FMCp - 6
Cellulose Ether Polymers as Optical Compensation Films for
LCDs High Birefringence and Tunable Optics
Zhuo Wang, Jason Folkenroth*, Weijun Zhou**, Yongwen Zhang, Xiuqin Shi
Dow Chemical, No. 936, Zhangheng Road, Pudong District, Shanghai 201203, China
*Dow Wolff Cellulosics, 1691 N. Swede Road, Larkin Laboratory, Midland, MI 48674, U.S.
**Dow Materials Science, Core R&D, 2301 N. Brazosport Blvd., Freeport, TX 77541, U.S.
Keywords: LCD, Compensation Film, Ethyl Cellulose, Silicone, Birefringence
ABSTRACT
ETHOCELΠethylcellulose polymers from Dow
Chemical provide an alternative material to tri-acetyl
cellulose (TAC) for LCD retardation films to increase
viewing angle and contrast through its unique
birefringence optics, excellent optical transparency, and
low haze. Moreover, a new ETHOCEL-based polymer
system has recently been developed with new optical
properties such as tunable birefringence and flat
wavelength dispersion.
molecular liquid crystals with rod-like or disk-like
orientation to achieve out of plane birefringence. Similar
with plasticizer, long-term stability is a big concern for
these additives. Recently, high molecular weight
polymers were blended with cellulose ester or cellulose
ether. For example, ethyl cellulose (EC) was blended
with CAP (cellulose acetate propionate), and a
copolymer of SMA (styrene maleic anhydrate copolymer)
[5-7]
was blended with ethyl cellulose.
However, the
miscibility is a big concern for these blending systems
and the haze is difficult to reduce to below 1%.
1. INTRODUCTION
The dominant LCD markets have different optical
property requirements for retardation films based on LCD
[1]
modes . As shown in Table 1, there are three general
types of LCDs, TN, VA and IPS modes, dominating the
LCD products such as TVs, PCs, monitors, tablet PCs,
phones, and others. The high-end products such as large
size TVs, tablet PCs, smart phones usually have more
demanding requirements for viewing angle and contrast,
as a result, require special in-plane (Ro) and out-of-plane
(Rth) retardation for optical films, as shown in Table 1:
In this report, ethyl cellulose (EC) and its polymer
blends are introduced with high and tunable retardation
values, respectively.
Table 1 LCD retardation film Ro/Rth requirements
Applications
Large size
TVs (>32”)
LCD
modes
VA mode
IPS mode
PCs, laptops
TN mode
High ending
Tablet PCs,
smart phones
VA mode
IPS mode
Ro needs
Rth needs
50 nm
0 nm (<10
nm)
125~570
nm
50 nm
0 nm (<10
nm)
130 nm
0 nm (<10
nm)
No needs
130 nm
0 nm (<10
nm)
To meet the current application targets, blending and
coating are effective ways to tune the optical properties.
For example, retardation adjusting additives and coating
[2-4]
technologies are widely used.
2. EXPERIMENTAL
Raw materials: ethylcellulose is from The Dow
TM
Chemical Company with the brand name of ETHOCEL
STD 100. Tri-acetyl cellulose is from ACROS, di-acetyl
cellulose L-50 is from Daicel, cellulose acetate
propionate CAP-381-0.5 and cellulose acetate butyrate
CAB-482-0.5 are from Eastman. Silicone solid flakes are
from Dow Corning Corporation.
Film preparation: ethylcellulose powders and silicone
flakes are first dissolved in a toluene/ethanol solvent
mixture with total solid content of 10 % by weight for film
preparation. The film was prepared by casting the dope
on a glass substrate by using an automatic casting
machine. After drying at room temperature for 15 hours,
the films were peeled off the substrate for performance
testing and structure characterization. The film thickness
was controlled for 90 micrometers.
Performance testing: Optical transparency and haze
properties were tested under ASTM 1003 by using
Transmittance & Haze meter (BYK 4727). Film
birefringence properties were collected by using Kobra
Automatic Birefringence Analyzer.
Most of these retardation additives are based on small
ISSN-L 1883-2490/19/0499 © 2012 ITE and SID
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3.1 Rth comparison
As shown in Fig. 1, the pure ethyl cellulose film shows
much higher Rth than cellulose ester films prepared
similarly, including cellulose acetate polymers of higher
substitution degree (TAC) and lower substitution degree
(di-acetyl cellulose, DAC), blends of acetate and
propionate/butyrate (CAP, CAB). So, high Rth is a
characteristic property for ethyl cellulose polymer. The
high Rth of ethyl cellulose makes it a promising ultra-thin
film material toward the trend of light and thin portable
devices.
500
470
4
400
3
300
200
148
1
0.5
0.6
45
0.3
0.9
100
0.648
0
DAC
TAC
CAP
CAB
Fig. 1 Ro, Rth of cellulose polymers
At the same time, adjustable Rth for ethyl cellulose
based films was also developed for existing technologies
to meet the various Rth needs in VA/IPS/TN mode LCDs.
As shown in Fig. 2, by addition of silicone additives with
ethyl cellulose, Rth of as-prepared films decreased from
470 nm to 1 nm. Rth can be easily controlled by the ratio of
ethyl cellulose to silicone.
