Self-cleaning glass

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Self-cleaning glass is a specific type of glass with a surface that keeps itself free of dirt and grime.

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The field of self-cleaning coatings on glass is divided into two categories: hydrophobic and hydrophilic. These two types of coating both clean themselves through the action of water, the former by rolling droplets and the latter by sheeting water that carries away dirt. Hydrophilic coatings based on titania (titanium dioxide), however, have an additional property: they can chemically break down absorbed dirt in sunlight.

The requirements for a self-cleaning hydrophobic surface are a very high static water contact angle θ, the condition often quoted is θ>160°, and a very low roll-off angle, i.e. the minimum inclination angle necessary for a droplet to roll off the surface. [1]

Self-cleaning surfaces

Several techniques are known for the patterning of hydrophobic surfaces through the use of moulded polymers and waxes, by physical processing methods such as ion etching and compression of polymer beads, and by chemical methods such as plasma-chemical roughening, which can all result in ultra-hydrophobic coatings. [2] While these surfaces are effective self-cleaners, they suffer from a number of drawbacks which have so far prevented widespread application. Batch processing a hydrophobic material is a costly and time-consuming technique, and the coatings produced are usually hazy, precluding applications on lenses and windows, and fragile materials. The second class of self-cleaning surfaces are hydrophilic surfaces which do not rely solely on the flow of water to wash away dirt. These coatings chemically break down dirt when exposed to light, a process known as photocatalysis. Despite the commercialization of a hydrophilic self-cleaning coating in a number of products, the field is far from mature; investigations into the fundamental mechanisms of self-cleaning and characterizations of new coatings are regularly published in the primary literature.

The discovery of self-cleaning behavior

The first self-cleaning glass was based on a thin film titania coating. [3] The film can be applied by spin coating of organo-titanate chelated precursor (for example titanium iso-tetrapropoxide chelated by acetylacetone), followed by heat treatment at elevated temperatures to burn the organic residues and to form the anatase phase. In that case, sodium might diffuse from the glass into the nascent titanium dioxide, causing a degradation in the hydrophilic/catalytic effect [4] unless preventive measures are taken. The glass cleans itself in two stages. The "photocatalytic" stage of the process breaks down the organic dirt on the glass using ultraviolet light and makes the glass superhydrophilic (normally glass is hydrophobic). During the following "superhydrophilic" stage, rain washes away the dirt, leaving almost no streaks, because water spreads evenly on superhydrophilic surfaces. [5]

The first commercial product

In 2001, Pilkington Glass announced the development of the first self-cleaning windows, Pilkington Activ™, and in the following months several other major glass companies released similar products. As a result, glazing is perhaps the largest commercial application of self-cleaning coatings to date. All of these windows are coated with a thin transparent layer of titanium dioxide. This coating acts to clean the window in two stages, using two distinct properties: photocatalysis and hydrophilicity. In sunlight, photocatalysis causes the coating to chemically break down organic dirt adsorbed onto the window. When the glass is wet by rain or other water, hydrophilicity reduces contact angles to very low values, causing the water to form a thin layer rather than droplets, and this layer washes dirt away.

The use of titanium dioxide in self-cleaning applications

Titanium dioxide has become the material of choice for self-cleaning windows, and hydrophilic self-cleaning surfaces in general, because of its favorable physical and chemical properties.[ citation needed ] Not only is titanium dioxide highly efficient at photocatalysing dirt in sunlight and reaching the superhydrophilic state, it is also non-toxic, chemically inert in the absence of light, inexpensive, relatively easy to handle and deposit into thin films and is an established household chemical that is used as a pigment in cosmetics and paint and as a food additive. [6]

The mechanism

The metastable anatase phase is generally considered to be the most photocatalytic among the polymorphic structures of titanium, possibly as the result of a typically higher specific surface area. [7] Moreover, ultraviolet irradiation creates surface oxygen vacancies at bridging sites, resulting in the conversion of relevant Ti4+ sites to Ti3+ sites which are favourable for dissociative water adsorption. [8] These defects presumably influence the affinity to chemisorbed water of their surrounding sites, forming hydrophilic domains, whereas the rest of the surface remains oleophilic. Hydrophilic domains are areas where dissociative water is adsorbed, associated with oxygen vacancies that are preferentially photogenerated along the [001] direction of the (110) plane; the same direction in which oxygen bridging sites align. [9]

