A coating is a covering that is applied to the surface of an object, usually referred to as the 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.
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 preventing corrosion and identification e.g. blue for process water, red for fire-fighting control. Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wettability, corrosion resistance, or wear resistance.[4] In other cases, e.g. semiconductor device fabrication (where the substrate is a wafer), the coating adds a completely new property, such as a magnetic response or electrical conductivity, and forms an essential part of the finished product.[5][6]
A major consideration for most coating processes is that the coating is to be applied at a controlled thickness, and a number of different processes are in use to achieve this control, ranging from a simple brush for painting a wall, to some very expensive machinery applying coatings in the electronics industry. A further consideration for 'non-all-over' coatings is that control is needed as to where the coating is to be applied. A number of these non-all-over coating processes are printing processes. Many industrial coating processes involve the application of a thin film of functional material to a substrate, such as paper, fabric, film, foil, or sheet stock. If the substrate starts and ends the process wound up in a roll, the process may be termed "roll-to-roll" or "web-based" coating.[7] A roll of substrate, when wound through the coating machine, is typically called a web.
Coating applications are diverse and serve many purposes.[4][8] Coatings can be both decorative and have other functions. 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, or a flat or matte appearance.[10]
A major coating application is to protect metal from corrosion.[11] This use includes preserving machinery, equipment, and structures.[12][13][14][15][16] Most automobiles are made of metal. The body and underbody are typically coated with underbody coating.[17] Anticorrosion coatings may use graphene in combination with water-based epoxies.[18]
Roof coatings are designed primarily for waterproofing and sun reflection to reduce heating. They tend to be elastomeric to allow for movement of the roof without cracking the coating membrane.[28][29][30]
The coating, sealing, and waterproofing of wood have been going on since biblical times, with God commanding Noah to build an ark and then coat it. Wood has been a key material in construction since ancient times, so its preservation by coating has received much attention.[31] Efforts to improve the performance of wood coatings continue.[32][33][34][35][36]
Automotive coatings are used to enhance the appearance and durability of vehicles. These coatings include primers, basecoats, and clearcoats, and they are applied using various techniques, including electrostatic and spray gun applications.[37]
Coatings are used to alter tribological properties and wear characteristics.[38][39] Other functions of coatings include:
Anti-reflective coatings for example on spectacles.[45]
Coatings that alter or have magnetic, electrical or electronic properties.[46][47][48]
Flame retardant coatings.[49][50][51] Flame-retardant materials and coatings are being developed that are phosphorus and bio-based.[52] These include coatings with intumescent functionality.[53]
The formulation of a coating depends primarily on the function required of the coating and also on aesthetics required such as color and gloss.[71] The four primary ingredients are the resin (or binder), solvent which maybe water (or solventless), pigment(s) and additives.[72][73] Research is ongoing to remove heavy metals from coating formulations completely.[74]
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 International80 (7): 324–328. doi:10.1007/bf02692680. ISSN1356-0751.
↑ 4.04.1Howarth 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 International80 (7): 324–328. doi:10.1007/bf02692680. ISSN1356-0751.
↑Howarth, G.A (1995). "5". in Karsa, D.R. Waterborne Maintenance Systems for Concrete and Metal Structures. 165. Cambridge, U.K: The Royal Society of Chemistry. ISBN0-85404-740-9.
↑S. Grainger and J. Blunt, Engineering Coatings: Design and Application, Woodhead Publishing Ltd, UK, 2nd ed., 1998, ISBN978-1-85573-369-5
↑Ramakrishnan, T.; Raja Karthikeyan, K.; Tamilselvan, V.; Sivakumar, S.; Gangodkar, Durgaprasad; Radha, H. R.; Narain Singh, Anoop; Asrat Waji, Yosef (2022-01-13). "Study of Various Epoxy-Based Surface Coating Techniques for Anticorrosion Properties" (in en). Advances in Materials Science and Engineering2022: e5285919. doi:10.1155/2022/5285919. ISSN1687-8434.
↑Mutyala, Kalyan C.; Ghanbari, E.; Doll, G.L. (August 2017). "Effect of deposition method on tribological performance and corrosion resistance characteristics of Cr x N coatings deposited by physical vapor deposition". Thin Solid Films636: 232–239. doi:10.1016/j.tsf.2017.06.013. ISSN0040-6090. Bibcode: 2017TSF...636..232M.
↑Gite, Vikas V., et al. "Microencapsulation of quinoline as a corrosion inhibitor in polyurea microcapsules for application in anticorrosive PU coatings." Progress in Organic Coatings 83 (2015): 11-18.
↑Žigon, Jure; Kovač, Janez; Petrič, Marko (2022-01-01). "The influence of mechanical, physical and chemical pre-treatment processes of wood surface on the relationships of wood with a waterborne opaque coating" (in en). Progress in Organic Coatings162: 106574. doi:10.1016/j.porgcoat.2021.106574. ISSN0300-9440.
↑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 Transactions59 (6): 1039–1047. doi:10.1080/10402004.2015.1131349.
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
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