With the recent boom in touch-based and head-mounted polymer displays, a need has arisen to improve the ability of polymer display substrates to resist fingerprints from users. To combat this issue, manufacturers have turned to the thin-film coating industry for a solution originally pioneered in the ophthalmic lens industry. The solution at present time is in the form of a fluorocarbon based thin-film coating referred to as hydrophobic or oleophobic.
Oleophobic literally means “fear of oil” and refers to the physical property of a molecule that is repelled from oil. The most common oleophobic substance is water, however for evaporative coating technologies fluorocarbons are deposited to the substrate to create a monolayer that resists common fingerprint oils. An additional benefit of evaporative oleophobic treatments is also hydrophobicity or water shedding capabilities. This benefit is true of all evaporative oleophobic treatments, but not necessarily inherent in all hydrophobic treatments. The key differentiator between oleophobic and hydrophobic treatments is measured through contact angle and surface energy. Oleophobic treatments routinely have a contact angle of 105-120° as measured with a goniometer, whereas hydrophobic coatings typically have a contact angle of ≤ 95°.
Although no true “Fingerprint rejecting” coating exists today, oleophobic treatments are the closest solution currently available. Despite the fact that these coatings do not “reject” fingerprints they do force the oils in the user’s fingerprints to bead for easier cleaning, and because the fingerprint does not stay fully intact due to oils beading on contact, the appearance of the fingerprints is much less noticeable on the display. Additionally, because the fingerprint cannot “fuse” to the display, cleaning fingerprint oils is far easier with a coating present. An inherent benefit of including an oleophobic coating on the air surface of your substrate is also a higher resistance to abrasion based on the high contact angle of the coating, essentially deflecting potential abrasives from the surface. Furthermore, the ease of cleaning an oleophobic coating lends to a polymer optic, one could gather that the user will be abrading the surface of the optic less periodically, leading to a longer lifecycle of effective use.
With the appropriate vacuum deposited adhesion layer, an oleophobic or hydrophobic coating can essentially be applied to any polymer or glass-based substrate. Typically, the treatment is being utilized on polycarbonate, PMMA, Zeonex E48R, and thin (≤0.5mm thick) chemically strengthened glass.
Although Polycarbonate, Zeonex E48R, OKP4, PMMA, CR-39 and glass are all typically accepted optical substrates, other oleophobic or anti-fingerprint coatings can also be applied to non-optical surfaces such as ABS and numerous non-optical polymers and materials.
Currently oleophobic and hydrophobic coatings are applied through vacuum and or vapor deposition processes. Vacuum deposition of oleophobic and hydrophobic materials occurs in a vacuum pumped chamber where an energy source (typically an electron beam) vaporizes a specific dielectric material that vacuum seals to the substrate’s outer layer. Typical production vacuum chambers have “pockets” or crucibles of varying evaporative materials that rotate to meet the predetermined layers of a coating stack. Once the appropriate thickness of dielectric material is deposited to the substrate, the pocket of dielectric material is then shut and the oleophobic material rotates into place and is vaporized. While airborne, the oleophobic evaporative material will create a permanent chemical bond with the already deposited dielectric material, therefore there is never a need to “reapply” the coating once it is in the field. Oleophobic and hydrophobic coatings form what is called a mono-layer over the dielectric adhesion layer, and due to the thin-nature of the coating, offer negligible effect to the optical properties of the substrate.
Many applications that require or call for fingerprint reducing coatings also require additional thin-film optical coatings such as anti-reflective treatments, and Indium Tin Oxide transparent conductive oxide coatings. In this application, the formula is designed in such a way that the air layer of the coating is calculated into the refractive index of the coating to not hinder the optical performance of the film, but to also lend itself so that it may act as a “catch” or adhesion layer for the oleophobic materials.
At the microscopic level, thin-film coatings are a series of peaks and valleys that promote destructive interference of light. The mono-layer oleophobic coating essentially fills these peaks and valleys, and because it is a mono-layer in terms of thickness the spectral performance of the coating is not compromised.
Design Considerations and Challenges
Among the most critical aspects of lens design when anticipating the need for an oleophobic or hydrophobic treatment is the geometry of the lens itself. Because oleophobic and hydrophobic coatings rely on a dielectric adhesion layer, optical lenses with steep attack angles and deep recesses do not lend themselves to some deposition techniques. Most electron-beam, ion-assisted vacuum deposition systems rely on “line of sight” deposition, meaning the surface of the substrate facing the evaporative material will receive the intended uniform deposition of material. Deep concave optical structures such as domes and other large significantly concave aspheric lenses do not lend themselves to the electron-beam, ion assisted deposition process because of “fall-off” of coating materials at the most oblique angles. Because this “fall-off” of material is not a uniform layer structure, adhesion in these specific zones is somewhat suspect and can sometimes lead to adhesion failure in either an isolated area of the film (nearest to the oblique angle of the substrate) or a catastrophic adhesion failure where the entire film structure is compromised due to poor edge adhesion. Most small aspheric lenses can be successfully coated on both the concave and convex surfaces.