Other applications

Other possible application areas are computer monitors and PDA screens, where fingerprints are undesirable. [10]

Titanium dioxide–based glass cannot decompose thick non-transparent deposits, such as paint or silicone, waterstop fingerprints or bleeding after weathering, or stucco dust produced during construction. [11]

Since 2001 the TC24 "Coatings on Glass" committee International Commission on Glass has been trying to set up test methods for evaluation of photocatalytic self-cleaning coatings on glass. [12]

Brands

See also

Related Research Articles

<span class="mw-page-title-main">Hydrophobe</span> Molecule or surface that has no attraction to water

In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.

<span class="mw-page-title-main">Rutile</span> Oxide mineral composed of titanium dioxide

Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.

<span class="mw-page-title-main">Anatase</span> Mineral form of titanium dioxide

Anatase is a metastable mineral form of titanium dioxide (TiO2) with a tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral forms) of titanium dioxide are known to occur naturally: brookite, akaogiite, and rutile, with rutile being the most common and most stable of the bunch. Anatase is formed at relatively low temperatures and found in minor concentrations in igneous and metamorphic rocks. Glass coated with a thin film of TiO2 shows antifogging and self-cleaning properties under ultraviolet radiation.

<span class="mw-page-title-main">Titanium dioxide</span> Chemical compound

Titanium dioxide, also known as titanium(IV) oxide or titania, is the inorganic compound with the chemical formula TiO
2. When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble in water, although mineral forms can appear black. As a pigment, it has a wide range of applications, including paint, sunscreen, and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million tonnes. It has been estimated that titanium dioxide is used in two-thirds of all pigments, and pigments based on the oxide have been valued at a price of $13.2 billion.

<span class="mw-page-title-main">Smart glass</span> Glass with electrically switchable opacity

Smart glass, also known as switchable glass, dynamic glass, and smart-tinting glass, is a type of glass that can change its reflective properties to prevent sunlight and heat from entering a building and to also provide privacy. Smart glass for building aims to provide more energy-efficient buildings by reducing the amount of solar heat that passes through glass windows.

<span class="mw-page-title-main">Photocatalysis</span> Acceleration of a photoreaction in the presence of a catalyst

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions." In many cases, the catalyst is a solid that upon irradiation with UV- or visible light generates electron–hole pairs that generate free radicals. Photocatalysts belong to three main groups; heterogeneous, homogeneous, and plasmonic antenna-reactor catalysts. The use of each catalysts depends on the preferred application and required catalysis reaction.

Superhydrophilicity refers to the phenomenon of excess hydrophilicity, or attraction to water; in superhydrophilic materials, the contact angle of water is equal to zero degrees. This effect was discovered in 1995 by the Research Institute of Toto Ltd. for titanium dioxide irradiated by sunlight. Under light irradiation, water dropped onto titanium dioxide forms no contact angle.

<span class="mw-page-title-main">Lotus effect</span> Self-cleaning properties

The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of Nelumbo, the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Ultrahydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia, Alchemilla, cane, and also on the wings of certain insects.

<span class="mw-page-title-main">Ultrahydrophobicity</span> Material property of extreme resistance to wetting

In chemistry and materials science, ultrahydrophobic surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can fully rebound like an elastic ball. Interactions of bouncing drops can be further reduced using special superhydrophobic surfaces that promote symmetry breaking, pancake bouncing or waterbowl bouncing.

