Coating Materials News Volume 10 Issue 4
CMN Wraps Up 10th Year With A Second Look At Coatings On Polymer Optics
Year 2000 issues of Coating Materials News (CMN) have been devoted to reviewing and updating some of the most popular topics that we have covered over the past decade. Issue 4 is concerned with coatings on polymer optics, including ophthalmic AR multi-layers. The challenges and differences between coating glass surfaces and polymeric materials are presented in the following discussion.
Coatings for Polymer Substrates
The acceptance of polymer materials as replacements for glass substrates in some applications has led to the requirement to deposit thin film coatings on their surfaces. Some uses for polymer materials include display screen covers, panels, protective overlays and lenses for cameras. Many square km of polymer film are used as window laminates and packaging for food and commercial goods. The surface might be coated to reduce reflection in an optical application or to reduce oxygen and water permeability in food storage. In the case of lenses, a major benefit has been the ability to employ lower cost mass production techniques such as injection molding. Examples of demanding optical quality in mass produced volumes is the aspheric lens assembly used in CD and DVD players and writers. Diffraction-limited beam quality is required to maximize data storage capacity. While plastic elements are being introduced as elements in photographic lenses, their inherent thermal instability related to high thermal expansion coefficient limits the consistency of the image quality possible. Another limitation is their basic softness. Acceptable surface polish can be obtained by replication in a highly polished master injection mold rather than direct polishing, which generally produces higher scatter. Low surface scatter is essential for high quality photographic and visual optics. .
Examples of polymer materials in film and rigid substrate form that are coated for various applications include PET, polycarbonate, polyimide, and acrylic [*]. Polyimide has the highest service temperature and acrylic the lowest. Applications include packaging and specially molded complex self-assembly components.
A common application for polymer optics is corrective eyewear. Until recently CR-39 was the material of choice over glass and polycarbonate. To obtain high polish (low scatter) surfaces CR-39 lenses must be cast in glass molds, an expensive process. CR39 displaced spectacle crown, and the CR39 was displaced by polycarbonate. CR39 is cast because the polymerization actually takes place during the casting process. The post-polishing step is not necessary for obtaining good surfaces, but normally only a spherical base curve is formed by casting. Often, the curve which imparts the prescription properties is generated by surfacing the rear face. CR-39 is not impact resistant and has a low refractive index of 1.49, making AR coating a challenge. The trend has been toward polycarbonate to take advantage of its high impact resistance and higher 1.57 index. However, polycarbonate is soft and scratches easily compared with glass and CR-39, so a durable hardcoat is needed to provide scratch resistance. The hardcoat application is a spin-on chemical solution followed by curing. Acrylic hardcoat treatment additionally enables surface adherence for the AR coating that otherwise was not obtainable.
The refractive indices of some plastics used in ophthalmic eyewear are higher than or similar to those of glass, so anti-reflective coatings are required to reduce glare and flare. This provides an advantage easily appreciated by a simple comparison between coated and uncoated eyeglasses. The index of polycarbonate is higher than that of glass, and an efficient AR coating can be applied to reduce the average reflection to ~0.5%. Dielectric multi-layer coatings can also provide some degree of scratch resistance, however the chemical "hard coat" of a few µm thickness provides most of the scratch resistance.
Ophthalmic lens coatings were discussed in a dedicated issue: CMN V7, Issue 4 (Oct.-Dec. 1997), and polymer coatings in CMN V8, Issue 3 (July-Sept. 1998). Rapid progress has since been made in the development of wide-band ophthalmic coatings for plastic eyewear, and it is now possible to order a plug-and-play coating system for the ophthalmic lab or production house. We discussed the special problems associated with coating the less-than-inert surface and chemistry of a polymer substrate. Preparation for coating included removal of surface and subsurface water, elimination of condensed contaminants and outgassing products such as low molecular weight components, and conditioning the molecular chain to form bonds to foreign atomic subgroups. In some cases a post-deposition treatment can be included to improve coating durability, however one tries to avoid that added process expense. Polymers absorb water continuously, and must be dried out after chemical / aqueous surface cleaning to insure good coating adhesion. Washing in special hot aqueous detergent and rinse cycles, followed by a dehydrating step is necessary. The lenses are then to be held at an elevated temperature to discourage further water absorption before loading to be coated. It is far easier to achieve adhesion on a hard-coated surface, especially given the rigorous environmental and durability requirements imposed on eyewear. The hard coated lens blank is supplied to the dispensing lab for AR coating, edging, tinting, and so on.