Secondary to the geometry of lens design is lens material. To date, vacuum deposited oleophobic and hydrophobic coatings have successfully adhered to polycarbonate, PMMA (acrylic), Zeonex E48R, Zeonex F52R, OKP4, and a few various grades of polystyrene. Some Cyclic Olefin Copolymers (COC) are already inherently hydrophobic, so gaining strong adhesion to these materials is quite challenging. Some promising results have been made with the use of pre- chamber plasma and corona discharge treatments to aid adhesion, but these methods are still in research and development stages and are not recommended for production-based programs.
Another design consideration and/or challenge is in the lens assembly process. Hydrophobic and oleophobic coatings in almost all cases resist standard bonding agents, so using basic optical epoxies when assembling treated lenses to frames or bezels is not straightforward. In the case where a treated lens will be assembled into a frame and or bezel it is important to specify that the clear aperture of the lens or non-critical area such as a flange or edge to the lens be masked during the deposition process. This masking technique can be either a true physical mask with a crude hand applied medium, or in production volumes this mask can be incorporated into the vacuum coating fixtures to reduce handling and make the clear aperture uniform among coated optics.
Measuring for Efficiency
To date there are few agreed upon national or international standards for measuring the efficiency of an oleophobic or hydrophobic coating. There are a few scientific and a few crude mechanical ways to measure how efficient an oleophobic or hydrophobic coating is performing, yet there is not an agreed upon standard in the optical coating industry.
The simplest way to measure the efficiency of an oleophobic or hydrophobic coating is through the use of a goniometer, which is an instrument that measures the contact angle of a drop of liquid. The contact angle of a drop of liquid can be measured by producing a drop of liquid on a solid. The angle formed between the solid/liquid interface and the liquid/vapor interface is referred to as the contact angle. The most widely-accepted method for measurement involves looking at the profile of the drop and measuring two-dimensionally the angle formed between the solid and the drop profile with the vertex at the three-phase line as shown in Figure 2.
This measurement is performed by utilizing Young’s Equation which defines the balance of forces caused by a drop of liquid on a dry (perfectly flat/planar) surface. In the case of a hydrophobic or oleophobic coated surface the contact angle of a drop of water will be larger. The Young equation is calculated as follows:
θ is the contact angle
γsl is the solid/liquid interfacial free energy
γsv is the solid surface free energy
γlv is the liquid surface free energy.
Standard grade hydrophobic coatings used in the ophthalmic industry typically have a contact angle of 90-95°. Oleophobic coatings typically have a contact angle of 110-115°.
One crude method to “measure” or display the efficiency of an oleophobic or hydrophobic coating is a simple “Post-It” note test. In simplest terms a standard office Post-It® note is applied to the lens. Because oleophobic and hydrophobic coatings have a high surface energy, the Post-It note should not adhere to the surface. In the case of a hydrophobic coating, the Post-It note may adhere but you will not be able to lift the lens with the Post-It note. In the case of an oleophobic coating, the Post-It note will not adhere at all to the substrate.
A second crude method to measure the efficiency of an oleophobic or hydrophobic coated substrate is the Sharpie Marker test. In this test you essentially attempt to write on the coated surface with a standard Sharpie® marker. Because the hydrophobic and/or oleophobic coating resists oils and moisture the marker will not successfully write on the coated substrate. As you apply ink to the coated substrate you’ll notice that the ink beads on contact and will not form a contiguous line. After applying the ink you’ll also notice that wiping the ink from the surface is extremely simple.
The third and final crude test for measuring the efficiency of an oleophobic or hydrophobic coated substrate is the “velvet board test”. This test is performed wherein two coated substrates are placed on a velvet lined board and the board is slowly lifted angularly from the ground or tabletop. The substrate with the more efficient coating will travel faster down the velvet lined board at a lower angle of incidence.
Obviously, the Velvet Board Test, the Sharpie Marker test and Post It Note test produce no quantitative data that can be used to empirically validate the performance of one hydrophobic treatment versus another. However, with the obvious performance that can be viewed from these aforementioned tests, it is usually quite clear which treatments perform to the levels of their claims.
- Glocker,and I. Shah (editors), [Handbook of Thin Film Process Technology, Vol.1&2] Institute of Physics (2 vol. set) (2002).
- Mahan, John E. [Physical Vapor Deposition of Thin Films] John Wiley & Sons, 110-118 (2000).
- Ohring, Milton [Materials Science of Thin Films: Deposition and Structure, 2nd Edition] Academic Press, 232-237 (2002).
- Tadmor, Rafael “Line energy and the relation between advancing, receding and Young contact angles”. American Chemical Society, Langmuir 20 (18): 7659 (2004).