Why Corona Treated Films Work Differently

What is Corona Treated Film? Where do you use this process and in what market does it appear. Will try to take a high-level look at what the corona treatment process is, its benefits and products that take advantage of this. Corona Treatment Definition Let's start with the basics first. How exactly is corona handled? Corona treatment systems are designed to increase the surface energy of plastic films, foils and papers to improve wetting and adhesion of inks, coatings and adhesives. With this process, the treated material will show improved print and coating quality, as well as greater lamination strength. The system consists of two main components: The power supply takes a standard power supply and converts it to a single-phase, higher frequency (10 to 40 kHz nominal) power supply to the processing station. The key to the whole system is the air gap. The processing station passes through an air gap, through a pair of electrodes, one at high potential and the other at the surface of the material, usually a roll that supports the material at ground potential. Only the side of the material facing the electrode has an effect on its surface. So how does it work? The voltage is applied to the 'top plate' which is the electrode. The dielectric element is usually a roll consisting of some type of roll cover and air. The final piece, or base plate, will take the form of an electrically grounded roller. In this case, the voltage build-up ionizes the air in the air gap, creating a corona that increases the surface tension across the ground roller. Adjustable Variables With a properly calibrated corona treater, the main variable under your control is the amount of treatment you can apply to the film/material. This is measured in watt density. This is where it can get a little technical, but it's the key to understanding how corona treatment is set up. The power density formula takes into account the speed of the film being treated, the number of sides being treated, the width of the treatment area and the kilowatt output of the corona treater. Just by looking at a piece of film, you can't tell if it's been finished, let alone to what extent. The way to check if a material is corona treated is to perform a dyne test. The conventional practice in the past was to process the film as it was extruded and then transport it to a converter for printing, coating and lamination without further processing. Since the film is easy to handle after extrusion, this has long been considered the sweet spot for handlers. In recent years there has been a trend towards in-line processing or reprocessing on coaters, laminators or printing presses. The main factor behind this trend is the reduction in handling levels over time and the diminished contact with the idlers during subsequent machine operations. There is also a practical limit to how much film can be treated before winding if clogging is to be avoided. Film suppliers typically offer treatment levels in the 36-42 dynes/cm range. In general, higher power densities result in higher dyne levels. But you can't use watt density to predict dyne levels. This is because the corona treatment system is not a smart device. Not sure if the movies released today are new or old, the additives are more or less than the ones released before. Example processing levels within this range are sufficient for flexographic printing using solvent-based linking, and are hardly acceptable for bonding using solvent-based adhesives. Since reducing pollution has always been a concern, this requires the installation of corona treatment equipment machines on many converters. This is partly due to the higher surface tension of the new inks and adhesives, which do not wet fibers and solvents, and partly due to the presence of fatty acid slip additives on the film surface. The initial treatment on the extruder takes place before the slip additive migrates to the surface. If the slip additive has relatively poor wetting characteristics and unless a second in-line corona treatment is performed, it can cause adhesion problems. Nonetheless, a high percentage of blow mills are equipped with corona treaters only as a quality assurance tool to avoid problems caused by suppliers not adequately treating films. There is an upper limit to the treatment level of a corona treatment system. Extremely high treatment levels create polar groups, which are hydrophilic and may absorb too much water, resulting in a reduced bonding layer on the substrate surface. Abstract Corona treaters use treatments measured in watt density to improve surface wettability. In general, higher power densities result in higher dyne levels, but this relationship is not always linear. Each film ultimately has a unique feel and signature to deal with different watt densities. It is strongly recommended that the testing department dial in the application form. Below are the areas to play when evaluating finishes. Material specifications (material supplier, material age, storage conditions), post-processing dyne level, initial dyne level, web width, air gap, power setting or power density and line speed.

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