With the surface prepared for coating as described, further steps are necessary to achieve a strong bond to the fundamentally weak polymer crosslinked groups. A process that promotes chemical bonding of O, C, or CH groups to metal oxides is the key to obtaining a strong bond. An activated electron or ion plasma in contact with the surface serves this purpose. The traditional "glow discharge" surface cleaning is well suited for this task when modified to produce the appropriate energies and exposure time without causing damage to the polymer surface. A plasma consisting of ~10% oxygen in Argon appears to be the correct mixture. In some cases pure Ar is used to first abrade the surface and O2 is admitted before deposition begins. As a further initiator of bonding strength, many AR designs include a very thin layer of a metal oxide such as chromium oxide. The layer is too thin to be optically important, but can provide the difference between long- and short-term mechanical / chemical durability.
Polymeric materials are ideal for creating molded mirror substrates of complex shapes. Advanced illumination systems in automobiles, aircraft, and compact display units make maximum use of the fit-to-form and lightweight advantages of plastic reflectors. Such surfaces, produced in high volume, are metallized (aluminum) and the metal is protected from abrasion and corrosion with a dielectric layer system (silicon monoxide or alumina). Generally, the rms surface roughness requirements are not as rigid as for refractive optics.
More than one deposition technique has been tried in the search to economize the coating deposition process without sacrificing durability or yield. Electron-beam and resistance-heated evaporation are proven favorites, but with these techniques, measures must be in place to avoid increasing the temperature of the plastic surface to the point where outgassing of unreacted monomers or actual decomposition occurs. The maximum temperature for coating polycarbonate and CR-39 surfaces is 100°C. Ion-assisted deposition (IAD) techniques are employed to achieve a balance of coating hardness and adhesion without exceeding the service temperature of the plastic. In order to prevent cracking, coatings of µm-thick alumina (employed on several different polymers for abrasion resistance) have been made using IAD to counterbalance the normally present tensive stress by introducing compressive stress. PET (polyethylene terepthalate), PC (polycarbonate), and PI (polyimide) materials have been successfully treated, but PVC (polyvinyl chloride) continued to exhibit coating cracking .
Sputter deposition processes can be automated more easily than the thermal processes and provide potentially better repeatability. Sputtering can deposit a fundamentally harder, more adherent coating, but other problems, such as layer stress and higher cost for lower batch throughput, reduce this potential benefit. Thus, standard thermal deposition technology is currently in place in the coating of ophthalmic lenses. An automated coating cycle consists of cleaning / conditioning of the surface by glow discharge exposure, followed by alternate cycling of each of the two-dielectric material layers, and finally, venting. The most commonly used metal oxide layers are TiO2 and SiO2, and compose the 4-or 5-layer AR stack. An optimum design requires an index intermediate to the TiO2 (n = 2.20) and SiO2 (n = 1.45) specifically 1.7. Such a design provides a high index contrast and a wide-band AR coverage. Rather than involve the additional difficulty of a third material and source, modern deposition automation permits the required intermediate 1.7 index to be produced as a combination of thin layers of the high- and low-index materials. We have presented the suggested deposition parameters for SiO2 and TiO2 in previous CMN issues along with a sample design and predicted performance. More efficient AR designs can be realized using non-oxides, for example CERAC’s CIROM®- IRX and MgF2, that provide nearly the optimum index values.
Polymeric materials have been substituted for glass in many applications: refractive and reflective as well as protective, all requiring some amount of optical or mechanical coating development. It is expected that continuing improvements in deposition technology and material selection will result in further improvements in quality.
*Newsletters beginning with Vol. 6 are available from the CMN archives page of this web site. Contact CERAC for printed copies of referenced newsletters prior to Vol. 6.
If you have a question or a topic you would like us to consider for a future issue of CMN, e-mail your requests to firstname.lastname@example.org or fax them to 414-289-9804.
(S.F. Pellicori is available for private consulting on matters concerning optical thin films. Please contact him directly for more information)
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