Coating

A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate. An example of all of these types of coating is a product label on many drinks bottles- one side has an all-over functional coating (the adhesive) and the other side has one or more decorative coatings in an appropriate pattern (the printing) to form the words and images.

Paints and lacquers are coatings that mostly have dual uses of protecting the substrate and being decorative, although some artists paints are only for decoration, and the paint on large industrial pipes is presumably only for the function of preventing corrosion.

Functional coatings may be applied to change the surface properties of the substrate, such as adhesion, wettability, corrosion resistance, or wear resistance. 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.

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. A roll of substrate, when wound through the coating machine, is typically called a web.

Coatings may be applied as liquids, gases or solids.

Functions of coatings

• Adhesive – adhesive tape, pressure-sensitive labels, iron-on fabric
• Changing adhesion properties
  • Non-stick PTFE coated- cooking pans
  • Release coatings e.g. silicone-coated release liners for many self-adhesive products
  • primers encourage subsequent coatings to adhere well (also sometimes have anti-corrosive properties)
• Optical coatings
  • Reflective coatings for mirrors
  • Anti-reflective coatings e.g. on spectacles
  • UV- absorbent coatings for protection of eyes or increasing the life of the substrate
  • Tinted as used in some coloured lighting, tinted glazing, or sunglasses
• Catalytic e.g. some self-cleaning glass
• Light-sensitive as previously used to make photographic film
• Protective coatings
  • Most surface coatings or paints are to some extent protecting the substrate e.g.
    • Sealing and waterproofing wood
    • Sealing the surface of concrete
      • Film-forming sealers and floor paint
      • Seamless polymer/resin flooring
      • Bund wall/containment lining
    • Waterproofing and damp proofing of concrete walls
    • Roof coating
    • Concrete bridge deck membranes
    • Sealing and waterproofing of masonry
    • Preserving machinery, equipment and structures ^[1]
      • Maintenance coatings/paints for metals, alloys and concrete
      • Chemical resistant coatings
    • Wear resistance
      • Anti-Friction, Wear and Scuffing Resistance Coatings for Rolling-element bearings^[2]
      • Hard anti-scratch coating on plastics and other materials e.g. of titanium nitride to reduce scratching and abrasion loss
      • Barrier coatings on concrete, metals and alloys subject to erosion/abrasive attack
  • Anti-corrosion ^[3]- ensure metal components have the longest possible lifespan.
    • Underbody sealant for cars
    • Many plating products
    • Preserving equipment and structural steel from degradation
    • Under thermal insulation and under protective fireproofing for structural steel
  • Passive fire protection
  • Insulation
  • Waterproof fabric and waterproof paper
  • Anti-graffiti
  • Antimicrobial surface
  • Foul release and anti-fouling
• Magnetic properties such as for magnetic media like cassette tapes, floppy disks, and some mass transit tickets
• Electrical or electronic properties
  • Conformal Antenna, e.g., metal coatings on plastic airframes
  • Conductive coatings e.g. to manufacture some types of resistors
  • Insulating coatings e.g. on magnet wires used in transformers
• Scent properties such as scratch and sniff stickers and labels
• Decorative- often to impart a specific colour, but also to create a particular reflective property such as gloss or matt.

Coating processes

Coating processes may be classified as follows:

See also

Look up coating in Wiktionary, the free dictionary.

References

• Titanium and titanium alloys, edited by C. Leyens and M. Peters, Wiley-VCH, ISBN 3-527-30534-3, 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, ISBN 0-8155-1492-1
• High-Performance Organic Coatings, ed. AS Khanna, Elsevier BV, 2015, ISBN 978-1-84569-265-0