ELECTRO-OPTICAL LIGHT MANAGEMENT MATERIAL: LOW
REFRACTIVE INDEX PRESSURE SENSITIVE ADHESIVES
E. P. Chang, Avery Research Center, Pasadena, CA
Daniel Holguin, Avery Dennison Performance Polymers, Pasadena, CA
Abstract
A prototype pressure sensitive adhesive which has a low refractive index (< 1.36) and is
optically clear has been developed. This pressure sensitive adhesive is particularly useful
for light transmitting devices.(1) The refractive index is a key feature in the application of
optical polymers. By controlling the refractive index of materials, one can manage the
optical properties. The lower the refractive index of the adhesive results in less distortion
of the transmitted light (Figure 1). Fluoro-polymers are known to have low refractive
index, but typically the fluoro-polymers are not sticky and are opaque due to crystallinity.
This pressure sensitive adhesive is a Fluoro-substituted mono-acrylate adhesive soluble in
common solvents (Figure 2). The design of this novel fluorosubstituted monoacrylate
adhesive polymer is based on the rheological marriage of fluoro-polymers and pressure
sensitive adhesives.
Introduction
The objective is directed to the development of optical adhesives useful in light
transmitting devices that help control light distribution, i.e., antiglare, anti-iridescence,
low reflectance and low interference, by employing adhesive polymers with low
refractive index (< 1.36) to obtain the desired light distribution. By combining chemistry,
physics, and adhesive performance the preparation of pressure sensitive adhesive
polymers was achieved having the desired physical properties of refractive index, plus
glass transition temperature, rheology, light transmittance, and adhesion (Figure 3).
An example of an application where a low refractive index adhesive has utility in
obtaining the desired light distribution is a rear projection screen for a television (back-lit
projection screen). The Fresnel collimating lens for use with a projection screen should
maintain the capability of effectively collimating light to provide more uniform
brightness across the screen. It is generally advantageous to reduce light distortion
between the Fresnel lens and other components to help control light distribution (Figure
4). This can be achieved by using a low refractive index adhesive to attach the secondary
component to the Fresnel lens. The Fresnel lens material is glass or poly methylmethacrylate with a refractive index of 1.53 and around 1.49 respectively..
Noting the refractive index of the Fresnel lens material, a significant difference would be
a refractive index less than 1.4. There is only air, water, and fluoro-polymers with a
refractive index less than 1.4 (Figure 5 and Figure 6).
Currently for applications where there is a pressure sensitive adhesive bond components
of the light transmitting devices; lens (or lenses), or other components (i.e. light
management films), the adhesive is acrylic with a refractive index of 1.46-1.47.
There is one light receiving device that uses water as a medium – the eye. The eye
actually uses a natural hydrogel as the medium and has a refractive index of 1.336.
Background
Low Refractive Index Hydrogels
We investigated and developed the use of hydrogels as a low refractive medium(2) .
Figure 7 shows the use of UV curable water soluble oligomers used to make hydrogels.
Water is very effective in making a low refractive index medium, but to get below 1.36
the hydrogel would have to contain more than 80% water. This is a very cost effective
product, but not a likely candidate for a long term pressure sensitive adhesive.
Refractive Index of Fluoro-polymers
Figure 8 shows the correlation of refractive index with the % fluorine in the polymer. It
can be observed that the higher the % fluorine, the lower the refractive index. In order to
achieve a polymer with refractive index < 1.36, the % fluorine content has to be > 57%.
Lorentz-Lorenz Equation and Correlation Model of Groh and Zimmerman
Relationship between refractive index with molar refraction and molar volume is shown
in Lorentz-Lorenz equation (3,4,):
(nD2-1)* MG = R L ------------------------------------------------------(1)
(nD2+2)* ρ
where nD is the refractive index
MG is the repeating unit molecular weight
ρ is the density
RL is the molar refraction
It can be observed from equation (1) that low refractive index can be achieved by either
lowering the molar refraction or increasing the molar volume (MG /ρ).
