue E-ink microcapsules 
[33] 
 
 
 
Fig. 8 – Relation between concentration of Span80 and the 
interfacial tension [33] 
 
 
 
Fig. 9 – SEM and particle size distribution before and after 
modification. (A) Raw materials. (B) Modified by ODA [9] 
 
of SEM images, the average particle size obtained. The 
particle size distribution of raw materials which was 
modified with ODA is shown in Figure 9. The distribu-
tion of particle sizes was from 100 to 1500 nm in the 
unmodifiedconditions while the extent of 80 to 220 nm 
in the modified particles could often be found. In this 
work, the average size of particles was 105 nm and 
they had a very narrow dispersion index equal to 0.068. 
Various modifiers were tested such as: SDS, CTAB, 
CH-6 and D-655.D-655 was thebest commercial disper-
sant according to the results of Figure 10. Sedimenta-
tion rate of the particles which was prepared at tetra-
chloroethylenewas less than 5 % in 12 days. Red parti-
cles which was obtained with white electrophoretic 
particles were used in this work to show the good con-
trast in electrophoretic display which had  200 ms  re-
covery time [9]. 
 
 
 
 
 
Fig. 10 – Effect of different dispersant on modified P.R.2 par-
ticles (A) SEM images (B) Sedimentation ratio within 12 days 
[9] 
 
 
 
Fig. 11 – Electrophoretic display suspension in the electric 
field. (A) Negative field is applied to the upper plate.(B) Posi-
tive field is applied to the upper plate [9] 
 
4. CONCLUSION 
 
As reviewed in this paper, composition of the E-
inkmicrocapsules were contained of materials, colored 
electrophoretic particles, media of dielectric, charge 
control agent and dispersant. Thus, a key factor in de-
termining image quality will electrophoretic particle 
properties. The Increaseimage quality requires high 
surface charge and responds faster to voltage applied to 
make exact image control. So a lot of researchhas been 
studied on themicroencapsolation process, modification 
of particles, surface morphology, surface charge and 
stability of the steric. 
 
B 
A 
 A REVIEW IN PREPARATION OF ELECTRONIC INK… PROC. NAP 3, 02NAESF06 (2014) 
 
 
02NAESF06-5 
 
Table 1 – The characteristics of electronic ink microcapsules prepared by different reacting conditions 
 
 
 
*[a] According to condition of preparation of the electronic ink microcapsules in this paper; [b] to substitute 0.3 mol/L of the 
citric acid-sodium hydroxide buffer solution by 10% of HCl solution; [c] reaction was performed for 1.5 h at room temperature 
directly; [d] volumes of the prepolymer were increased to 20 mL [33] 
 
Although, black and white electrophoretic displays 
microcapsules was produced with commercial success 
but itmay notmet monochromeEPDdisplay market-
requirements. Thus purpose of the progresses and 
technologies is produce color displays. 
 
 
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A Review In Preparation of Electronic Ink for Electrophoretic Displays

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