ABSTRACT: Nano-Composite Polymer Optical Coatings Tom Faris Vampire Optical Coatings, Inc. P.O. Box 240 Kirkersville, Ohio 43033 (740)-927-5257 f(740)-927-5032 vampirecoatings@earthlink.net Traditionally broadband anti-reflection coatings and index matching coatings for use with high performance transparent conductive oxide coated films have been prepared via multi-layer dielectric materials deposited under vacuum. These coatings typically have thicknesses between 850 and 2000 Angstroms, with relatively low production rates and a concomitant high cost. 1,2 Advances in nano-particle technology have enabled the development of high refractive index polymer based nano-composite coatings that are processable at normal atmospheric pressures, can be applied, with the required precision at high rates, using conventional wet roll-to-roll coating equipment. These nano-composite based polymer coatings have been used to manufacture 2-layer low reflectance coatings and in combination with thin sputtered metal oxide layers to prepare broadband anti-reflection coatings or as index matching layers to produce high light transmission transparent conductive oxide coated films. In this paper we will report the development of stable nano-composite coatings, which exhibit high refractive indices that are processable at high rates, with high precision at low costs. In addition, we will show that these new polymer coating materials have been combined with traditional vacuum sputtering methods to produce high light transmission ITO coated films and high performance antireflection coated films. INTRODUCTION: In today s world of mobile computing, mobile entertainment systems, cellular phones, global positioning systems, touch panels and low profile instrument panels high contrast in all light environments is a must. Reflection reducing coatings are necessary for these display devices to deliver optimal performance in the high ambient light conditions inherent in automobiles and other mobile displays; however, the high cost of anti-reflection coating/index matching coatings have precluded their use in these displays on a large scale. Light is reflected from each interface whenever there is a change in the refractive index; in addition, the greater the change in refractive index the greater the reflectance. The light transmission of a clear substrate is given by LT(%) = 100 R(%)-Abs(%) 2 For instance, the reflection from PET film is between 5.50 to 6.00 percent per side, which limits the total transmission of PET film to 88 percent. This situation is exacerbated when multiple air/substrate interfaces are present within the display structure; such as, a resistive touch panel. (Figure 1)
Air Gap Figure 1: Reflections from a Resistive Touch Panel R1 R2, R3 R4 R5 PET film ITO coating Display Typically, multi-layer interference coatings are used to reduce the intensity of the light reflected from the screen or panel. Anti-Reflective coatings improve transmission and reduce reflection from each interface coated; thereby, improving the performance of the display. While the display performance is clearly improved with incorporation of AR or other reflection reducing coatings, most displays do not use these coatings due to cost considerations. Typically, there are several routes for production of AR coatings on plastic substrates: A) One can use vapor phase deposited coatings, which consist of alternating layers of high and low refractive index materials. Due to the necessity of an ultra-vacuum, slow deposition rates and an additional anti-smudge coating step, coatings of this type are quite expensive. B) On rigid plastic substrates, one can employ sol-gel chemistry in conjunction with dip coating to apply thin alternating layers to form a multi-layer stack. Dip coating typically requires large tanks of liquid with low usage rates that must be isolated from vibration to produce high quality parts. Coatings produced via this method require a slow thermal cure step that increases the cycle time, and a concomitant increase in the final cost. Finally, these coatings also need an additional step to apply the low energy anti-smudge layer to the AR coating s surface adding to the already high costs. 3 C) What is likely the most common approach today is use of low refractive index fluorinated monomers (Fluoro Link ) and fluoropolymers (Cytop, TeflonAF ) to produce low reflectance surfaces. While coatings produced via this method not only exhibit low surface reflection and also have inherent anti-smudge properties due to fluoro-chemical s low surface energy, it is difficult to produce hard, scratch and solvent resistant coatings or to adhere other coatings to surfaces prepared from these materials. EXPERIMENTAL: Metal oxide nano-particle dispersions were prepared, cast from solution as thin films onto 100 micron PET film, haze, light transmission, reflectance and optical properties measured (Table 1). These coatings were then used to prepare multi-layer thin film interference filters. (Table 2)
