Why coroplast signs need to treated?
Coroplast sheets, which are made in polypropylene, have chemically inert and nonporous surfaces with low surface tensions causing them to be non-receptive to bonding with printing inks. Although results are invisible to the naked eye, surface treating modifies surfaces to improve adhesion. These inks react with the surface to provide a cross-linked chemical bond; in effect becoming part of the sheet.
Sheets after treatment are suitable nearly all kinds of printing, flexo printing, screen printing, offset printing and UV printing.
The effects of corona treatment will slightly diminish over time. So it is better for the sheets to used within one year.
What is corona treatment?
Corona treatment (or air plasma) is a kind of electric shock treatment, which makes the surface of the substrate have higher adhesion. Most plastic polymers are non-polar polymers, and known inks and adhesives with low surface tension cannot adhere firmly to them. Therefore, corona treatment should be performed on the surface to make the plastic molecules the chemical bond breaks and degrades, increasing the surface roughness and surface area. A large amount of ozone is also produced during discharge. Ozone is a strong oxidant, which can oxidize plastic molecules to produce carbonyl groups and peroxides, and other polar groups, thereby increasing its surface energy.
The principle is to use high-frequency and high-voltage corona discharge (high-frequency AC voltage as high as 5000-15000V/m2) on the surface of the plastic to be treated to generate low-temperature plasma, which causes a radical reaction on the plastic surface and crosslinks the polymer. The surface becomes rough and increases its wettability to polar solvents-these ionic bodies break their molecular structure by electric shock and penetration into the surface of the printed body, and then oxidize and polarize the molecules on the surface to be treated, and ion shock erodes the surface. So as to increase the adhesion of the surface of the substrate.
How to test corona treatment?
Dyne test pen (50 mN/m) can be used as a rapid test tool for surface energy after corona treatment. When the test pen draws a line on the corona treated surface, if it is continuous in a line, it means that the surface energy of the material is not less than 50 mN/m. And if it is intermittently not connected in a line, it means that the surface energy of the material is less than 50 mN/m. Insufficient or even untreated, which does not meet the requirements of printing processing.
How did Coropak do the treatment?
As far as we know, corona treatment will deteriorate slightly over long periods of time, maybe one year, maybe more, according to environmental changes. Sometimes, The dyne index cannot be stably maintained at 50 mN/m using one single machine.
So, what did Coropak do? We have double-double corona treatment. Not only treated on both sides of the sheet, but also treated twice. After the treatment, the dyne level can reach up to 58 mN/m. And the corona treatment effect stays longer than usual.
Feel free to contact us for more details. Thanks for your reading.