Download The preparation of environment friendly gelatin-gum arabic

The preparation of environment friendly gelatin-gum arabic
microcapsule for electrophoretic display
Hongli Liu , Shirong Wang , Xianggao Li , Yin Xiao *
(School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, PR
China)
Abstract: The microencapsulated display device is the main choice for electrophoretic display.
The application of environment friendly dispersion medium is very critical for the
commercialization of this technology. The electrophoretic microcapsules using Isopar L, which is
non-toxic, low aromatic concentration and volatility, as dispersion medium were prepared by
complex coacervation between gelatin and acacia. The influence of the weight ratio of capsule
material to core material, pH value, dispersion time and rate on the properties of microcapsules
was investigated. Optimized technology was defined as follows: the ratio of capsule material to
core material is 1:4, the microcapsule wall is cured at pH=8.5 with the temperature of 45 °C, the
disperse time and rate was 20 min and 800 rpm, respectively. The particle size and yield of the
microcapsule prepared under optimized conditions were 30-60 μm and 83.88%, respectively. The
contrast ratio of the electrophoretic display device used the prepared microcapsules was as high as
2.09.
Key Words: Electrophoretic microcapsules; Electrophoretic display; Isopar L; Environmental
friendly
1. Introduction
Electrophoretic display (EPD) which bases on the theory of electrophoresis was
first introduced by Ota in 1972 [1]. EPD is non-light-emitting display [2-4] and works
on the principle that electrophoretic particles with opposite charge dispersed in
nonpolar liquid media migrate to the corresponding electrode when an electric filed is
applied [5]. Microencapsulated electrophoretic display has attracted much attention
due to the combination of the advantages of both electronic display and conventional
paper [7], such as wide-viewing angle, flexibility, high contrast ratio, lightweight,
portability and low cost [8-13]. The microcapsules encapsulate the electrophoretic
suspension containing particles with different color or electric property into individual
microcapsule. This technology could overcome many disadvantages during display
process, such as the sedimentation of electrophoretic particles, low reflectance and
*
Corresponding author. Tel: +86 22 2740 4208. E-mail: [email protected] (Yin Xiao).
transition state (grey state). Currently, it has been commercialized and has achieved
mass market success [14].
Nowadays, environmental protection has attracted more and more attention and
great effort has been devoted to the exploration of eco-friendly materials. Herein, the
development of environmentally friendly materials is one of the key challenges to the
further application of EPD. The microcapsules consist of capsule material,
electrophoretic particles, dispersion medium, charge control agent (CCA) and
dispersant.
The
halogenated
hydrocarbon
(tetrachloroethylene
[15],
polytrifluorochloroethylene, carbon tetrachloride) and epoxide (toluene, naphthaline,
et al) [16] have been often used as dispersion medium. However, such reagents are
harmful to both human health and environment because of their high volatility and
toxicity.
In this work, Isopar L was selected as the desired dispersion medium due to its
merits of insulativity, reactionlessness, low volatility and toxicity. The environmental
friendly microcapsules were prepared from gelatin and gum arabic, using inorganic
materials, that is TiO2 and carbon black, as white and black pigment, respectively.
Innocuity surfactants (T151 and CH-5) were employed as charge control agent and
dispersant. Capsule material was prepared from gelatin-gum arabic which are
non-toxic water-soluble protein by complex coacervation. The operation parameters
such as capsule material/core material ratio, pH value, dispersion time and stirring
rate were optimized to obtain microcapsules with high yield and well-distributed
particle size.
2. Experimental
2.1 Materials and apparatus
Gelatin and gum arabic were pharmaceutical grade and purchased from Bai Ling
Wei Technology Co., Ltd., China. Carbon black (SB4) purchased from Degussa Co.
was dried under 120 °C for 3h before use. Titanium dioxide (TiO2) was supplied by
Beijing Mengtai Youyan technology development center. Isopar L was purchased
from Shanghai Huishuo Chemical Co., Ltd China. Glacial acetic acid, sodium dodecyl
sulfate (SDS), glutaraldehyde (50%) and sodium carbonate were analytical grade and
purchased from Guangfu Chemical Regent Co., Ltd., China. T151 and CH-5 used as
charge control agent and dispersant are supplied by Wuxi south oil additive Co., Ltd
and Shanghai Sanzheng Co., Ltd., China, respectively. Perfluorobutyl sulfonyl
fluoride (HX-8) was purchased from Hubei Hengxin Chemical Co., Ltd., China.
A4300 (fatty acid derivatives) was purchased from Tianjin Haiguang Chemical Co.,
Ltd., China.
The morphology of the microcapsule was observed by BX51 transmission
optical microscope (Olympus Corporation, Japan). Contrast ratio of EDP prototype
used the prepared microcapsules were measured by Eye-one pro colorimeter (X-Rite,
USA). The procedure of coating the microcapsules onto the ITO film was processed
by blade coater (home-made).