Groh and Zimmerman plotted the ratio of molar refraction RL to molar volume VL for
different atoms present in organic polymers (5) using previously published data (6-8) . In
spite of the broad range of values for each atom due to different binding structures, and
varying chemical environments, it is apparent that fluorine and to a lesser extent oxygen
lowers the refractive index of a compound (Figure 6).
Experimental
Materials
Based on the above concept, as well a the material properties requirements of pressure –
sensitive adhesives, the preferred molecular structure to investigate was to have
fluorination on the bulky polymer side chain (to increase the molar volume and to
decrease the molar refraction by fluorination) rather than the more typical fluorinated
polymer main chain (poly-tetra-fluoro-ethylene). Also a fluoro-polymer with the side
chain fluorinated was needed for the polymer to be soluble in non-halogenated solvent
for processing (Figure 9).
The fluoro-acrylate monomers used in this work were selected based on commercial
significance (cited on the Environmental Protection Agency TSCA Inventory list):
PDFA = 1H,1H-pentadecafluorooctyl acrylate (Synquest Laboratories, Alachua, FL)
HFBA = 1H,1H-heptafluorobutyl acrylate (Synquest Laboratories)
TFA = 2,2,2-trifluoroethyl acrylate (Synquest Laboratories)
HDFA = 1H,1H,2H,2H-heptadecafluorodecyl acrylate (Synquest Laboratories)
HFIPA = hexafluoroisopropy acrylate (Synquest Laboratories)
TDFA = 1H,1H,2H2H-tridecafluorooctyl acrylate (Synquest Laboratories)
TAN = blend of 1H,1H,2H,2H- fluoroalkyl acrylates (Zonyl TA-N from E. I. DuPont de
Nemours)
and
AA = acrylic acid (Sigma-Aldrich, Milwaukee, WI)
H
O
H
F
F
F
F
F
F
F
C
C
C
C
C
C
C
C
O
F
F
F
F
F
F
F
F
C
H
C
C
H
1H,1H-Pentadecafluorooctyl Acrylate (PDFA)
H
H
O
H
F
F
F
C
C
C
C
O
F
F
F
F
C
H
C
C
H
H
1H,1H-Heptafluorobutyl Acrylate (HFBA)
H
O
H
H
F
F
F
F
F
F
F
F
C
C
C
C
C
C
C
C
C
C
O
H
F
F
F
F
F
F
F
F
F
C
H
C
C
H
1H,1H,2H,2H-Heptadecafluorodecyl Acrylate (HDFA)
H
F
F
F
C
H
C
C
F
O
F
Hexafluoroisopropyl Acrylate (HFIPA)
O
F
C
H
C
H
C
H
O
O
C
C
H
H
H
F
F
F
F
F
F
C
C
C
C
C
C
C
C
H
H
F
F
F
F
F
F
H
1H,1H,2H,2H-Tridecafluorooctyl Acrylate (TDFA)
C
H
O
OH
C
H
Acrylic Acid (AA)
C
C
H
F
H
Refractive Index and Glass Transition Temperature
Table I shows screening experiments using high fluorine content acrylic monomers along
with TFA to disrupt the tendency of high fluorine content acrylic monomers to
crystallize, and AA was used to improve polymer solubility in organic solvent and
provide a site for subsequent ionic crosslinking with AAA (aluminum acetoacetonate).
Pressure sensitive adhesives typically have a Glass Transition Temperature (Tg) of less
than 0oC, therefore HFBA (Experiment #2) and PDFA (Experiment #1) appeared the
most promising candidates. HFBA with the lowest polymer Tg and PDFA with the lowest
refractive index with a reasonably low Tg
Table I: Refractive Index of Different Fluoro-acrylic Copolymers
Table I
Screening Fluoroacrylic Copolymers
Experiment
Copolymers
Number
Ratio 88/10/2
1
PDFA/TFA/AA
2
HFBA/TFA/AA
3
HDFA/TFA/AA
4
HFIPA/TFA/AA
%F
58.9%
49.7%
58.5%
48.8%
RI
polymer
1.359
1.374
1.371
1.376
DSC
o
Tg C
11
0
51
32
Table II is a series of experiments using PDFA and HFBA at various ratios along with
AA. The data shows that for the target Refractive Index (RI < 1.36) the upper limit of
HFBA is 30% along with 2% AA (Experiment #9). But 49% HFBA (Experiment #10)
was needed for a low Tg. Figure 10 shows the effects of PDFA/HFBA/AA composition
on Tg as well as refractive index. It is apparent that as the refractive index decreases, the
Tg increases correspondingly.
