What is Corona treated films? What and where do you use this process and what are the markets this shows up in. Will try and take a high level view of what the Corona treatment process is, its benefits and the products that take advantage of this.
Corona Treatment Defined
Need to start with the basics here first. What exactly is Corona treated? The Corona treatment system is designed to increase the surface energy of plastic films, foils and paper in order to allow improved wettability and adhesion of inks, coatings and adhesives. When this process is applied, the materials treated will demonstrate improved printing and coating quality, and stronger lamination strength.
The system is made up of two major components:
- The power supply
- The treater station
The power supply takes standard power and converts it into single phase, higher frequency (nominally 10 to 40 kHz) power that is supplied to the treater station.
The key to the whole system is the air gap. The treater station applies this prover to the surface of the material, through an air gap, via a pair of electrodes, one at high potential and the other; usually a roll that
supports the material, at ground potential. Only the side of the material that is facing the electrodes has its surface effected.
As you can see from the diagram below the major components in their correct working location.
So how does it actual work? Voltage is applied to the “top plate” which is the electrode. The dielectric component, often a roll, is made up of some type of roll covering and air. The final component, or bottom plate, would take the form of an electrically grounded roll. With this in place, the voltage buildup ionizes the air in the air gap, creating a corona that will increase the surface tension of the surface passing over the electrically grounded roll.
Variables That You Can Tweak
With a properly calibrated corona treater, the primary variable under your control is the amount of treatment that you can apple to the film/material. This is measured as Watt Density. (Refer to this link for explanation of Watt Density –https://en.wikipedia.org/wiki/Cartridge_heater).
This is where it can get a little technical, but this is key to understanding how a corona treatment is set up. The Watt Density formula takes into consideration 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.
The formula is the following:
Power Supply Output (watts)
# of treated sides * Line Speed (ft/min or meters/minute) * Electrode Width (ft or meters)
As a basis point, for most converting applications, they are sized between a watt density of 2 and 3 watts per square foot per minute.
What to Look For
Simply looking at a film you will not be able to determine if it has been surface treated, let alone to what level. The way you check to see if a material has been corona treated is performing a Dyne test.
Dynes are a unit of measurement that indicates the surface energy of a film. The dyne test results on the proper care in running the test and interpreting the results. Keep in mind, when you take your dyne test before the treatment and after the treatment, you are only testing a small part of your material. Even with all this said, the only true way to test to make sure you have the correct adhesion is to run your application with the material. And remember different types of film with create different surfaces with the same amount of watt density.
Corona Treating Applications
Normal practice in the past has been to treat film at the time of extrusion and ship it to the converter for printing, coating and laminating without further treatment. Since film is easier to treat immediately after extrusion, this has always been considered the optimum location for the treater.
In recent years there has been a trend toward in-line treat mentor retreatment on the coating, laminating, or printing machine. The main factor for this trendis the fact that treatment levels decay over time and can be weakened by contact with idler rolls during subsequent machine operations. There is also a practical limit on the treatment level that can be applied to a film prior to winding if blocking is to be avoided.
Film suppliers typical provide treatment levels in the 36-42 dynes/cm range. Generally speaking, higher watt densities produce higher dyne levels. However you cannot use watt density to predict dyne levels. This is due to the corona treating system is not an intelligent device. It has no idea that the film running today is new or old, has more or less additives than he film ran previously.For example treatment levels in this range are adequate for flexographic printing using solvent-based links and are just barely acceptable for bonding with solvent-based adhesives.
With pollution reduction being a constant concern, this necessitated the placement of corona treating equipment on the many converting machines.This is partly due to the new inks and adhesives have a higher surface tension and don’t wet the fil as well as solvents, and partly because of fatty-acid slip additives present on the surface of the film.
Initial treatment on the extruder is one before the slip additive has migrated to the surface. Where the slip additive has relatively poor wetting characteristics and ca cause problems in bonding unless subjected to a second in-line corona treatment. With this said, a high percentage of converting facilities were equipped with corona treaters simply as a quality assurance tool for avoiding problems caused by inadequately treated film from a supplier.
Corona treatment system have an upper limit on the treatment level. Very high treatment levels create polar groupings that are hydrophilic and may absorb excessive water, causing a reduced bonding layer at the substrate surface.
Summary
Corona treaters apply treatment measured in watt density to improve surface wettability. In general higher watt densities produce higher dyne levels, but the relationship is not always linear. Each film ends up having a unique feel and signature in response to various watt densities. Testing is highly advised to dial in the needs for your application.
The following are the areas the come in to play when you are evaluating surface treatment.
Material specification(material supplier, age of the material, storage conditions), post treatment dyne level, initial dyne level, web width, air gap, kW setting or watt density and line speed.
Resources:
- Thomas I. Butler Film Extrusion Manual, Second Edition and ”Why all films do not treat the same”, Enercon Industries. Information was summarized to produce this white paper.