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 finish may be decorative, functional, or both. The finish itself may be an all-over finish, completely covering the substrate, or it may only cover parts of the substrate.
Functional coatings may be applied to change the surface properties of the substrate, such as applying a pre-coating to fabric before printing, adding a non-slip function and many others.
The finish is usually applied with a controlled thickness, and a number of different processes can be used to achieve this. A further consideration for partial coating is the precise control of where the finish is to be applied. A number of these non-all-over coating processes are printing processes, such as non-slip including a client logo.
In general, in order for a thin uniform coating layer to be applied, the application must be done at low or moderate viscosities. In many cases this low viscosity is achieved by dissolving or dispersing the desired product in a liquid such as water or a solvent, but this then necessitates the subsequent removal of the liquid by, for instance, a drying process. Hot-melt coating achieves this low to moderate viscosity by melting the desired material before applying it to the substrate. The substrate and coated layer are then cooled, generally by passing over a chilled roller. There is no liquid to remove, so the process is, in principle, much faster than water- or solvent-based equivalents.
Many industrial coating processes involve the application or bonding of a thin film of functional material to a substrate, such as paper, fabric, film or foil. If the substrate starts and ends the process wound up in a roll, the process may be termed ‘roll-to-roll’ coating or converting.