Table II: Physical Properties of Varying PDFA/HFBA/AA Copolymers
Table II
Experiment PDFA/HFBA/AA
Number
Copolymers
5
93/5/2
6
88/10/2
7
83/15/2
8
78/20/2
9
68/30/2
10
49/49/2
11
23/75/2
12
0/98/2
13
48/48/4
Varing Copolymer Composition Ratio
%F
60.9%
60.4%
59.8%
59.4%
58.3%
56.3%
53.6%
51.2%
55.2%
RI
polymer
1.356
1.356
1.357
1.357
1.359
1.362
1.366
1.370
1.365
DSC
o
Tg C
21
14
10
7
1
-3
-1
-1
7
DMA
o
Tg C
30
25
22
13
13
6.5
1
6
20
G'
Dahlquist Surface
o
at 20 C
Contact
Energy
o
dynes/cm2 Temp, C dyne/cm
3.3E+07
30.0
9.6
8.00E+06
24.0
7.6
2.00E+06
18.0
5.5
1.50E+06
14.0
3.9
1.30E+06
14.0
4.2
2.00E+06
14.0
4.1
1.60E+06
14.0
5.1
3.00E+06
20.0
4.1
1.00E+07
30.0
13.5
% Transmittance
94.2
92.7
94.2
93.9
93.1
93.7
94.0
93.7
93.0
Table III reduces the amount of AA to 0.2% and we see that a 49.9% HFBA. 49.9%
PDFA, 0.2%AA polymer (Experiment 14) now has a RI of 1.358 meeting the target goal.
Other polymers were made replacing the long fluorine side chain monomer PFDA with
other monomers: TAN (Experiment #15), TDFA (Experiment #16), and HDFA
(Experiment #17), but only the TAN polymer gave a RI < 1.36.
Table III: Physical Properties of Different Fluoro-acrylic/HFBA/AA Copolymers
Experiment Table III
Number
Copolymers
Ratio 49.9/49.9/.2
14
PDFA/HFBA/AA*
15
TAN/HFBA/AA*
16
TDFA/HFBA/AA*
17
HDFA/HFBA/AA*
%F
57.4%
56.8%
54.6%
56.1%
Varying Copolymer Composition
G'
Dahlquist Surface
o
at 20 C
RI
DSC DMA
Contact
Energy
o
o
o
polymer Tg C Tg C dynes/cm2 Temp, C dyne/cm
1.358
-8
1
4.00E+05
3
1.358
-5
1
3.50E+05
2
12.9
1.362
-8
-0.5
5.30E+05
5
9.5
1.359
-5
0.5
4.20E+05
4
9.2
% Transmittance
93.5
92.0
93.2
93.1
Rheology of Fluoro-Acrylate Pressure Sensitive Adhesives
Figures 11 and 12 compare, respectively, the temperature dependence of the dynamic
shear storage modulus, G’, and viscoelastic index, tan δ, for three PDFA/HFBA/AA
polymers with varying ratios of 93/5/2 (i.e., experiment 5), 49/49/2 (i.e., experiment 10,
and 23/75/2 (i.e., experiment 11). It can be observed that the experiment 5 polymer shows
a Tg of 30oC based on the tan δ peak (Figure 13), and the room temperature (20oC) G’ is
3.3x107 dynes/cm2, which does not satisfy (i.e., is higher than) the Dahlquist contact
Efficiency Criterion of 3x 106 dynes/cm2. However, even though the Tg and room
temperature G’ of experiment 11 appear to be the most favorable for a PSA, its refractive
index is slightly too high (1.366). In view of this, experiment 10 polymer was chosen for
further improvement of refractive index and adhesion performance.
