Many flexographic substrates have non-porous surfaces such as the most commonly used polyethylene and polypropylene substrates. Non-porous surfaces have low adhesion energy and are not easily adhered to printing inks, coating materials and adhesives. Other materials, such as metal foils and some papers, also have low adsorption energy, and the same problems will occur. For these reasons, the surface energy of the substrate should be changed in order to obtain the desired flexo printability.
The purpose of corona treatment is to change the surface energy of many substrates, making it easy to bond with printing inks, coating materials, and adhesives. All substrates have better adhesion properties after some processing in the manufacturing process. Corona treatment is post-processing. It should be pointed out that corona treatment is not the only treatment that can be used to change the surface energy of a substrate when it is used to produce substrates. Other treatments include flame treatment and coating treatment. The specific treatment method used depends mainly on the structure of the substrate.
Many people believe that corona treatment roughens the surface of the substrate, which makes it easier to absorb printing inks and adhesives, but this view has been negated by observations made using scanning electron microscopy. The current fluent theory holds that corona treatment rearranges the molecular structure on the surface of the substrate, creating more polar sites and facilitating the attachment of foreign objects. The surface energy is measured in dyne. All liquids as well as most substrates (except porous) can be measured for their dyne. In order for the printing ink to adhere well to the surface of the substrate, the dyne value of the substrate should be 10 dyne higher than the dyne of all inks.
The surface energy of water-based inks is higher than the surface energy of solvent-based inks, so the substrate must also have a higher surface dyne value. Everything in nature has the property of returning to its original state. The higher the dyne value the paper product processor wants to achieve, the faster the processing energy decays. Therefore, when the aqueous ink is printed on films, metal foils, and some papers, it should be processed twice before starting printing. The use of a corona treatment device on the press (in proper match) can increase the processing power of the film to the original energy level (or slightly higher).
As mentioned earlier, the processing level decays with time. The secondary treatment can remove dirt on the surface of the film, not only helping to increase the degree of adhesion of the printing ink, but also improving the visual effect. In view of this, experts recommend secondary corona treatment of the substrate surface when using solvent-based inks, water-based inks or UV inks for printing films, metal foils, or some paper printing jobs.
Packaging materials and their structure continue to evolve with various sealing method lamination processes, printing and ink technical specifications. The related substrates are also being improved, and the technical difficulty is even greater. In this context, corona treatment will play a greater role.