Anti-fog coatings are also known as non-mist coatings and have grown in use in eyewear and headgear in the last few years. Fogging by moisture condensation on transparent substrates presents a major challenge in several optical applications that require excellent light transmission characteristics, such as eyeglasses and vehicle windshields, and can lead to serious hazards involving in blurred vision, light scattering, energy consumption and safety hazard during the usage process of transparent glass and plastics. These problems limit the uses of transparent polymeric materials.
Their development has accelerated though breakthroughs in the use of inorganic materials such as TiO2, or SiO2, polymers containing polar functions such as hydroxyl (OH), carboxyl (COOH), and ester groups (COOR), and textured or porous surfaces.
Applications that benefit from anti-fog treatments include:
- eyewear (e.g., safety goggles, face shields)
- optical instruments (e.g., cameras, microscopes, endoscopic instruments)
- externally located gauges and signs
- visors or sport goggles.
- display screens (e.g., computer monitors, mobile device displays)
- military helmets
- photovoltaic modules
- car windshields and lamp casings.
Types of anti-fog coatings
Undesirable surface behaviour can create problems in a range of optical applications. The utilization of advanced surface coating technologies can be used to address a wide variety of these problems. Examples include:
- Cleaning optical surfaces is time consuming, expensive, or impossible.
- Fingerprints negatively impact the performance of optics.
- Functional issues due to liquid behaviour on surfaces.
- Contamination and fouling materials negatively impact optical behaviour.
- Improved adhesive/bonding characteristics are desired on optical surfaces.
- Surface is not lubricous enough.
- Wettability of an optical surface is not ideal.
- Fogging & moisture build up negatively impact optical performance.
There are two main types of anti-fog coatings:
- Hydrophobic and superhydrophobic coatings that repel water, making it bead and run off of the lens.
- Hydrophilic and superhydrophilic coatings that form a thin coating of water over the lens.
Combinations of both have also been developed.
Most anti-fog coatings on the market rub off after several cleanings or fog up after only a few seconds. The best anti-fog coatings are applied during the manufacturing process. This process allows the anti-fog properties to last several minutes; potentially long enough for temperature variances to equalize. Condensation on eyewear lenses occurs naturally because of temperature variances – in winter especially – but a premium anti-fog coating will usually provide clear visibility through a lens for more than 3 minutes (an average lens will fog up after about two seconds).
Table 1. Types of anti-fog solutions.
|Liquid cleaning solutions|| Liquid cleaning solutions are provided by various manufacturers and can be applied on various surfaces; however they are readily removed by standard washing operation. Spray treatments often lose their effect over time as the anti-fog film dries up or becomes unevenly distributed. Anti-fogging sprays do not withstand washing and must be reapplied regularly.
• Defogging sprays
• Anti-fog wipes
• Anti-fog markers.
|Films||• Films obtained by UV or thermic polymerization of monomer solutions are applied to visors, helmets, ski and protection goggles but they also show limited resistance and their anti-fog properties rapidly fade out.|
|Durable anti-fog coatings||• Companies are now developing effective and durable anti-fog coatings that are compatible with a wide range of substrate materials and geometries with minimal modification to manufacturing conditions.
• Anti-fog coating that is both hydrophobic (repels moisture, making it bead and run off the lens, providing the user with a clearer field of view), and hydrophilic (attracts moisture and forms a thin coating of water over the lens. This can also reduce the scattering of light and allowing the wearer to see more clearly).
Despite the development of several anti-fog coating strategies, the long-term stability, adherence to the underlying substrate, and resistance to cleaning procedures are not yet optimal. There are also issues with designing the materials structures with different roughness and developing AF materials with other functions such as wear resistant and high transparency.
The use of anti-fog coatings neutralizes many of the deficiencies of existing methods for removing fog. Their use relies on the on the interactions between fog drops and solid surfaces via the design of surface geometries and chemical composition to achieve appropriate wettability.
AF coatings mainly work along the following principles:
- The fog evaporates quickly.
- Reducing the quantity of fog produced on materials surfaces per unit time.