AA is non fluorinated and has a higher refractive index than fluorinated acrylics. It is,
however, needed for improved polymer solubility and cross-link sites, as well as for
specific adhesion to polar surfaces such as glass and stainless steel. To reduce the
refractive index further, the concentration of AA was decreased from 2 to 0.2% with a
0.1% AAA (aluminum acetoacetonate) cross-linker to provide good cohesive strength.
The rheological properties of these two copolymers (i.e., experiment 10 and experiment
12) are compared in Figure 11. It is noteworthy that in experiment 12, with lower AA, the
refractive index was reduced (1.362 to 1.358), as were both the Tg (6.5 to 1oC based on
peak) and the room temperature G’ (3.3x 106 to 4x 105 dynes/cm2).
From the bulk properties viewpoint, the lower Tg and lower room temperature G’ would
predict a better PSA. This is reflected in the increase in peel adhesion against low
surface-energy substrates high density poly(ethylene) (HDPE) (0.2 to 0.3 lb/in) and
Teflon (0.1 to 0.4 lb/in) in Table II experiment #10 and experiment #14. However, lower
peel adhesion against high-surface-energy substrates such as glass (2.2 to 1.6 lb/in) and
Stainless Steel (1.9 to 1.4 lb/in) was also observed in Table II experiment #10 and
experiment #14. This reversed trend is consistent with at 10-fold decrease in AA, and
adhesion promoter.
Tables II and III also show the optical clarity (% light transmitted) needed for light
transmitting materials.
Adhesive Properties of Fluoro-acrylate Pressure Sensitive Adhesives
The adhesive properties on Table IV show the polymer from Experiment #10 to have the
best adhesive properties, but the Refractive Index (1.362) is a bit above the target (<1.36).
By reducing the amount of AA in the polymer to 0.2% in Example #14 the Refractive
Index is reduced to 1.358. For use in light management applications the adhesive needs to
be crosslinked. Example #14 show adhesive properties after crosslinking with 0.1% AAA
to have adhesive properties below Example #10, but by reducing the crosslinker level to
0.05% AAA (Example #18) the adhesive properties improve considerably.
Table IV: Adhesion Performance of Different PDFA/HFBA/AA Copolymers
Table IV
Experiment PDFA/HFBA/AA
Number
Copolymers
5
93/5/2
6
88/10/2
7
83/15/2
8
78/20/2
9
68/30/2
10
49/49/2
11
23/75/2
12
0/98/2
13
48/48/4
Copolymers
Ratio 49.9/49.9/.2
14
PDFA/HFBA/AA*
15
TAN/HFBA/AA*
16
TDFA/HFBA/AA*
17
HDFA/HFBA/AA*
18
PDFA/HFBA/AA**
Adhesive Properties
o
Initial, lb/in 50 C***
o
o
90 Peel 90 Peel
RI
%F
polymer
Glass
Glass
60.9%
1.356
0.0 cl
0.1 jp
60.4%
1.356
0.4 jp
0.3 jp
59.8%
1.357
0.5 jp
0.4 jp
59.4%
1.357
0.6 jp
0.5 jp
58.3%
1.359
0.7 jp
1.7 jp
56.3%
1.362
1.3 cl
2.2 cl
53.6%
1.366
1.2 cl
1.5 cl
51.2%
1.370
1.5 cl
1.2 cl
55.2%
1.365
0.6 jp
0.5 jp/tr
57.4%
56.8%
54.6%
56.1%
57.4%
1.358
1.358
1.362
1.359
1.358
1.0 cl
1.0 cl
1.1 cl
1.0 cl
1.3 cl
1.0 cl
na
na
na
1.6 cl
o
o
o
50 C***
o
90 Peel
HDPE
0.0 jp
0.1 jp
0.1 jp
0.1 jp
0.1 jp
0.2 jp
0.2 jp
0.1 jp
0.1 jp
50 C***
o
90 Peel
Teflon
0.0 jp
0.1 jp
0.1 jp
0.1 jp
0.1 jp
0.1 jp
0.1 jp
0.1 jp
0.0 jp
50 C***
o
90 Peel
SS
0.1 jp
0.5 jp
0.5 jp
1.7 jp
1.6 jp
1.9 cl
1.1 cl
1.5 cl
0.5 jp/tr
0.4 jp
0.4 cl
0.2 jp
0.3 jp
0.3 jp
0.3 jp
0.4 cl
0.3 jp
0.4 jp
0.4 jp
1.1 cl
1.2 cl
1.4 cl
1.1 cl
1.4 cl
Shear
minutes
85
22
50
85
15
2
Adhesive coated onto PET film at a coat weight of 25-30 g/m
"cl" indicates clean peel
"jp" indicates jerky peel
"tr" indicates that the adhesive was transferred to the test panel for the PET film
"sp" indicates that the adhesive split apart, leaving residue on the test panel and/or PET film
* with 0.1% by weight Aluminum Acetoacetonate crosslinker
** with 0.05% by weight Aluminum Acetoacetonate crosslinker
*** samples were laminated to test substrate at 50oC, then tested at ambient conditions
Because of the success of Example #18 a larger lab-batch polymer was made and the
physical properties are shown as follows (Figure 13):
• 2 liter lab batch
– PDFA/HFBA/Acrylic Acid 49.9/49.9/0.2
– 69% polymer in Ethyl Acetate solvent
– Viscosity = 3290 cPs
•
Coated Polymer
– Refractive Index = 1.358
– % Light Transmission = 92%
– Surface Energy = 10 dynes/cm
– Tg = -8oC
It is noteworthy because we see that fluoro-acrylate polymers can be made in nonhalogenated solvents with a viscosity in the range for industrial coaters.
Figure 14 shows the adhesion performance of the above adhesive for shear and peel
testing with heat lamination. It is noteworthy that because of its low surface energy (~10
dynes/cm), about half a pound of peel adhesion is achieved against a low surface energy
(~ 18 dyne/cm) Teflon substrate.
Refractive Index Limits of Fluoro-acrylate Polymers
Figures 15 and 16 shows the predicted refractive of a homologous series of fluoroacrylate with increasing the repeating CF2 unit in the side chain based on Groh and
Zimmerman’s model. The lower refractive index with the number of CF2 units
approaching infinity is 1.29. This work demonstrated that a good balance of low
refractive index with good pressure-sensitive adhesion is achieved through the interplay
of both the polymer chemistry and physics.
Conclusions
•
•
•
•
A good balance of low refractive index (<1.36) and good pressure sensitive
adhesion has been achieved through the interplay of polymer chemistry
(employing fluoro-acrylates) and physics (material properties of Tg and
rheology).
Crystallization of polymer (main or side chain) will increase the predicted
refractive index as well as decrease the contact efficiency of PSA’s
Lower refractive index with higher fluorine % raises the Tg
The theoretical lower refractive index limit of organic polymers is predicted to be
around 1.29. Acrylate- based polymers is around 1.33 , while PSA polymers is
around 1.35
References
1. D. Holguin, E. P. Chang, US Patent #6703463, “Optical Adhesive Coating
having Low Refractive Index”, March 19, 2002
2. H.P. Barker, E.P. Chang, D. Holguin, US Patents # 6668431 B2, “Optical Coating
having Low Refractive Index” October 25, 2001
3. H.A. Lorentz., Wied. Ann. Phys. 9 , p. 641 , 1880
4. L.V. Lorenz, Wied. Ann. Phys. 11 , p. 70 , 1880
5. Groh and Zimmermann, 24, p.6660, Macromolecules, 1991
6. American Institute of Physics Handbook, 3rd ed., McGraw Hill, New York, 1972
7. D.W. van Krevelen, P. J. Hoftyzer, J. Appl. Polym. Sci. 13, p.871, 1969
8. D.W. van Krevelen, Properties of Polymers; Elsevier Scientific Publishing Co. ,
Amsterdam, 1976
AVERY RESEARCH CENTER
Why look at refractive index?
Things you can do if you control the refractive index of materials
•
Manage optical properties of transmitted light.
•
Low refractive index material has less light distortion.
Figure 1
AVERY RESEARCH CENTER
Three key aspects for low refractive index
pressure sensitive adhesives
•
A need for amorphous fluoro-polymer with low
modulus and non-crystalline nature (Fluoro side
chain).
Aspect 2
•
The use of fluorinated acrylic monomer to make
Fluoro-Acrylate based polymers.