Nanotechnology is impacting the field of consumer goods, several products that incorporate nanomaterials are already in a variety of items; many of which people do not even realize contain nanoparticles, products with novel functions ranging from easy-to-clean to scratch-resistant. Examples of that car bumpers are made lighter, clothing is more stain repellant, sunscreen is more radiation resistant, synthetic bones are stronger, cell phone screens are lighter weight, glass packaging for drinks leads to a longer shelf-life, and balls for various sports are made more durable. Using nanotech, in the mid-term modern textiles will become "smart", through embedded "wearable electronics", such novel products have also a promising potential especially in the field of cosmetics, and has numerous potential applications in heavy industry. Nanotechnology is predicted to be a main driver of technology and business in this century and holds the promise of higher performance materials, intelligent systems and new production methods with significant impact for all aspects of society.

<span class="mw-page-title-main">Water-repellent glass</span>

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), and fluorocarbons. In order to prepare WRGs, sol-gel processes involving dual-layer enrichments of large size glasses are commonly implemented.

Anti-fog agents, also known as anti-fogging agents and treatments, are chemicals that prevent the condensation of water in the form of small droplets on a surface which resemble fog. Anti-fog treatments were first developed by NASA during Project Gemini, and are now often used on transparent glass or plastic surfaces used in optical applications, such as the lenses and mirrors found in glasses, goggles, camera lenses, and binoculars. The treatments work by minimizing surface tension, resulting in a non-scattering film of water instead of single droplets. This works by altering the degree of wetting. Anti-fog treatments usually work either by application of a surfactant film, or by creating a hydrophilic surface.

<span class="mw-page-title-main">Akira Fujishima</span> Japanese chemist (born 1942)

Akira Fujishima (藤嶋 昭, Fujishima Akira, born March 10, 1942) is a Japanese chemist and president of Tokyo University of Science. He is known for significant contributions to the discovery and research of photocatalytic and superhydrophilic properties of titanium dioxide (TiO2), which is also known as the Honda-Fujishima effect.

Hydrophobic silica is a form of silicon dioxide that has hydrophobic groups chemically bonded to the surface. The hydrophobic groups are normally alkyl or polydimethylsiloxane chains. Hydrophobic silica can be processed in different ways; such as fumed silica, precipitated silica, and aerosol assisted self assembly, all existing in the form of nanoparticles.

TioCem is a specialized cement with photocatalytic features, used on the surface of buildings to reduce air pollution caused by exposure of the cement to ultraviolet light (UV).

An antimicrobial surface is coated by an antimicrobial agent that inhibits the ability of microorganisms to grow on the surface of a material. Such surfaces are becoming more widely investigated for possible use in various settings including clinics, industry, and even the home. The most common and most important use of antimicrobial coatings has been in the healthcare setting for sterilization of medical devices to prevent hospital associated infections, which have accounted for almost 100,000 deaths in the United States. In addition to medical devices, linens and clothing can provide a suitable environment for many bacteria, fungi, and viruses to grow when in contact with the human body which allows for the transmission of infectious disease.

Photocatalyst activity indicator ink (paii) is a substance used to identify the presence of an underlying heterogeneous photocatalyst and to measure its activity. Such inks render visible the activity of photocatalytic coatings applied to various "self-cleaning" products. The inks contain a dyestuff that reacts to ultraviolet radiation in the presence of the photocatalytic agent in the coating. They are applied to the coated product and show a color change or disappearance when exposed to ultraviolet radiation. The use of a paii based on the dye resazurin forms the basis of an ISO standard test for photocatalytic activity.

Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories:

  1. superhydrophobic
  2. superhydrophilic
  3. photocatalytic.
<span class="mw-page-title-main">Titanium dioxide nanoparticle</span>

Titanium dioxide nanoparticles, also called ultrafine titanium dioxide or nanocrystalline titanium dioxide or microcrystalline titanium dioxide, are particles of titanium dioxide with diameters less than 100 nm. Ultrafine TiO2 is used in sunscreens due to its ability to block ultraviolet radiation while remaining transparent on the skin. It is in rutile crystal structure and coated with silica or/and alumina to prevent photocatalytic phenomena. The health risks of ultrafine TiO2 from dermal exposure on intact skin are considered extremely low, and it is considered safer than other substances used for ultraviolet protection.