- Prolonging the condensation time of the fog.
In general, superhydrophilicity can be obtained by various chemical and physical methods. One common approach is to coat hydrophilic inorganic nanoparticles such as SiO2 and TiO2. However, the fabrication methods for these materials involve multiple steps including seed growth, one dimension nanomaterial prefabrication and post treatments.
Most commercially available anti-fog products in the automotive industry are spray on liquid-solutions (e.g. Rain-X). Automotive manufacturers are also developing anti-fog coatings at the manufacturing stage for application on optical polycarbonate sheets or components.
Anti-fogging coatings are employed on solar panels to prevent performance loss, especially in humid environments. Use of anti-fog coatings allow for improved photovoltaic operational efficiency. A significant part of the incident solar energy is lost due to the reflection at the air/glass interface as well as the accumulation of dusts on the modules. Fog on
solar cells will significantly reduce the conversion efficiency of photovoltaic cells.
The market for anti-fog coatings in healthcare and medicine has grown over the last year due to the increasing use of face shields. Traditional googles used in healthcare usually incur serious fogging problems with long-term wear. In laparoscopic surgery, obscured vision during surgery is hazardous and caused typically by laparoscopic lens fogging.
Fogging is the number one cause of workers not wearing or removing their safety glasses – and wearing safety eyewear at all times is the only way to ensure eye protection in hazardous environments. During cold weather, moving from outdoors to inside and back again can cause lenses to fog. Workers in hot humid environments are also highly susceptible to these issues. Changes in body temperature due to the type of work being performed may also result in fogging.
Therefore, a significant market demand has arisen for safety glasses with lenses that have been treated with an effective anti-fog coating.
Food packaging & agricultural films
Several large chemicals and materials producers develop anti-fogging additives for agricultural and food packaging applications. By adding anti-fogging additives to food packaging films, condensed water droplets are spread into a thin transparent layer rather than droplets. In general, food packaging only requires short-term anti-fogging performance that lasts the lifetime of the packaged food.
The use of anti-fogging additives in agricultural films give a number of benefits, including:
- Improved light transmission resulting in higher plant growth rates, higher crop yield per plant and earlier crop maturity.
- Reduced burning of plants and crop spoilage.
- Reduced constant water dripping.
Anti-fog coatings and film manufacturers
Advanced Nanotechnologies S.L.
Spin-off company from the University of Barcelona (UB). The company manufacture coatings using Plasma Deposition Reactors, CVD and PECVD, Sputtering and Thermal treatments. Anti-Fog treatment X-Fog TM is a permanent nano-coating for plastic and glass substrates with anti-fog materials. The company uses Physical Vapor Deposition (PVD) to deposit the anti-fog material. To maintain the anti-fog effect on the treated substrate, it must be cleaned with a special wipe that has been developed by the company.
Adaptive Surface Technologies
Adaptive Surface Technologies (formerly SLIPS Technologies) is a spin-out from Harvard University. SLIPS (Slippery Liquid-Infused Porous Surfaces) changes the surface of a solid material into a microscopically thin and ultra-smooth (friction-free) immobilized “sea” of lubricant. The result is a robust and self-healing super-slippery surface that is highly repellent to virtually any environmental challenge such as crude oil, cement, water, ice, bio- fouling, chemicals, paints, oils, and insects.
The company produces Akita SpektraShield™ Antifog Coatings. In the presence of water and water vapor it is hydrophilic, keeping the water from beading and fogging the surface. In the presence of oils and other smearing contaminants the surface is oleophobic, keeping the oil from coating the surface and rendering the anti-fog material ineffective. SpektraShield is available in a solvent-free formulation that can be dipped, sprayed, spin or flow coated.
FSI Coating Technologies, Inc.
FSI Coating Technologies, Inc. (FSICT) offers a wide range of coatings under the Visgard® and Vistex® product brands. The products are permanent water-washable anti-fog coatings and standard/ customized anti-fog coated film and sheet products.
The Global Market for Anti-Fog Coatings and Films