Aspect 3
•
The need to make Fluoro-Acrylate based polymer in a
common solvent.
Aspect 1
Figure 2
Contents:
AVERY RESEARCH CENTER
1
• Background and Introduction
2
• Polymer chemistry
3
• Physics
– Choice of fluoro-acrylate monomers
– Material properties of Refractive Index,
T g and Rheology
4
Figure 3
• Adhesive Performance
AVERYAdhering
RESEARCH CENTER
a light
management film to a
Fresnel lens
Lens - Adhesive – Light Management Film
Viewer
Light
Source
Figure 4
AVERY RESEARCH CENTER
Refractive index of materials
Material
•
•
•
•
•
•
•
•
•
•
•
Figure 5
Air (vacuum)
Water
Poly(tetrafluoroethylene)
Poly(dimethylsiloxane)
Poly(oxyethylene)
Polyacrylates
Poly(ethylene)
Poly(butadiene-co-styrene)
Glass
Polycarbonate
Poly(ethylene terphthalate) PET
Refractive Index
1.00
1.33
1.35-1.38
1.43
1.45
1.46-1.47
1.51
1.53
1.53
1.59
1.64
AVERY RESEARCH CENTER
Refractive Index of UV-Cured Hydrogel
as a function of % water
Refractive Index
Oligomer
Ratio #2/#1
1.43
1.42
1.41
1.4
1.39
1.38
1.37
1.36
1.35
1.34
1.33
20
30
40
50
60
70
80
90
100
Percentage Water in Gel (%)
Figure 7
AVERY
RESEARCH CENTER
Correlation
of Experimental
Refractive Index
with % Fluorine
RI
% F
Correlation of Refractive Index with % Fluorine
% Fluorine
1.356
1.362
65
1.361
1.359
60
1.357
1.357
1.356
55
1.366
1.37
50
1.365
1.35
1.367
1.356
Figure 8
60.4
56.3
56.3
58.3
59.4
59.8
60.9
53.6
51.2
55.2
1.355
52.3
57.5
1.36
1.365
Refractive Index
1.37
1.375
Theoretical Lowest Refractive Index of fluoro-acrylate homologs
H
C
O
R
CH2
n
C
CH2
Calculated
Refractive Index
Refractive Index
estimated on %F
O
Side Chain R =
CF3
C=1
C=4
CF2
F 2C
CF3
CF2
1.400
1.393
1.352
1.363
CF2 CF3
CF2 CF2
C=8
CF2 CF2
F 2C
CF2 CF3
CF2
1.330
1.357
1.320
1.345
CF2 CF2
CF2 CF2
CF2 CF2
F 2C
CF2 CF3
CF2 CF2
C=12
CF2 CF2
CF2
CF2 CF2
CF2 CF3
CF2 CF2
CF2 CF2
CF2 CF2
CF2 CF2
CF2 CF2
C=16
F 2C
CF2 CF2
CF2 CF2
1.311
1.341
CF2 CF2
CF2
CF2 CF2
CF2 CF2
CF2 CF2
C=20
Figure 15
F 2C
CF2 CF2
1.311
1.339
CF2
Theoretical lower refractive index limit of fluoroacrylates based on Groh and Zimmerman’s model.
MG, Mol.
RL Molar VL Molar
%
Fraction Volume Wt repeating
3
Flourine cm /mole cc/mole unit g/mole
RL/VL
Density
g/cc
MG/VL
Estimated
Calc'd. nD nD from %
Refractive flourine
Index experiment
CF3
37.0
24.9
102.2
154
0.244
1.51
1.400
1.393
C4F9
56.3
40.0
185.1
304
0.216
1.64
1.352
1.363
C8F17
60.1
60.3
295.4
504
0.204
1.71
1.330
1.357
C12F25
67.5
80.5
405.8
704
0.198
1.74
1.320
1.345
C16F33
69.4
100.8
516.8
904
0.193
1.75
1.311
1.341
C20F41
70.6
121.0
627.3
1104
0.193
1.76
1.311
1.339
0.183
1.81
1.293
1.329
#C's approaches
infinity
Figure 16
76