Rth-hypothetical
400
300
200
y = -4.7x + 470
y = -10.133x + 481.48
R² = 0.9958
100
0
0
20
40
60
80
100
Silicone ratio in total solid, %
Fig. 3 Linear relationship of Rth and additive
dosages
This interaction is also helpful for the good miscibility
of ethyl cellulose and silicone polymers. The miscibility
could also be indirectly observed by transparency and
haze testing. As shown in Figure 4, EC/silicone blend
films exhibit very high transparency and ultra low haze.
Across the wide blending ratio 5% to 70%, all blend films
show transparency higher than 92% and haze lower
than 0.5%, which also supports the excellent miscibility
of ethyl cellulose and silicone polymers.
EC/Silicone: Transparency & Haze
Transparency, %
EC/Silicone: Rth
600
500
Rth(60um), nm
Rth-exp
500
9
0
EC
EC/Silicone: Rth
600
400
300
100
5.0
80
4.0
60
3.0
40
2.0
20
1.0
0
0.0
0
200
Haze, %
2
Rth(60um), nm
Ro(60um), nm
5
silicone polymer. By comparison of the experimental
data with the ideal physical blending hypothesis for Rth,
the actual Rth decreases much faster than hypothetic
data. This fast decrease of Rth might be caused by the
strong interaction between the blending polymers, which
weakens the orientation of ethyl cellulose to an isotropic
status.
Rth(60um), nm
3. RESULTS AND DISCUSSION
5
10
20
30
40
50
70
Silicone ratio in total solid, %
100
R² = 0.9995
0
0
20
40
60
Fig. 4 Film transparency and haze
80
Silicone ratio in total solid, %
Fig. 2 Rth of EC films with additives
Although Rth doesn’t decrease linearly with silicone
ratio across the whole range from 0% to 70%, it was found
that Rth first decreases linearly with the increase of the
blending ratio of silicone from 0% to 40%, as shown in
Figure 3. The linear relationship between Rth and blending
ratio reflects the good miscibility of ethyl cellulose and
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Refractive index is a critical parameter to high
transparency and low reflection of films. As shown in
Figure 5, the refractive index of EC/Silicone films is very
similar to pure ethyl cellulose film and only slightly
increases with blending ratio of silicones. The similar
refractive index of ethyl cellulose and silicone resin and
continuous variation with the blending ratio further
supports the excellent miscibility of ethyl cellulose and
silicone resins.
In Plane Retardation: Ro
1.520
1.510
1.500
1.490
1.480
1.470
1.460
Ro, nm (60um thickness)
Refractive Index
EC/Silicone: Refraction Index
R² = 0.9896
0
20
40
60
80
500
400
300
200
100
0
0
20
100
R² =60
0.9885 80
40
100
Stretching ratio, %
Silicone blending ratio, %
Fig. 7 Stretching on Ro properties
Fig. 5 Film refraction index
Ro needs to be lower than 10 nm in IPS mode LCD
retardation films, so EC/Silicone blend film can meet
Ro/Rth requirements in IPS mode LCD very well. However,
in VA mode and TN mode LCD retardation films need a
larger Ro such as 50nm, 125 nm, or higher. Usually, the
large Ro comes from in-plane refractive index difference
which is achieved by thermal stretching of TAC, PC, and
COP films. In this study, film thermal stretching was also
tried to confirm the Ro adjustment capability for ethyl
cellulose films.
In LCD displays, three colors of light with different
wavelength will pass through retardation films and other
films associated with the polarizer. So, flat wavelength
dispersion is preferred for retardation film to avoid color
shifting.
As shown in Figure 8, the EC/Silicone blend film
shows flat wavelength dispersion, which is similar to the
COP film and much smaller than PC and TAC
retardation films. The VA TAC reference film shows
reverse wavelength dispersion, which might be induced
by additives.
As shown in Figure 6, stretched films become white in
cross section polarizing light, while the unstretched part at
each edge of the film is still black, which means the
stretching is effective to increase Ro of EC/Silicone films.
Ro/Ro(590)
Wavelength dispersion
1.1
1.08
1.06
1.04
1.02
1
0.98
0.96
0.94
EC/Silicone
VA TAC
1/4ʄPC
1/4ʄCKP
400
500
600
700
800
Wavelength, nm
Fig. 8 Wavelength Dispersion
Fig. 6 Stretched film under cross polarizing light
Ro is tested by using Kobra birefringence analyzer. As
shown in Figure 7, Ro of the films increase linearly with the
stretching ratio from 0% to 100%. An Ro of 50 nm for VA
mode LCDs will be achieved at stretching ratio of 10% and
125 nm Ro for TN mode 3D LCDs will be achieved at
stretching ratio of 26%. Higher Ro can be achieved via
higher stretching ratio for various TN mode LCD
applications.
4. CONCLUSION
In this study, ethyl cellulose based optical films are
reported for their high out-of-plane retardation and
tunable optics including Ro and Rth. The out-of-plane
retardation of blend films, based on a normal 60 μm
thickness, is tunable from 470 nm to 1 nm. The EC blend
films are also capable of increasing Ro from 0 nm to
more than 400 nm via mechanical stretching technology.
The tunable optics of EC blends make it a viable
compensation film for various LCD modes including VA
mode, IPS mode, and TN/STN mode as well as a
conventional polarizer protection film. Moreover, ethyl
cellulose based films show flat wavelength dispersion
properties which are helpful to reduce the color shift.
Furthermore, the high transparency and low haze
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properties provide ethyl cellulose based films huge
potential in high-end optical film applications.
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[4] Y. Kaneko, et. al, JP2009122172A, KONICA
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MINOLTA OPTO INC, 2007
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