Corona treatment is one of the most commonly used methods in adhesion processes for preparing materials like film and polymers manufactured on large rolls. Corona treatment is used to activate the surface, or create a molecularly amenable condition on the surface for adhesion, of a rolled material requiring coating, printing, laminating or painting.
The treatment works by discharging high-voltage, high-frequency electricity from an electrode in a ceramic tube that runs the length of the roll of material needed to be activated. The electricity is sent through the material to an electrically-grounded, metal roll called the treater roll, that the material is wrapped around. This interaction between the electrode and the metal roll creates a visible flash on the surface of the material roll as it moves between the two components. The results, however, are completely invisible to the human eye. Like was stated earlier, this treatment is altering the surface at a chemical level. Therefore, there is no visual test that could ever offer confidence that the treatment was successful at creating a chemically clean surface. Only a measurable, quantitative inspection gives the data necessary to take action on.
There are many different providers of corona treatment systems each with variations to the standard setup. Even with the diversity of systems available there are some variables in every corona treatment process that need to be fully understood and controlled in order to get the best results.
In most corona treatment systems, the material roll moves over the stationary treater roll at a consistent rate in order to ensure a uniform activation of the surface. However due to electrical pinning - a phenomenon where the treater roll rotates when the treatment is applied due to a static electrical force being created and causing the film or material to wrinkle – additional parameters need to be adjusted or implemented. A drive shaft and a nip roll are devices that control the tension and wrap of the material on the treater roll to prevent or mitigate wrinkling by compensating for electrical pinning.
Often wrinkling is undetectable by merely observing the roll through a visual inspection because the rotating of the treater roll is so subtle it creates minute waves in the roll that still have real impact on the uniformity of the corona treatment. So, once again, it is always necessary to measure the treatment level across the entire roll and track changes so adjustments can be made accordingly.
Similarly, modifications to the treatment power (measured in watt density, which is the watts per foot per minute of treatment) are necessary depending on what the coating or adhesive being applied to the film is made up of. The watt density of the corona treatment controls the level of surface energy of the material being treated. So when this parameter is adjusted it determines the condition on the surface of that is optimal for bonding.
Water-based coatings require a higher surface energy than solvent-based ones. UV coatings require an even higher surface energy than water-based coatings. And not all coatings that fall under each of these categories have the same treatment needs. Not knowing what the exact range of treatment is necessary to guarantee adhesion can lead to under or over-treatment. Over-treating a surface can be just as detrimental to adhesion as not prepping the surface enough. It is critical to implement quantifiable supply chain controls to eliminate the risk of treating outside of established treatment specifications.
Additionally, it’s necessary to establish and measure the baseline, chemical state of a material before treatment so that can be compensated for when it’s time to apply the treatment.
Subtle changes of the material from the supplier or something occurring during the storage of these materials and coatings can impact how effective the same level of treatment that had always been used will be. Just because treatment worked the first thousand times is not an indicator that it will work the thousand and first time. Continuous monitoring is crucial to ensure on-going success.
BTG Labs has developed the Surface Analyst tool perfectly designed for fast, easy, accurate and non-destructive monitoring of surfaces most commonly treated using corona. The Surface Analyst 2001 takes the guess work out of determining if the watt density is properly
set, if the drive shaft and nip roll are sufficiently controlling the tension and if the necessary surface energy level is achieved after treatment. All of these considerations are critical to take control of but are almost always left up to chance and trial and error.
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