User:Ajmellquist/Water-repellent glass

Water-repellent glass is often used to coat automobile windshields to increase visibility in rainy conditions.

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Water-repellent glass (WRG) is a transparent coating film fabricated onto glass, enabling the glass to exhibit hydrophobicity and durability. WRGs are often manufactured out of materials including derivatives from per- and polyfluoroalkyl substances (PFAS), tetraethylorthosilicate (TEOS), polydimethylsilicone (PDMS), aluminum oxides (Al2O3), and fluorocarbons. In order to prepare WRGs, sol-gel processes involving dual-layer enrichments of large size glasses are often implemented.

WRGs are most commonly used commercially for automobile windows to increase visibility in precipitous weather conditions and nighttime driving. In industry, WRG was first used by Volvo Cars first on their late-2005 vehicles, and has also been used by Japanese automobile makers such as Toyota, Honda, and Mazda . Additionally, WRG has other practical applications such as eyewear and cover glasses for solar cells.

To achieve enhanced visibility in wet weather, windows enriched with WRG wick away rainwater, snow, and dirt due to hydrophobic properties. These properties are achieved through high contact angles with water drops (over 100°), high water-sliding property, and high durability against both chemical and mechanical attack. According to a study on WRG, surface roughness of the film contributes to a high mechanical durability. This prevents films from abrasion due to windshield wipers, rainwater, and other weather conditions.

Properties

Mechanical Durability

Water-repellent films' mechanical durability properties are dependent on state of the surface roughness of the film and density of adsorbed water-repellent molecules. Mechanical durability of WRG can be characterized as wear and weather resistance, an important attribute for manufacturing automobile windows.

The greater the surface roughness, the more resistant the film will be to abrasion. Average surface roughness of a glass substrate indicates the size of surface particles, is measured using an atomic force microscope (AFM), and recorded in nanometers. A study analyzed different samples of silica films and found minimum and maximum surface roughnesses of 0.4 and 16.1 nm respectively. Surface roughnesses greater than 8 nm are considered large. After rubbing each sample with a flannel cloth, the study was able to determine each's resistance against wear. Films with higher surface roughnesses exhibit the highest mechanical durability. Additionally, films formed on top of silica were more durable than films formed on soda-lime glass.

The WRG's mechanical durability can also be increased by a larger density of reaction sites per surface area. An increased density of reaction sites on the film is also a result of a higher surface roughness. This works to increase durability since a higher density means more rigid chemical bonds. For instance, forming a WRG film out of polyfluoroalkyl isocyanate creates a surface with siloxane bonding. There exists a direct correlation between the density of silanol groups on the film surface and the adhesion density of the film.

Hydrophobicity

Hydrophobic properties of WRG glass windows are crucial to its repellency abilities.

The contact angle of a water droplet on a glass surface determines how easily the water droplet can slide off of the glass. Higher contact angles indicate less contact between the water and glass, contributing to hydrophobicity.

High water-sliding property of WRG films is necessary for hydrophobicity. The higher the water-sliding angle, the easier a water drop can slide down the film surface. A film's water-sliding angle is often dependent on the film coating substance. For instance, a study revealed that coating a WRG film with Fluoroalkylsilane (FAS) produced a higher water-sliding angle than coating with Polydimethylsilicone (PDMS).

High contact angles of over 100 degrees are associated with more effective water-repellency properties. The greater the contact angle between the water droplet and glass surface, the less the contact between the water and the glass, and the easier the water droplet can slide off of the glass. This can be achieved by increasing surface roughness, since the contact angle becomes larger as surface particles become larger.

Production

Sol-Gel Process

The sol-gel process is a common method of preparing water-repellent glass coating films done with various materials and often resulting in dual-layer films. This process is advantageous for automobile window applications since it works with large, curved safety glasses and allows qualities such as durability and hydrophobicity to be controlled .

In a study done by the University of Massachusetts, the sol-gel process was employed to prepare a dual-layer film using layers composed of silica and fluorocarbon. The silica layer was selected to enhance durability and placed at the glass-film interface, while the fluorocarbon layer was placed at the film-air interface and incorporated a specific surface roughness into the design. The process involved the following distinct steps: preparing both the silica sol and water-repellent solutions, spraying the solution onto the glass, treating the glass through drying, and treating through heating.

In addition, the Nippon Sheet Glass Co. in Japan discussed a sol-gel treatment involving fluoroalkylsilane (FAS) and polydimethylsilicone (PDMS). Both materials were mixed with catalysts in solvents, fabricated onto glasses, and dried. The use of the sol-gel treatment allowed for flexibility in experimenting with contact angle, sliding angle, and durability. The study pointed out that this process could be also used in automobile industry.

Applications

Automobile Windows

WRG can has been most notably used in the front windows of automobiles. Several millions of WRG windows have already been installed in industry.

WRG has been applied on some glass windows of various automobile producers:

• Honda (Legend)
• Mazda (Family Wagon)
• Nissan (Stagea, Cefiro, Cima)
• Toyota (Celsio and Soarer)
• Volvo

Solar Panel Covers

A WRG film can also be added on top of solar panels in order to increase their efficiency. However, it is important that the cover glass is self-cleaning so it does not hinder light transmission into the solar cell. The hydrophobic film acts as a barrier that causes water droplets to roll off the solar panel, rather than adhering and blocking sunlight from being absorbed. Solar panels enhanced with anti-reflective, water-repellent layers show a 6.6% increase in performance when compared to those without a coating.