The surface energy of the plastic materials, measured in dynes/cm, must be at least 10 dyne/cm higher than the surface tension of the liquid that is to be applied.
Generally plastic films have chemically inert and non-porous surfaces with low surface tension that make them receptive to the bonding of substrates such as printing inks, adhesives and coatings.
Among the various plastic films, polyethylene and polypropylene are those that have the lowest surface energy and are the two materials most often subjected to corona treatment to improve their adhesion characteristics.
However, the surface treatment with corona effect is not limited to these two materials and can also be used to improve the adhesion capacity of virtually all plastics as well as of some non-plastic materials (aluminium and paper) or metallised in general.
Sectors of use
These materials can be treated: plastic films, foils and sheets, aluminium foil, metallised films and paper.
Film and foil extrusion
The main application of corona treatment is on the lines of extrusion (blown film extrusion, cast film extrusion, foil and sheet extrusion in general).
It increases the surface tension of plastic materials to prepare them for subsequent processing (printing, laminating, adhesive coating, etc.)
There are two main applications:
- treatment of solid sheet for refrigerators, which is treated to facilitate the bonding of polyurethane foam
- treatment of hollow sheet to facilitate the bonding of the print.
Coating for paper and aluminium foil
The corona treatment before the extrusion coating of paper and/or aluminium foil promotes good bonding of the polyethylene and speeds up the production process.
Converting for plastic and metallised film and aluminium foil
The application of corona treatment on printing and laminating lines is required for the following reasons:
- in the interval of time that passes between the extrusion phase and the converting phase, the surface tension of plastic film may decrease due to migration on the surface of additives, lubricants, master-batches and antistatic material contained in the material
- the use of inks or adhesives without solvent and/or water-based require levels of surface tension higher than those based on solvent.
How does it work
A station for corona treatment consists essentially of a high-frequency generator, a transformer voltage booster which is connected to a straight and fixed electrode by a roller which is electrically grounded, coated by an insulating material.
It is in the air space (air-gap) between the film and the electrode that the regime of effluvium discharges is established and creates the so called "corona effect" that leads to activation of the film.
Duration of treatment
The treatment is normally carried out on the extrusion lines of the film, but its effectiveness tends to diminish over time, so we resort to corona treatment also at the entrance of the printing presses and coating and laminating lines to reinstate the material.
Corona treatment originally on extrusion lines:
- blown film extrusion
- cast film
- foam sheet extruder
- sheet extruder.
Treatment reinstatement on converting lines:
Other methods of increasing surface tension
The other methods by which a surface treatment is carried out are the following: