To solve such problems relating to oil and water repellency, various stain-proofing agents have been proposed. One proposal was a stain-resistant antireflective coating obtained by surface treating a substrate with a perfluoroalkyl-group-containing compound. Another approach was a stain-resistant, low-reflection plastic that had a polyfluoroalkyl group containing mono and disilane compounds and halogen, alkyl, or alkoxysilane compounds as a surface-modification coating. A third option that has been proposed is forming a copolymer of perfluoroalkyl (meth)acrylate and an alkoxy silane group-containing monomer on an optical thin film mainly consisting of silicon dioxide. However, these coatings have insufficient stain-resistance properties, especially on the most important stains such as fingerprints, skin oil, sweat and cosmetics.
Recently, researchers and development partners at Dow Corning Corp. and Daikin Industries, Ltd. determined that obtaining the desired high water and oil contact angles and low sliding angles necessary for high antistaining performance required chemical modification of a linear perfluoropolyether. The resulting technology can be used to create innovative stay-clean and easy-to-clean surface properties on glass and metal, in addition to many common plastics, such as acrylic, poly(methyl methacrylate) (PMMA) and polycarbonate.
Surface Property EvaluationThe researchers developed four alkoxysilyl perfluoropolyether coatings for testing (see above image for synthesis). All four antistaining coatings were applied on glass test pieces using chemical vapor deposition (CVD) and dip coating in a dilute solution, as described in the Summary of Application Methods sidebar at the end of this article.
The impact of the alkoxysilyl structure on the modified perfluoropolyether was evaluated while keeping the polyether chain length constant (n = 20) using the dip coating application method. The surface properties of the treated glass substrates were compared using the static contact angle and sliding angles of water and oil (n-hexadecane).
As can be seen in Table 1, high water and oil contact angles were observed in all cases. The simpler alkoxysilyl group in Coatings 1 and 3 gave slightly higher static contact angles and slightly lower sliding angles for both water and oil when compared with Coatings 2 and 4. The organic linking group between the silicon and the perfluoropolyether backbone also had a slight impact on the results, with the amide-containing group (Coatings 3 and 4) giving lower water contact angles and higher oil contact angles. The results suggest that a highly stain-resistant surface can be achieved on glass. In all cases, the degree of surface hysteresis (the ability of oil and water to slide over the treated surface) was small, as shown by the low sliding angles.
The effect of perfluoropolyether molecular weight was evaluated by adjusting the degree of polymerization of the polyether backbone in Coating 1. No clear trend was seen for the water and oil contact angles, although the data did suggest that as the perfluoropolyether molecular weight increased, the sliding angle for water also increased. For the oil sliding angle, there was no observable relationship with polymer molecular weight.
Coating ComparisonThe product recently developed as a result of the alkoxysilyl perfluoropolyether synthesis* was compared to a common antistaining material (perfluoroalkylsilane) on a glass substrate and to a control sample with no surface treatment. Measurements of the water and oil contact angle and sliding angle gave the results shown in Table 3.
The effect of rubbing durability is commonly the key parameter when considering the use of the akoxylsilyl perfluoropolyether coating on surfaces that are subjected to physical wear. Applications such as ophthalmic lenses, camera lenses, portable electronic devices and auto interiors require resistance to rubbing to ensure long-term hydrophobic (water-repelling) and oleophobic (oil-repelling) properties. Durability testing typically is carried out using a device that applies a constant pressure on a uniform surface area while cycling from side to side across the surface. The glass test pieces were subjected to a 500-gram force applied by a cotton cloth fixed to an aluminum block. The water-contact angle was measured after various intervals to obtain the relationship with rubbing cycles. A comparative method was used to assess durability, as there are no absolute pass/fail values.
Improved Fingerprint ResistanceCoatings based on alkoxysilyl perfluoropolyethers show significant promise for preventing fingerprints and smudges on a variety of consumer products. Creating an antifouling surface has many advantages, from a reduced need for cleaning to improved safety. Weather resistance and durability can be achieved by adding alkoxy silane-reactive components to the patented polymer composition. A wide range of uses can benefit from these surface improvements, including applications such as consumer appliances, optical components (e.g., antireflective films, optical filters, ophthalmic lenses, mirrors), electronic display screens (e.g., liquid crystal displays, CRT displays, plasma displays, projection TVs), automotive interiors, treated glass and earthenware.
For more information, visit www.dowcorning.com.
Author’s acknowledgement:The following individuals also contributed to this article: Peter Hupfield, senior development specialist, Dow Corning Ltd., United Kingdom; Dr. Yasuo Itami, chief researcher, Daikin Industries Ltd., Japan; Eiji Kitaura, application engineer, Dow Corning Toray Ltd., Japan; Dr. Don Kleyer, associate research scientist, Dow Corning Corp., U.S.; Dr. Tetsuya Masutani, business development manager, Daikin Industries Ltd., Japan; and Dr. Yasuhiro Nakai, research chemist, Daikin Industries Ltd., Japan.
Editor’s note:This article is based on the paper “New Antifingerprint Coatings,” presented at the 11th Annual International Coatings for Plastics Symposium, co-hosted by Paint & Coatings Industry and Finishing Today in April 2008.
SIDEBAR: Summary of Application Methods for Glass Test PiecesDip Coating Method
1. Clean surface with suitable solvent.
2. Dip glass test piece in a 0.1% solution of the PFPE-silane chemical for 30 seconds.
3. Dry at room temperature and 50% relative humidity for eight hours (alternatively, dry at 122°F [50°C] and 50% relative humidity for one hour).
4. Rinse substrates in a sonication bath.
5. Dry for one hour at room temperature.
Chemical Vapor Deposition (CVD) Method
1. Evaporate diluted PFPE silane on a pourous pellet and place in CVD chamber.
2. Place glass test piece in CVD chamber.
3. Evacuate chamber to a pressure of E-5 Torr.
4. Apply current to pellet.