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What to remember when running water- based inks or solventless adhesives
By Corona Supplies
Oxfordshire, UK-based Corona Supplies, manufacturer of corona treaters and solutions
provider for plastic film extrusion, conversion and narrow web printing, says that water-based printing inks have been run successfully on all printing presses 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 can be summarised as: • More energy is required to evaporate the water, and this means modification to drying tunnels.
• 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 need replacing. • Presses must be cleaned quickly so that ink is not allowed to set hard on the Anilox. • For good adhesion 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. The problem of surface tension however, if too low, represents an inherent defect in the incoming material that can only be overcome by pre-treating on 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, believed to be due to the non-polar structure. The chemical changes caused by Corona Treatment include polarity and therefore increased wettability on the material surface. With polyolefins, the surface treatment polarises
10 November 2015
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, even where the surface energy level has decayed from the original treatment level. This permits smaller corona treatment equipment to be fitted to the press than would be the case with material not treated at the manufacturing stage.
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. 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.
High frequency corona generators - reactive power Reactive power systems provide a quantitative measure of the impedance match between the load and the generator. Impedance matching is the procedure of maximising the available power, delivered to the discharge electrode by tuning the inductance and the capacitance of the generator and electrode station to resonance. The importance of impedance matching cannot be over-emphasised. Where major components, inductors and capacitors in a power supply cannot be readily changed, one major factor can, the frequency at which the corona operates. Fine
adjustment of the output frequency can ensure that the match between power supply and discharge station is maintained at optimum conditions. The end result of this process is that equivalent treat levels are achieved with significantly less power absorbed. With this knowledge now well understood, it is not a problem to design display panels on corona treater generators that make repeatability of power used very clear to machine operators, a significant step towards more efficient, more reliable corona treatment.
Reactive power supplies, such as the GXR range from Corona Supplies (see below), are fitted with sophisticated matching circuitry that automatically tunes to load conditions, using output frequencies in the range of 10- 25kHz. This enables the power supply to become ‘plug and play’ as connection to a different treater station, or load condition, does not require any set-up time.
T: +44 (0)1844 261779
www.coronasupplies.co.uk
www.convertermag.co.uk
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