2.2 Preparation of the microcapsules
2.2.1 Preparetion of electrophoretic liquid
Carbon black 0.5 g, hyperdisperant T151 0.1 g and CH-5 0.2 g were dispersed in
10 mL Isopar L as black electrophoretic slurry. Then the black electrophoretic liquid
were ball-milled for 36 h at 300 rpm. The white electrophoretic slurry consist of TiO2
(36 g), T151 (1.2 g) and CH-5 (2.4 g) and Isopar L (120 mL) were ball-milled for 48
h at 300 rpm. Then, the black and white electrophoretic slurry were mixed together
according to a certain proportion. Furthermore, 1.91 wt.% perfluorobutyl sulfonyl
fluoride and 0.75 wt.% A4300 served as dispersant and charge control agent were
added to the electrophoretic liquid and then the mixture was sonicated for 2 h in a
bath sonicator to ensure dispersion.
2.2.2 Preparation of gelatin-gum arabic Microcapsules
Gelatin (2.5 g) and gum acacia (2.5 g) were dissolved in 125 mL deionized water
with stirring at 45 °C for 30 min. 15 g of the electrophoretic fluid served as core
material was dispersed at 45 °C in gelatin aqueous solution which commixed with 0.1
g SDS. The emulsion was agitated at 800 rpm for 20 min. Gum acacia aqueous
solution blended with 0.1 g SDS was added to the above solution at 45 °C. Then the
pH value was adjusted to 5.0 with 5 wt.% acetic acid to induce complex coacervation.
After cooling to 10 °C with continuous vigorous stirring, 100 mL glutaraldehyde
aqueous solution (5 wt.%) was added dropwise to crosslink the microcapsule and the
reaction mixture was stirred at 0 °C for 2 h. Afterward, the pH value was regulated to
8.5 using 10 wt.% sodium carbonate aqueous solution. Then the reactant was stirred at
45 °C for 70 min. The microcapsules were washed by deionized water several times,
classified into different size by sieve and then collected from the water by filtration.
The yield of the microcapsule (Y) is defined as below:
=
Y
Wcapsule
Wgelatin + W gum acacia
× 100%
in which Wcapsule was the mass of the microcapsule with the diameter in 30-60 μm;
Wgelatin and Wgum acacia were the mass of gelatin and gum acacia, respectively.
2.3 Preparation of prototype EPD
Microcapsules with diameter in 30-60 μm were uniformly coated on indium tin
oxide (ITO) with 611 (polyurethane) served as adhesive by blade coater. The weight
ratio of microcapsule to 611 was 2:1. After drying at room temperature, the
microcapsule coating film was pasted onto a patterned electrode coated with
anisotropic conductive adhesive. The microcapsule EPD was obtained after
employing 1.5 kg /cm2 of vertical pressure on it and drying at room temperature for
24 h.
3. Result and discussion
3.1 Preparation of microcapsules
In this paper, Isopar L with low toxicity, low aromatic concentration and
volatility was employed as dispersion medium, which permit the prepared
microcapsules is environmental friendly while suffering no yield penalty.
For the EPD, preferable display image will be obtained when the microcapsule
size in 30-60 μm [17]. In order to improve the yield of microcapsule with diameter in
30-60 μm, the ratio of capsule material (gelatin + gum acacia) to core material
(electrophoretic fluid) was studied.
Size
Standard Deviation
75
75
60
60
45
45
30
30
15
15
6
0
1
2
3
4
5
Core material/Capsule material
b
70
90
Standard deviation/μ m
Particle size/μ m
‫ء‬Particle
60
Yield/%
a
90
50
40
30
0
1
2
3
4
5
6
Core material/Capsule material
Fig. 1 Effect of the ratio of core material to capsule material on the particle size (a) and yield
(b) of microcapsules.
Fig. 1 (a) represents the effect of capsule material/core material ratio on
microcapsule size. The particle size of microcapsules decreases and then increases
with the increase of core material. When the ratio is 1:3 and 1:4, the particle size of
microcapsule is mainly in the range of 30-60 μm. Especially, the standard deviation
reaches the local minimum at the ratio of 1:4. More capsule material could deposit on
the interface of core material as less core material used, which lead to the increase of
microcapsule size. While if the core material is too much, the microcapsules diameter
also increases because the core material in the capsule was more. The yield of the
microcapsule increases as the capsule material/core material ratio changes from 1:1 to
1:5, as shown in Fig. 1 (b).
Fig. 2 shows the contrast ratio of the prototype EPD used microcapsules
prepared at different capsule material/core material ratio. It is found that the highest
point reached when the core material is 1:4.
2.2
Contrast ratio
2.0
1.8
1.6
1.4
1.2
1.0
0
1
2
3
4
5
6
Core material/Capsule material
Fig. 2 Effect of the ratio of core material/capsule material on the contrast ration of prototype
EPD.