Coating Table 1: Nano-Particle Coating Dispersions & Properties Wt% Nanoparticle* Metal Oxide Type Metal Oxide Size Appearance 80.0 ZrO2/Sb2O5/ZnO 20 nm Dry Coating is clear _C 85.0 ATO 15nm Clear Gray/Green coating _S 55.0 Sb2O5 30nm Dry Coating is clear Refractive Index (550nm) 1.7950 1.7455 1.6835 IM002.15 25.0 SiO2 20nm Clear 1.485 IMESD07 70.0 ATO 15nm Gray/Green 1.685 *-Nano-particle wt% is given as a percent of dry coating weight Figure 2: Coated Film Examples A B 3.0 um HC PET Film C Table 2: Example Organo-Nano Composite Coatings ID A B C %LT %Haze %R (Y) %R (450-650nm) VAR1251S N/A IM002.15 (95nm) (80nm) >93.0 % <1.00 % 1.40 % 1.80 2.0 PET N/A N/A N/A 89.0 % <1.00% 5.00% 5.0 5.50 VAR751H MHC (95nm) IMAR ITO (22nm) TiO2 (18nm) IM002.15 (45nm) LR N/A Fluoro (100nm) (102nm) (75nm) (80nm) >93.0% <1.00% 0.80% <1.00 91.0 % <1.00 % N/A N/A >93.0 % <1.00 % 1.00 % 1.35 1.50
RESULTS & DISCUSSION: Stable coatings containing high weight percentages (25 to 85 wt%) of nano-particle metal oxides were prepared and used to manufacture various multi-layer interference filters using commercially available roll-to-roll coating equipment. 1) High Refractive Index Coatings (): Coatings of this type were found to be quite useful as replacements layers for thick vacuum deposited layers in multi-layer broadband Anti-Reflective coatings, as a high refractive index layer for Low Reflectance coatings and as a high refractive index layer in index matched ITO coated films. When used in combination with a thin vacuum deposited layer, in a design such as [UV acrylate/tio2/], acts as a buffer layer in the multi-layer interference filter design; thusly, several thick vacuum deposited inorganic metal oxide layers are eliminated and a broadband Anti-Reflection coating is produced. (Table 2 & Figure 3) When used in combination with a thin vacuum deposited layer, in a design such as [ITO/IM002.15/], a high transmission low reflectance ITO coated film is produced. As compared to the traditional method used to produce high transmission ITO film, several thick vacuum deposited inorganic metal oxide layers are eliminated. (Table 2 & Figure 4) When used in combination with low refractive index outer layers, coatings can be used to produce Low Reflectance coated films. These coatings exhibit low reflectance, high transmission and excellent durability. 2) Low Refractive Index Coatings (IM002.15): Coatings of this type were found to be useful as thick UV cure abrasion resistant coatings, the outer layer in Low Reflectance coatings and as a low refractive index material in the production of index matched ITO coated films. Used in combination with, the Low Reflectance films produced exhibit high abrasion resistance, low reflectance and good solvent/chemical resistance. Depending on the requirements of the application, coatings of this type can include anti-fingerprint additives; such that, an AFP treated Low Reflectance film can be produced without need of an additional processing step. Additionally, when used with in combination with ITO and to produce high transmission low reflectance conductive films, IM002.15 can be formulated such that a high surface energy surface is provided to enhance the adhesion and durability of the ITO layer. 3) Other Coatings/Combinations: Coatings incorporating Antimony Tin Oxide have been prepared. These coatings are useful in multi-layer interference filter design (Electro Static Dissipative (ESD) and High Refractive index function), infrared-blocking filters and as ESD coated films. Multi-layer heat mirrors have been prepared in the lab using only and IM002.15 as the layers; however, these designs and processes require further optimization to be practical on a production basis.
Figure 3: Anti-Reflection, Low Reflection and Vacuum AR comparison % specular reflectance 12 11 10 9 8 7 6 5 4 3 2 1 0 VAR 1251S Y<1.50 %(measured) VAR 751H Y<0.80% (measured) 4-layer stack Y<0.70% (measured) 400 450 500 550 600 650 700 750 800 wavelength(nm) Figure 4: 300 Ohm/sq ITO and Index Matched ITO Film ITO Coated Film Light Transmission % Light transmission 94 92 90 88 86 84 82 80 78 300 Ohm/sq ITO 76 IMAR ITO 74 400 450 500 550 600 650 700 wavelength
CONCLUSION: Metal oxide nano-particle coatings can be used to substitute for or as a compliment to vacuum deposited metal oxides in multi-layer structures such as, Anti-Reflective and index matching coatings. Combinations of wet, UV curable high refractive index coatings with vacuum deposited coatings allow substantial productivity improvements. Coated films have been successfully prepared on a production scale with these coatings that exhibit excellent optical, mechanical and temperature/humidity resistance. REFERENCES: 1) US Patent 6,623,513; Choi, Hyung Chul, Lindholm, Edward P., Smyth, William K., Nagarkar, Pradnya, V.; 3M Company 2) Macleod, HA; Thin Film Optical Filters; 3 rd Edition; 1986 3) US Patent 5,586,018; Chen, Den Guo, Yan, Yongan; Yazaki Corporation