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CORONA TREATMENT: A must when runing water based inks or adhesives
suitable for paper, board and polyolefin substrates. But certain problems must be addressed, more specifically with polyolefin - filmic materials than with more porous substrates. These problems are that more energy is required to evaporate the water, and this means modification to drying tunnels and the viscosity of the water-based ink needs close attention. On longer runs the pH of the system needs to be monitored as the volatile amine will slowly evaporate and needs to be replaced, and presses must be cleaned relatively quickly so that ink is not allowed to set hard on the Anilox. Finally, for good adhesion to be obtained, it is to ensure a high level of surface tension on the film surface. Most of these matters relate to know-how
and to the skills available in the printing plant to overcome them. The problem of surface tension however, if too low, represents an inherent defect in the incoming material, which can only be overcome by pre-treating the press with an additional piece of equipment, namely a corona treater.
ADHESION Adhesion between surfaces can be understood when roughness provides a mechanical key for the adhesive. When surfaces are smooth, one can still speculate about the reasons why adhesion occurs with some materials, and not with others. Generally it is accepted that a strong bond can be made between two polar materials and a bond of a lower strength results if one or both surfaces are non-polar. Polyolefins do not have the necessary chemical structure to make pre-treatment by mechanical means easy, and the difficulty encountered is believed to be due to the non-polar structure of polyolefins. The chemical changes caused by Corona Treatment include polarity and therefore increased wettability on the material surface. With polyolefins, the surface treatment polarises the long chain polymer molecules into shorter chain fragments and oxidises the ends of these smaller sections, creating mainly ether and hydroxyl groups. Treatment occurs as a very thin and ordered surface layer to the film. If the surface tension of a polymeric film is raised by corona treatment to a higher level, then adhesion properties can be improved. Polymeric films treated at the
maker/extrusion stage often re-treat more easily than totally non-treated materials,
8 November 2018
By Stuart Greig of Corona Supplies Ltd W
ater based printing inks have been run successfully on all printing presses
even where the surface energy level has decayed from the original treatment level. The polymer blend, particular in relation to additive content, has a major influence on achievable surface energy levels. Additives such as slip or Ti02 will inhibit the corona process, necessitating higher power levels to achieve a given surface energy level. The decay rate of the surface energy level may also be accelerated by the presence of additives, particularly those designed to bloom on the film surface, such as slip.
CORONA PRE-TREATMENT Corona Discharge Treatment at atmospheric pressure for improving adhesion of various polymer substrates is now standard practice. Corona is generated by applying a high frequency, high voltage signal to an electrode separated from an earthed plane by an air gap, the substrate and a layer of dielectric material.
A conductive substrate moving through this mechanical construction will provide a grounding path from itself to all the grounded path rollers in the production line. When the corona treater is energised, this prevents the establishments of the high tension required to strike the corona arc. To overcome this problem, the insulating medium is moved from the base roll to the discharge electrode which then becomes the insulated electrode and the base roll remains metallic thus maintaining the capacitor construction. This is known as the Bare Roll treater construction and will handle both conductive and non- conductive substrates. It should be noted that Bare roll treater systems are not as effective on difficult films as dielectric covered roll treaters.
Not all corona systems are the same,
and many factors play an important role in a correct and effective corona discharge, such as corona power supply, frequency of operation, dielectric materials, discharge electrode configurations and air gap settings.
BARE ROLL OR COVERED ROLL The fact that there is a need for two quite distinct types of corona discharge stations is explained by the presence of two groups of substrates used within the packaging industry, namely Conductive materials and Non-Conductive materials.
CONDUCTIVE & NON-CONDUCTIVE Traditionally in the corona system the dielectric insulation medium was always situated on the backing roll – Covered Roll Station. This mode is only suitable for non- conductive substrates as they provide no grounding path when the electrical discharge takes place between the conditioning electrode and the insulated coated electrode which is the dielectric roll.
DISCHARGE MECHANISMS In covered roll corona systems, the preferred discharge mechanism for full width, edge to edge treatment is the multi-blade discharge bar, allowing surface area to be varied to suit different kW rating, and provide excellent dwell time for the corona for substrates running at high line speeds. But where lanes of non-treatment are
required across the web width then the segmented electrode should be used. Mounting a treater in-line on the press or the laminator, gives the converter a form of insurance that desired results will be obtained. It also gives added flexibility regarding jobs using inks, adhesives without solvents, and difficult materials so that adhesion will always be acceptable.
ucoronasupplies.co.uk convertermag.co.uk
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