Silane-Based Coatings

May 1, 2010
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The preservation and protection of glass and AR coatings is critical to maintaining the lifetime and functionality of products such as lenses, windows and display screens.



Optical glass surfaces are present in a variety of critical indoor and outdoor service environments over a wide range of temperatures and humidity levels. For example, they are present in applications from industrial gauges to binocular and telescopic lenses for sporting and hunting, as well as automotive windows, technical glass such as touchscreens, display glass, and architectural glass.

Glass is renowned for its exceptional transparency and wide service temperature range, and it features excellent hardness and resistance to scratching compared to optically transparent polymers. Despite these qualities, glass is not a perfect material. It is made up of fused silica and, as such, is susceptible to weathering and scratching from abrasives like sand and sharp metal or mineral-containing objects. It is also subject to chemical pitting and degradation as a result of exposure to water, salt or caustic materials. Once scratched or pitted, the optical clarity of the glass is lost and the lens, window or display surface must be replaced, often at great expense. Preservation of the pristine surface of the glass extends the useful lifetime of the object.

Glass is also moderately reflective; visible light transmission drops by 4% at each glass/air interface. To improve light transmission for critical optical applications, antireflective (AR) coatings are applied to each surface, generally by chemical vapor deposition (CVD) or physical vapor deposition (PVD) methods. AR coatings are made up of either multi-layer stacks of SiO2 and a higher refractive index material or a single-layer coating commonly made of MgF2.

Silane Monolayers

The porous nature of AR coatings makes them susceptible to contamination; exposure to oils and soils such as simple fingerprints leaves an indelible deposit of material in the AR layer that cannot be removed without destroying the coating. Protection of the AR layer is key to the coatings' use and acceptance in many user environments such as on eyeglasses or flat panel television screens. Preservation and protection of glass and AR coatings is also critical to maintaining the lifetime and functionality of products such as lenses, windows and display screens. Similarly, "glass-like" surfaces such as ceramic tile and ceramic-glazed objects also benefit from protection from mechanical abrasion and chemical pitting.

A leading method for protecting these silica-containing surfaces is through the application of hydrophobic silane monolayers. These monolayers consist of an inorganic head group that binds covalently to the silaceous surface and an organic tail that self-organizes to form a dense network of carbon chains. Because these coatings are only one monolayer thick, their dimensions are no more than 10 nm in thickness. They are completely "invisible."

The chemistry of self-assembled silane monolayers on glass has been studied for several decades. Organic-functionalized silanes with three hydrolysable groups, such as a halogen or alkoxide, are applied to a glass surface via immersion, vacuum evaporation, spraying or wiping. The silane head group forms a covalent bond to the native hydroxyl groups on the surface of the glass, expelling HCl, in the case of a trichlorosilane RSiCl3, or alcohol, in the case of a trialkoxysilane (RSi(OR')3). Self-condensation between adjacent silane molecules cross-links the monolayer into a durable two-dimensional film.

The chemistry of the functional group R determines the macroscopic properties of the coating. A long alkyl chain (C8 or greater) can form regular packing arrangements along the Cn bonds, terminating in a near-perfect CH3 field.1 These monolayers are significantly hydrophobic, with static water contact angles approaching a value of 120° under ideal deposition conditions.2,3

The addition of fluorine groups along the alkyl chain increases the water contact angle and improves the thermal resistance of the monolayer.4 The functionalization of the end group of the chain with a reactive OH or NH2 group can lead to a hydrophilic surface,2 while the inclusion of polymerizable groups provides the opportunity for cross-linking of the chains at both the foot and head of the monolayer.

In addition to protecting large surfaces such as windows or display screens, the small size of a silane monomer allows the self-assembled monolayers to conform to almost any geometry, as long as a silica-type surface is present. For example, silane self-assembled monolayers have long been used to coat the curved surfaces of silica nanoparticles to alter the functionality and polarity of the nanoparticle surface. Functionalized nanoparticles can then be cross-linked with acrylic monomers when coated with methacryloxypropylsilane or with epoxy monomers when coated with glycidylpropylsilane, resulting in hybrid silica-embedded organic coatings.5

Figure 1. Intensity of 2916 cm-1 CH2 peak in mapping FTIR spectrum, normalized to 1.

