L’coating is a surface treatment which consists in applying a generally liquid coating (lacquer, paint, oil, etc.) on a support (paper, textile, plastic film, metal, etc.). Generally, the coating is followed by drying and possibly crosslinking.
The different coating methods are classified according to whether it is a free coating, a self-controlled coating or a pre-controlled coating..
Free coating[modifier | modifier le code]
In this category, the substrate is immersed in a bath. The parameters which make it possible to determine the thickness of the fluid on the substrate are mainly: substrate speed, viscosity of the fluid, density of the fluid, angle between the substrate and the surface of the bath, the surface tension.
Self-controlled coating[modifier | modifier le code]
Self-controlled coating techniques are an evolution of free coating in that they involve free coating which is subsequently controlled in order to reduce fluid thickness or achieve higher speeds. Each technique has its own methods of achieving the desired thickness at the desired speed. In this category, we can cite roller coating, engraved roller coating, doctor blade coating, air knife coating (air knives) and centrifugal coating.
Pre-controlled coating[modifier | modifier le code]
Pre-controlled coating methods allow the desired amount of fluid to be applied directly to the substrate. These techniques require the use of a good quality pump which will feed a die in order to distribute the fluid evenly over the entire desired width. In this way, the applied thickness is easily controlled by the ratio of the linear flow rate versus the speed of the substrate.
In this category, we can distinguish three types of processes:
Metallurgy[modifier | modifier le code]
In metallurgy, lacquers and paints are mainly applied in order to increase the anti-corrosive properties (painted sheets) or to protect food (cans).
We can mainly distinguish two coating methods used in the metallurgical industry:
- L’roller coating, which is used to apply pretreatments, paints or lacquers;
- the hot dip galvanizing, which applies the zinc coating to the steel. This technique involves a coating using air knives to adjust the thickness of liquid zinc.
Plastics[modifier | modifier le code]
In plastics processing, coating concerns the production of flat products, in particular PVC floor and wall coverings and films for flexible packaging. Among countless possibilities:
- an ordinary film (PE) will be coated with a material which will give it oxygen barrier properties, for example PVDC, to manufacture food packaging;
- a metallized PET will be coated with a hot-melt lacquer to produce a lidding film;
- an oriented and lightened PP (cavity OPP) will be coated with a cold glue (cold seal) to allow wrapping of chocolate bars.
Textiles[modifier | modifier le code]
In the field of textiles, the coating can be applied by calendering, blade, doctor blade, immersion, foam or spraying. A substance (lacquer, oil, resin, vinyl, etc.) is deposited on one side of the fabric, usually to make it waterproof or stain resistant.
The coated textile is a reinforced composite which finds its use in the manufacture of flexible tanks, truck covers, swimming pool covers, etc.
- H. Benkreira, Classification of coating flows, Thin Film Coating, 1993, Royal Society of Chemistry