<span class="mw-page-title-main">Liquid marbles</span>

Liquid marbles are non-stick droplets wrapped by micro- or nano-metrically scaled hydrophobic, colloidal particles ; representing a platform for a diversity of chemical and biological applications. Liquid marbles are also found naturally; aphids convert honeydew droplets into marbles. A variety of non-organic and organic liquids may be converted into liquid marbles. Liquid marbles demonstrate elastic properties and do not coalesce when bounced or pressed lightly. Liquid marbles demonstrate a potential as micro-reactors, micro-containers for growing micro-organisms and cells, micro-fluidics devices, and have even been used in unconventional computing. Liquid marbles remain stable on solid and liquid surfaces. Statics and dynamics of rolling and bouncing of liquid marbles were reported. Liquid marbles coated with poly-disperse and mono-disperse particles have been reported. Liquid marbles are not hermetically coated by solid particles but connected to the gaseous phase. Kinetics of the evaporation of liquid marbles has been investigated.

References

  1. ↑ Marmur, A. Langmuir 20, 3517–3519, (2004).
  2. ↑ Roach, P., Shirtcliffe, N. J. & Newton, M. I. Soft Matter 4, 224–240, (2008).
  3. ↑ Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. (1995-11-01). "Photooxidative self-cleaning transparent titanium dioxide films on glass". Journal of Materials Research. 10 (11): 2842–2848. Bibcode:1995JMatR..10.2842P. doi:10.1557/JMR.1995.2842. ISSN   2044-5326. S2CID   138230137.
  4. ↑ Paz, Y.; Heller, A. (1997). "Photo-oxidatively self-cleaning transparent titanium dioxide films on soda lime glass: The deleterious effect of sodium contamination and its prevention". Journal of Materials Research. 12 (10): 2759–2766. Bibcode:1997JMatR..12.2759P. doi:10.1557/JMR.1997.0367. ISSN   0884-2914. S2CID   135908071.
  5. ↑ "Eco glass cleans itself with Sun". 2004-06-08. Retrieved 2022-11-08.
  6. ↑ Titaniumdioxide mchnanosolutions.com [ dead link ]
  7. ↑ the Anatase to Rutile Transformation ART, J.Mat.Sci.2011
  8. ↑ Assadi, MHN (2016). "The effects of copper doping on photocatalytic activity at (101) planes of anatase TiO 2: A theoretical study". Applied Surface Science. 387: 682–689. arXiv: 1811.09157 . Bibcode:2016ApSS..387..682A. doi:10.1016/j.apsusc.2016.06.178. S2CID   99834042.
  9. ↑ Wang, Rong; Hashimoto, Kazuhito; Fujishima, Akira; Chikuni, Makota; Kojima, Eiichi; Kitamura, Atsushi; Shimohigoshi, Mitsuhide; Watanabe, Toshiya (1997). "Light-induced amphiphilic surfaces". Nature. 388 (6641): 431–432. Bibcode:1997Natur.388..431W. doi: 10.1038/41233 . ISSN   0028-0836.
  10. ↑ "Self-cleaning glass". Engadget. Retrieved 2022-11-08.
  11. 1 2 Neat Glass Archived 2008-10-06 at the Wayback Machine , Cardinal CG Technical Service Bulletin # CG05-06/06.
  12. ↑ "TC24 reports".
  13. ↑ "Pilkington Activ™ Self-Cleaning Glass". www.pilkingtonselfcleaningglass.co.uk.
  14. ↑ "SunClean glass info".
  15. ↑ "Saint Gobain launches self-cleaning glass"
  16. ↑ "Saint-Gobain Glass launches 2nd generation self-cleaning glass".
  17. ↑ "Self-cleaning Glass - Saint-Gobain Glass UK". www.selfcleaningglass.com. Retrieved 2017-07-16.
  18. ↑ Hyoumenkagaku Vol. 26, No. 11, pp. 700―703, 2005, T. Anzaki, movie Archived 2011-07-22 at the Wayback Machine
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