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 methyl-methacrylate 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,): (nD

2-1)* MG = R L ------------------------------------------------------(1) (nD

2+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)

CO O

C

CC

C C C

F

F F

F F

F

C C

F

F F

C

F

C

F

F

H

H

HH

H

F

F

F

1H,1H-Pentadecafluorooctyl Acrylate (PDFA)

1H,1H-Heptafluorobutyl Acrylate (HFBA)

CO O

C

CC

C C C

F

F F

F F

F

F

H

H

HH

H

F

F

FF

F

CO O

C

CC

C C C

H

H F

F F

F

C C

F

F F

C

F

C

F

F

H

H

HH

H

F

C

F

C

1H,1H,2H,2H-Heptadecafluorodecyl Acrylate (HDFA)

Hexafluoroisopropyl Acrylate (HFIPA)

FF

F

CO O

C

CC

C F

F

FC

H

HH

H

CO

O C

C

C

C C C C C C C F

H

H

H

H

H

H

H F

F

F

F

F

F

F

F

F

F F

F

1H,1H,2H,2H-Tridecafluorooctyl Acrylate (TDFA)

CO OH

CC H

H

H

Acrylic Acid (AA)

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 RI DSCNumber Ratio 88/10/2 %F polymer TgoC

1 PDFA/TFA/AA 58.9% 1.359 112 HFBA/TFA/AA 49.7% 1.374 03 HDFA/TFA/AA 58.5% 1.371 514 HFIPA/TFA/AA 48.8% 1.376 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 Varing Copolymer Composition RatioG' Dahlquist Surface

Experiment PDFA/HFBA/AA RI DSC DMA at 20 oC Contact Energy % Trans-Number Copolymers %F polymer TgoC TgoC dynes/cm2 Temp, oC dyne/cm mittance

5 93/5/2 60.9% 1.356 21 30 3.3E+07 30.0 9.6 94.26 88/10/2 60.4% 1.356 14 25 8.00E+06 24.0 7.6 92.77 83/15/2 59.8% 1.357 10 22 2.00E+06 18.0 5.5 94.28 78/20/2 59.4% 1.357 7 13 1.50E+06 14.0 3.9 93.99 68/30/2 58.3% 1.359 1 13 1.30E+06 14.0 4.2 93.1

10 49/49/2 56.3% 1.362 -3 6.5 2.00E+06 14.0 4.1 93.711 23/75/2 53.6% 1.366 -1 1 1.60E+06 14.0 5.1 94.012 0/98/2 51.2% 1.370 -1 6 3.00E+06 20.0 4.1 93.713 48/48/4 55.2% 1.365 7 20 1.00E+07 30.0 13.5 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 Varying Copolymer Composition

Number G' Dahlquist SurfaceCopolymers RI DSC DMA at 20 oC Contact Energy % Trans-

Ratio 49.9/49.9/.2 %F polymer TgoC TgoC dynes/cm2 Temp, oC dyne/cm mittance14 PDFA/HFBA/AA* 57.4% 1.358 -8 1 4.00E+05 3 93.515 TAN/HFBA/AA* 56.8% 1.358 -5 1 3.50E+05 2 12.9 92.016 TDFA/HFBA/AA* 54.6% 1.362 -8 -0.5 5.30E+05 5 9.5 93.217 HDFA/HFBA/AA* 56.1% 1.359 -5 0.5 4.20E+05 4 9.2 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 Adhesive PropertiesInitial, lb/in 50oC*** 50oC*** 50oC*** 50oC***

Experiment PDFA/HFBA/AA RI 90o Peel 90o Peel 90o Peel 90o Peel 90o Peel ShearNumber Copolymers %F polymer Glass Glass HDPE Teflon SS minutes

5 93/5/2 60.9% 1.356 0.0 cl 0.1 jp 0.0 jp 0.0 jp 0.1 jp6 88/10/2 60.4% 1.356 0.4 jp 0.3 jp 0.1 jp 0.1 jp 0.5 jp7 83/15/2 59.8% 1.357 0.5 jp 0.4 jp 0.1 jp 0.1 jp 0.5 jp8 78/20/2 59.4% 1.357 0.6 jp 0.5 jp 0.1 jp 0.1 jp 1.7 jp9 68/30/2 58.3% 1.359 0.7 jp 1.7 jp 0.1 jp 0.1 jp 1.6 jp

