A coating is a covering that is applied to the surface of an object, or substrate.[1] The purpose of applying the coating may be decorative, functional, or both.[2] Coatings may be applied as liquids, gases or solids e.g. powder coatings.
Paints and lacquers are coatings that mostly have dual uses, which are protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is for identification (e.g. blue for process water, red for fire-fighting control) in addition to 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.[3] 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.[4][5]
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.
"Roll-to-roll" or "web-based" coating is the process of applying a thin film of functional material to a substrate on a roll, such as paper, fabric, film, foil, or sheet stock. This continuous process is highly efficient for producing large volumes of coated materials, which are essential in various industries including printing, packaging, and electronics. The technology allows for consistent high-quality application of the coating material over large surface areas, enhancing productivity and uniformity.[6]
Applications
Coatings can be both decorative and have other functions.[3][7] 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.[8] A decorative coating can offer a particular reflective property, such as high gloss, satin, matte, or flat appearance.[9]
A major coating application is to protect metal from corrosion.[10][11][12][13][14] 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.[15]
The body and underbody of automobiles receive some form of underbody coating.[16] Such anticorrosion coatings may use graphene in combination with water-based epoxies.[17]
Most roof coatings are designed primarily for waterproofing, though sun reflection (to reduce heating and cooling) may also be a consideration. They tend to be elastomeric to allow for movement of the roof without cracking within the coating membrane.[27][28][29]
Wood has been a key material in construction since ancient times, so its preservation by coating has received much attention.[30] Efforts to improve the performance of wood coatings continue.[31][32][33][34][35]
The formulation of a coating depends primarily on the function required of the coating and also on aesthetics required such as color and gloss.[69] The four primary ingredients are the resin (or binder), solvent which may be water (or solventless), pigment(s) and additives.[example needed][70][71] Research is ongoing to remove heavy metals from coating formulations completely.[72]
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).[73]
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.
Slot Die coating - Slot die coating was originally developed in the 1950s.[76] Slot die coating has a low operational cost and is an easily scaled processing technique for depositing thin and uniform films rapidly, while minimizing material waste.[77] Slot die coating technology is used to deposit a variety of liquid chemistries onto substrates of various materials such as glass, metal, and polymers by precisely metering the process fluid and dispensing it at a controlled rate while the coating die is precisely moved relative to the substrate.[78] The complex inner geometry of conventional slot dies require machining or can be accomplished with 3-D printing.[79]
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.
^Howarth, G A; Manock, H L (July 1997). "Water-borne polyurethane dispersions and their use in functional coatings". Surface Coatings International. 80 (7): 324–328. doi:10.1007/bf02692680. ISSN1356-0751. S2CID137433262.
^ abHowarth G.A "Synthesis of a legislation compliant corrosion protection coating system based on urethane, oxazolidine and waterborne epoxy technology" Master of Science Thesis April 1997 Imperial College London
^Howarth, G A; Manock, H L (July 1997). "Water-borne polyurethane dispersions and their use in functional coatings". Surface Coatings International. 80 (7): 324–328. doi:10.1007/bf02692680. ISSN1356-0751. S2CID137433262.
^Howarth, G.A (1995). "5". In Karsa, D.R; Davies, W.D (eds.). Waterborne Maintenance Systems for Concrete and Metal Structures. Vol. 165. Cambridge, U.K: The Royal Society of Chemistry. ISBN0-85404-740-9.
^"The benefits of liquid roofing". Why use liquid waterproofing. Liquid Roofing & Waterproofing Association. Archived from the original on 1 October 2011. Retrieved 12 September 2011.
^Mutyala, Kalyan C.; Singh, Harpal; Evans, R. D.; Doll, G. L. (23 June 2016). "Effect of Diamond-Like Carbon Coatings on Ball Bearing Performance in Normal, Oil-Starved, and Debris-Damaged Conditions". Tribology Transactions. 59 (6): 1039–1047. doi:10.1080/10402004.2015.1131349. S2CID138874627.
Titanium and titanium alloys, edited by C. Leyens and M. Peters, Wiley-VCH, ISBN3-527-30534-3, table 6.2: overview of several coating systems and fabriction processes for titanium alloys and titanium aluminides (amended)
Coating Materials for Electronic Applications: Polymers, Processes, Reliability, Testing by James J. Licari; William Andrew Publishing, Elsevier, ISBN0-8155-1492-1
High-Performance Organic Coatings, ed. AS Khanna, Elsevier BV, 2015, ISBN978-1-84569-265-0