Coating

Source: Wikipedia, the free encyclopedia.
Lacquer being sprayed onto a cabinet

A coating is a covering that is applied to the surface of an object, or substrate.[1][2] The purpose of applying the coating may be decorative, functional, or both.[3] Coatings may be applied as liquids, gases or solids e.g. powder coatings.

preventing corrosion. Along with corrosion resistance, functional coatings may also be applied to change the surface properties of the substrate, such as adhesion, wettability, or wear resistance.[4] In other cases the coating adds a completely new property, such as a magnetic response or electrical conductivity (as in semiconductor device fabrication, where the substrate is a wafer), and forms an essential part of the finished product.[5][6]

A major consideration for most coating processes is controlling coating thickness. Methods of achieving this range from a simple brush to expensive precision machinery in the electronics industry. Limiting coating area is crucial in some applications, such as printing.

fabric, film, foil, or sheet stock.[7]

Applications

Coatings can be both decorative and have other functions. [4][8] A pipe carrying water for a fire suppression system can be coated with a red (for identification) anticorrosion paint. Most coatings to some extent protect the substrate, such as maintenance coatings for metals and concrete.[9] A decorative coating can offer a particular reflective property, such as high gloss, satin, matte, or flat appearance.[10]

A major coating application is to protect metal from corrosion.[11][12][13][14][15][16] Automotive coatings are used to enhance the appearance and durability of vehicles. These include primers, basecoats, and clearcoats, primarily applied with spray guns and electrostatically.[17] The body and underbody of automobiles receive some form of underbody coating.[18] Such anticorrosion coatings may use graphene in combination with water-based epoxies.[19]

Coatings are used to seal the surface of concrete, such as

bridge decks.[25][26][27][28]

Most

elastomeric to allow for movement of the roof without cracking within the coating membrane.[29][30][31]

Wood has been a key material in construction since ancient times, so its preservation by coating has received much attention.[32] Efforts to improve the performance of wood coatings continue.[33][34][35][36][37]

Coatings are used to alter

rolling-element bearings[40]

Other

Other functions of coatings include:

Analysis and characterization

Numerous destructive and non-destructive evaluation (NDE) methods exist for characterizing coatings.

transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning tunneling microscope (STM), and Rutherford backscattering spectrometry (RBS). Various methods of Chromatography are also used,[69] as well as thermogravimetric analysis.[70]

Formulation

The

example needed][72][73] Research is ongoing to remove heavy metals from coating formulations completely.[74]

For example on the basis of experimental and epidemiological evidence, it has been classified by the IARC (International Agency for Research on Cancer) as a human carcinogen by inhalation (class I) (ISPESL, 2008).[75]

Processes

Coating processes may be classified as follows:

Vapor deposition

Chemical vapor deposition

Main article: Chemical vapor deposition
  • Metalorganic vapour phase epitaxy
  • Electrostatic spray assisted vapour deposition
    (ESAVD)
  • Sherardizing
  • Some forms of Epitaxy
    • Molecular beam epitaxy

Physical vapor deposition

Main article: Physical vapor deposition

Chemical and electrochemical techniques

Spraying

Roll-to-roll coating processes

Common

roll-to-roll
coating processes include:

  • Air knife coating
  • Anilox coater
  • Flexo
    coater
  • Gap Coating
    • Knife-over-roll coating
  • Gravure
    coating
  • Hot melt coating- when the necessary coating viscosity is achieved by temperature rather than solution of the polymers etc. This method commonly implies slot-die coating above room temperature, but it also is possible to have hot-melt roller coating; hot-melt metering-rod coating, etc.
  • Immersion
    dip coating
  • Kiss coating
  • Metering rod (Meyer bar) coating
  • Roller coating
  • Silk Screen
    coater
    • Rotary screen
  • 3-D printing.[81]
  • Extrusion coating - generally high pressure, often high temperature, and with the web travelling much faster than the speed of the extruded polymer
    • Curtain coating- low viscosity, with the slot vertically above the web and a gap between slot-die and web.
    • Slide coating- bead coating with an angled slide between the slot-die and the bead. Commonly used for multilayer coating in the photographic industry.
    • Slot die bead coating- typically with the web backed by a roller and a very small gap between slot-die and web.
    • Tensioned-web slot-die coating- with no backing for the web.
  • Inkjet
    printing
  • Lithography
  • Flexography

Physical

See also

Look up coating in Wiktionary, the free dictionary.

References

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Further reading

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