Cover Story
CORONA PRE-TREATMENT – UPDATE FOR RUNNING OF WATER BASED INKS OR SOLVENTLESS ADHESIVES
By Stuart Greig, Corona Supplies W
ater 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 to be replaced; Presses must be cleaned relatively quickly so that ink is not allowed to set hard on the anilox; For good adhesion to be obtained it is necessary to ensure a high level of surface tension on films. 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 on the press with an additional piece of equipment, namely a corona treater. 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. Polyolefin 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 polyolefin. The chemical changes caused by Corona Treatment include polarity and therefore increased wettability on the material surface. With polyolefin, 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. Polyolefin such as Polypropylene film has
the lowest surface tension of 29 dynes/cm, while Polyamide (nylon) has the higher surface tension, at 46 dynes/cm. The various solvents in printing inks range from 18-29 dynes/cm, while water has a surface tension of 72 dynes. Water molecules will prefer to cling to them-selves rather than to a low energy, weakly attractive surface such as a polymeric 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
No treat online/ Pre treated on extruder (left) vs treated online (right)
8 September 2020
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 permit’s smaller corona treatment equipment to be fitted to the press than would be the case with material not treated at the manufacturing stage. 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 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. The fact that there is a need for two
distinct types of corona station, bare roll & covered roll, is explained by the use of conductive & non-conductive films within the packaging industry. In Bare Roll stations the preferred choice of insulated electrode is the ceramic dielectric tube. Our unique Triple-treat ceramic electrode provides increased corona output, excellent dwell time at high speeds on difficult to treat materials, yet does not require additional space.
CONCLUSION We cannot control dyne level drop-off over time, due to the effects of additives, crystallinity, and storage conditions. Storage time can be reduced in order to minimise drop-off. However, this still does not guarantee that good adhesion will be obtained on the printing press or the solvent-less laminator. 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. After all, the cost of one treater system, is approximately equal to that of a few returned reels, without the dent in reputation.
ucoronasupplies.co.uk convertermag.com
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