Fig. 3 represents the morphology of the microcapsules prepared at different
stirring rate. It shows that the microcapsules possess the uniform particle size and
little fragments when prepared at 700 and 800 rpm.
A
B
C
D
Fig. 3 Optical microscopy images of microcapsules prepared with the stirring rate of (A)600
rpm, (B)700 rpm, (C)800 rpm and (D)900 rpm.
As shown in Fig. 4, the particle size was mainly in the range of 30-60 μm when
the stirring rate was 700 and 800 rpm. Big particle size of the microcapsules at low
stirring rate can be attributed to the weaker shear force between agitator blade and
microcapsules, and vice versa. At the stirring rate of 800 rpm, the standard deviation
is as small as 27.14 μm and the yield is up to 83.40%.
a
Particle Size
Standard Deviation
90
90
Standard deviation/ μ m
75
75
75
60
60
60
45
45
30
30
15
15
0
15
600
700
800
900
Rotational speed/rpm
Yield/%
Particle size/μ m
90
b
45
30
600
700
800
Rotational speed/rpm
900
Fig. 4 Effect of stirring rate on the particle size (a) and yield (b) of microcapsules.
The size of microcapsules mainly distributes in 30-60 μm when the dispersion
time is 20 min, as shown in Fig. 5. The particle size of microcapsules is oversize for
short dispersion time, which could be ascribed to that the microcapsules do not be
dispersed adequately by the shear force, and vice versa.
45
85
40
40
80
35
35
30
30
25
25
65
20
60
Particle size/ μ m
45
20
Particle Size
Standard Deviation
10 15 20 25 30 35 40
Dispersion time/min
Yield/%
90
a
Standard deviation/ μ m
50
50
b
75
70
10
15 20 25 30 35
Dispersion time/min
40
Fig. 5 Effect of dispersion time on the particle size (a) and yield (b) of microcapsules.
According to the reaction process, as shown in equation (2) [17], the capsule
wall of microcapsule is cured by the crosslinking between formyl group of
glutaraldehyde and amino group of gelatin. The curing process will not complete until
the formation of network polymer [18].
NHR2 NHR2
2R2NH2
2R1NH2
OHC(CH2)3CHO
R1N-HC(CH2)3CH-NR1
OH
OH
CHOH
CHOH
R1HN-HC(CH2)3CH-NHR1
2OHC(CH2)3CHO (CH )
2 3
(CH2)3
CHO
CHO
NHR2 NHR2
R1N-HC(CH2)3CH-NR1
5R3NH2
NHR2 NHR2
gelatin-NH3
R1N-HC(CH2)3CH-NR1
NHR3
R3HN CH
glutaraldehyde-CHO
(CH2)3
(CH2)3
CH
R3HN CH
2OHC(CH2)3CHO
(H2C)3
CHOH
HOHC
CH
(CH2)3
CHNR
RNHC
N
R3
N
R3
NHR3
(2)
Fig. 6 indicates that when the pH is 8.5, the yield of the microcapsule is as high
as 83.88%. Low yield at low pH could be dues to the fact that some amino groups of
gelatin still exist as -NH3+ and hence cannot crosslink with formyl group. While parts
of gelatin will deteriorate if the pH is too high, this is responsible for the low yield of
the microcapsules.
90
80
Yield/%
70
60
50
40
7.0 7.5 8.0 8.5 9.0 9.5 10.0
pH
Fig. 6 Effect of curing pH on the yield of microcapsules.
3.2 Preparation of EPD device prototype
Fig. 7 represents the optical microscopy images of the microcapsules prepared
under the optimized condition discussed above. It is clearly found the microcapsules
show smooth surface, plump granule, uniform particle size distribution and no
adhesion.
Fig. 7 Optical microscopy images of microcapsules prepared under the optimum technology.
Fig. 8 is a clock display prototype exhibiting the black patterns with white
background (a) and white patterns with black background (b). The prototype is driven
by direct current (DC) with 5V. It is found that the image shown by the display is
clear and can change with the time.
Fig. 8 The images of clock display prototype driven under 5 DC.
4. Conclusion
Smooth, plump and uniform microcapsules with Isopar L which is environmental
friendly as dispersion medium, selecting gelatin-gum acacia as capsule wall were
prepared by complex coacervation technique. It is observed that the capsule
material/core material ratio, dispersion time, stirring rate and curing pH deeply affect
the particle size distribution and yield of the microcapsules. The highest yield and
uniform particle size distribution is obtained under the optimized technology: the ratio
of capsule material to core material is 1:4; the disperse time and stirring rate was 20
min and 800 rpm, respectively; the microcapsules wall is cured at pH=8.5 with the
temperature of 45 °C. The clock display panel prototype prepared by the prepared
microcapsules can display clear image when operated under 5V DC.
Acknowledgements
This research was supported by the Key Projects in the Science & Technology Pillar Program of
Tianjin (Grant No: 11ZCKFGX01700).
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