Formulation and Application

A key component to the success of these coatings is a highly reactive formulation that is applied and cured under ambient conditions without any external catalyst such as heat or acid.* The coating initially grafts to the substrate within seconds after it is applied at room temperature. While a moderate amount of humidity is required to facilitate cross-linking, most users can obtain near-complete grafting and cross-linking within minutes of application at temperatures of 55-80°F and 30-70% RH. Many commercial products that promise similar hydrophobic effects are based on a chemistry that requires heat, an acid catalyst or significantly longer curing times.

A second key feature of the coatings is the ease with which they can be applied. Using a carefully selected combination of carrier and silane, the coatings can be applied at room temperature by a variety of simple methods, including wiping on/off or spraying the surface to be treated. Long immersion times are not required.

Researchers at Pennsylvania State University undertook a study to determine the uniformity of the coating on a glass surface. Using a mapping Fourier transform infrared (FTIR) spectroscopy technique, the integrated area of a diagnostic peak was determined over a 100 μm2 area, moving the objective in increments of 100 μm for a total of 20 measurements. The intensity of the peak is proportional to the concentration of the group at the probe site and thus is indicative of coating thickness. Shown in Figure 1 is a plot of the integrated signal, normalized to 1.

The coating thickness is consistent over a wide range, varying no more than 10% below the average. These maps could be recreated at any random stripe across the surface of the substrate, indicating the overall uniformity of the coating despite the "non-ideal" coating conditions.

*Developed by Nanofilm.

Figure 2. View through a coated (left side) and uncoated (right side) windshield on a rainy day.

Practical Uses

A critical factor in the usefulness of any coating is its durability and performance under expected operating conditions. Silane self-assembled monolayers are especially well-suited to forming durable coatings as a consequence of the covalent bonds formed both between the silanes and the native hydroxyl groups on the glass, as well as bonds formed between neighboring silanes, leading to a dense two-dimensional network. Depending on the application, coatings may require resistance to elements such as sunlight, abrasion, salt water or elevated temperatures.

Ultraviolet Radiation
Coatings expected to be used outdoors over prolonged times need to be resistant to the effects of ultraviolet (UV) light from the sun. One coating was developed as a hydrophobic and UV-resistant coating for automobile windshields.** During a rainstorm, the hydrophobic coating causes the raindrops to bead up and roll off the windshield instead of sheeting out on the naturally hydrophilic glass (see Figure 2).

**Nanofilm's Clarity DefenderTM

Figure 3. Water contact angle vs. time in QUV accelerated weatherometer test.

Due to the natural UV-resistance of the hydrophobic tail, the coating generally lasts from 6 to 12 months without needing to be refreshed. Figure 3 (p. 26) shows the stability of the water contact angle during a 6000-hour accelerated weathering test (QUV), which simulates rain and UV exposure. The water contact angle (WCA, an indicator of the state of the coating) is hardly affected by the exposure.

Abrasion Resistance
Because the silane monolayer is covalently bound to the glass, it resists removal by abrasion, such as the wiping action of the windshield wiper blades. Measurement of the WCA of the coating can indicate the integrity of the coating; the UV-resistant coating maintains its WCA over the course of thousands of wiper/windshield cleaner cycles.

In addition, when the coating does eventually wear away as a result of abrasive friction or very long outdoor exposure, it can easily be replenished by simply applying more of the coating solution over the surface. No surface preparation to remove the old coating is necessary; the silane molecules graft to any exposed glass. Excess coating will not build up over the old coating, since there is no site for bonding, and will simply be washed away.

Figure 4. Water contact angle vs. time for coated plates immersed in 85°C salt solutions. Mixed salts indicates a 4.5% NaCl, 0.8% Na2HPO4 solution, pH adjusted with 1M HCl or 1M NaOH as needed; NaCl indicates a 5.0% NaCl solution, pH adjusted with 1M HCl as needed.

Salt and Acid Resistance
The resistance of silane self-assembled monolayers to salt is highly dependent on the pH of the salt solution. Shown in Figure 4 are the results of immersing coated glass panels in various salt solutions at 85°C. Salt solutions that are acidic have little to no effect on the WCA of the coating, indicating the resistance of the coating to damage by the salt solution.