10 49/49/2 56.3% 1.362 1.3 cl 2.2 cl 0.2 jp 0.1 jp 1.9 cl11 23/75/2 53.6% 1.366 1.2 cl 1.5 cl 0.2 jp 0.1 jp 1.1 cl12 0/98/2 51.2% 1.370 1.5 cl 1.2 cl 0.1 jp 0.1 jp 1.5 cl13 48/48/4 55.2% 1.365 0.6 jp 0.5 jp/tr 0.1 jp 0.0 jp 0.5 jp/tr

CopolymersRatio 49.9/49.9/.2

14 PDFA/HFBA/AA* 57.4% 1.358 1.0 cl 1.0 cl 0.4 jp 0.3 jp 1.1 cl 8515 TAN/HFBA/AA* 56.8% 1.358 1.0 cl na 0.4 cl 0.4 cl 1.2 cl 2216 TDFA/HFBA/AA* 54.6% 1.362 1.1 cl na 0.2 jp 0.3 jp 1.4 cl 5017 HDFA/HFBA/AA* 56.1% 1.359 1.0 cl na 0.3 jp 0.4 jp 1.1 cl 8518 PDFA/HFBA/AA** 57.4% 1.358 1.3 cl 1.6 cl 0.3 jp 0.4 jp 1.4 cl 15

Adhesive coated onto PET film at a coat weight of 25-30 g/m2

"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 non-halogenated 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 fluoro-acrylate 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

• Manage optical properties of transmitted light.

• Low refractive index material has less light distortion.

Why look at refractive index?

Things you can do if you control the refractive index of materials

Figure 1

AVERY RESEARCH CENTER

• A need for amorphous fluoro-polymer with low modulus and non-crystalline nature (Fluoro side chain).

• The use of fluorinated acrylic monomer to make Fluoro-Acrylate based polymers.

• The need to make Fluoro-Acrylate based polymer in a common solvent.

Three key aspects for low refractive index pressure sensitive adhesives

Aspect 1

Aspect 2

Aspect 3

Figure 2

AVERY RESEARCH CENTER

• Background and Introduction

• Polymer chemistry– Choice of fluoro-acrylate monomers

• Physics– Material properties of Refractive Index,

Tg and Rheology

• Adhesive Performance

Contents:

1

2

3

4

Figure 3

AVERY RESEARCH CENTER

Lens - Adhesive – Light Management Film

LightSource

Viewer

Figure 4

Adhering a light management film to a Fresnel lens

AVERY RESEARCH CENTER

• Air (vacuum)• Water• Poly(tetrafluoroethylene)• Poly(dimethylsiloxane)• Poly(oxyethylene)• Polyacrylates• Poly(ethylene)• Poly(butadiene-co-styrene)• Glass• Polycarbonate• Poly(ethylene terphthalate) PET

Refractive Index

Refractive index of materials

1.001.331.35-1.381.431.451.46-1.471.511.531.531.591.64

Material

Figure 5

AVERY RESEARCH CENTER

1.331.341.351.361.371.381.391.4

1.411.421.43

20 30 40 50 60 70 80 90 100

Percentage Water in Gel (%)

Ref

ract

ive

Inde

x

Oligomer Ratio #2/#1

Refractive Index of UV-Cured Hydrogel as a function of % water

Figure 7

AVERY RESEARCH CENTERCorrelation of Experimental Refractive Index with % Fluorine

RI % F

1.356 60.41.362 56.31.361 56.31.359 58.31.357 59.41.357 59.81.356 60.91.366 53.61.37 51.2

1.365 55.21.367 52.31.356 57.5

Correlation of Refractive Index with % Fluorine

50

55

60

65

1.35 1.355 1.36 1.365 1.37 1.375

Refractive Index

% F

luor

ine

Figure 8

of fluoro-acrylate homologs

C

O

O

CH2

C CH2

H

R

n

CF3

CF2CF2

CF3F2C

Calculated

Refractive Index

Side Chain R =

CF2 CF2

CF2 CF2

CF2 CF3

CF2F2C

F2C CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF3

F2C CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF3

F2C CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF2

CF2 CF3

Refractive Index

estimated on %F

C=4

C=8

C=12

C=16

C=20

C=1 1.400 1.393

1.352 1.363

1.330 1.357

1.320 1.345

1.311 1.341

1.311 1.339

Theoretical Lowest Refractive Index

Figure 15

Theoretical lower refractive index limit of fluoro-acrylates based on Groh and Zimmerman’s model.

Figure 16

EstimatedRL Molar VL Molar MG, Mol. RL/VL Density Calc'd. nD nD from %

% Fraction Volume Wt repeating g/cc Refractive flourineFlourine cm3/mole cc/mole unit g/mole MG/VL 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

#C's approaches 76 0.183 1.81 1.293 1.329infinity