On the other hand, salt solutions with pH above 7 (caustic) readily damage the coating, resulting in rapid loss of WCA within one hour at 85°C. In another test, coated glass panels were exposed to a concentrated acidic solution of various alkali and alkaline earth salts (pH = 1) for 30 minutes at 80°C. The WCA of the panels was unchanged.

Figure 5. Water contact angle vs. time for coatings as a function of temperature.

Temperature Stability
The temperature stability of the self-assembled monolayers is strongly dependent on the chemistry of the silane tail group. The tail group of most commercially available silanes consists first of a three-carbon-atom chain separating the silicon atom from further functional groups. These silanes are called gamma-substituted silanes, and they can withstand long-term continuous exposure to 160°C.

Silanes with an aromatic group bound to the silicon atom have even higher thermal stability. The electron-withdrawing or electron-donating abilities of further substituent groups likewise reduce or enhance thermal stability.6 Figure 5 illustrates the thermal stability of two coatings at 200°C.

Stain Resistance
Silane self-assembled monolayers are frequently used as a protective topcoat on anti-reflective coated glass or plastic. An antireflective stack, which must end in an SiOx layer to react with the silane, is quite porous. Staining and contamination of these surfaces is an ongoing problem. The application of silanes either by solution (dipping, wiping or spraying) or by vacuum evaporation exposes the entire SiOx surface to the silane monomers. Once bound to the surface, the monomers are then cross-linked to form a durable protective coating.

Figure 6 features a pair of glass plates with antireflective coatings. A top-coating* was applied to the plate on the left, while the plate on the right remained unprotected. Both plates were subjected to a chemical exposure challenge by saturating a spot with a variety of liquids such as lens cleaner, isopropanol, Windex® and 10% acetic acid for over 24 hours. The AR coating appears undamaged by visual inspection.

*Nanofilm's Ultraseal AB5

Figure 7. The same panels after standing for 3 minutes with blue food dye and being blotted gently with a paper towel.

Blue food dye was then applied to both plates at the test locations and left for three minutes. The plates were blotted dry with a paper towel (see Figure 7). As can be seen in the picture on the left, the coated plate repelled all stains, while the unprotected antireflective layer was damaged and allowed penetration of the dye.

Table 1.

Coefficient of Friction
Self-assembled coatings also reduce the coefficient of friction on glass and ceramics. In a nano-scratch test performed by Micro Photonics, Inc., coated glass plates required significantly more force to scratch the plate when coated than plain glass. Table 1 shows the results for two coatings in the nano-scratch test, while Table 2 shows one coating's measured coefficient of friction.

Table 2.

The phenomenon is evident simply by touching the coated glass or ceramic object; the coated surface is noticeably smoother and more slippery. One can surmise that coated pieces will incur less damage as a result of being stacked or moved against one another.

Broad Opportunities

Silane self-assembled monolayers can be used to create a durable protective coating on glass or ceramic surfaces. The covalent bonding between the coating and the surface gives the coating excellent resistance to damage by abrasion and erosion, while the chemistry of the tail group can alter the coating's thermal, ultraviolet and chemical resistance.

Additional applications of silane monolayer coatings include easy clean-up of graffiti, hydrophilic anti-fog treatments, ease of cleaning of oily residues, and acid resistance. Industrially, silanes are used for applications such as the coating of chromatography columns, the reduction of protein adsorption, the enhancement of corrosion resistance and the dispersion of pigments. Self-assembled silane monolayers can preserve and protect most glass and ceramic surfaces with a durable and covalently bonded coating.

For additional information regarding silane-based coatings, contact Nanofilm at 10111 Sweet Valley Dr., Valley View, OH 44125; (216) 447-1199; fax (216) 447-1137; e-mail info@nanofilmtechnology.com; or visit www.nanofilmtechnology.com.

Acknowledgements

The author would like to acknowledge Tad Daniels, Ph.D., and Professor Carlo Pantano of the Pennsylvania State University for running and interpreting the FTIR experiments, as well as for many helpful discussions.

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