The impact of corona treatment on seal strength
How does Corona Treatment effect seal strength?
Answer
We need to start by reviewing the definition of what “treatment” is. The film industry generally refers to “treatment” as a method of changing the surface tension of a film. Creating a higher surface tension on the film surface will help to overcome the natural surface tension of a liquid to itself. Polyethylene film that is not treated will have a surface tension typically less than 34 dynes/cm. Since most inks and adhesives have natural surface tensions that are higher than this level, they will tend to bead on the surface rather than wetting out on the film. Treatment is the process of increasing the film surface tension to get closer to the natural liquid surface tension.
The most common method used to increase the surface tension of film is Corona treating. This is accomplished by passing the film through a highly charged electrical field that bombards the film surface in the presence of oxygen (air). This bombardment converts the oxygen into ozone, which oxidizes the surface of the film, creating sites for anchorage of adhesives, inks or coatings. Higher power levels in the field create higher levels of oxidation, which result in higher surface tension of the film. Typically a surface tension of at least 38 dynes/cm. is required for proper adhesion to solvent-based inks, while 40 dynes/cm. is required for adhesives and water based inks.
Oxidizing of the film by treatment also creates a surface that is “hard” and less prone to flow when softened. Thus if an attempt is made to heat seal two corona treated surfaces (or sealing the treated side to itself) as in a pouch, the material will not flow in the seal region sufficiently to bond well to the other surface. The result is a very weak seal that will pull apart in sort of a peel seal mechanism. This is also the case if an attempt is made to seal the treated surface to an untreated surface. Only with the use of sophisticated sealant technology can an acceptable seal be achieved.
In most cases this does not create a problem in that the material is either surface printed on the outside of the pouch with the untreated inside providing the sealability, or it is laminated so that the treated side provides the lamination bond and is buried from the seal surface. As long as an attempt is not made to seal to a treated surface, the corona treatment does not play a role in the seal strength.
Under normal circumstances the corona treatment will only be on one surface of the film. Films can of course be treated on both sides if that is desirable however for the purposes of this discussion assume that we are only considering one-side treated films. There can however be instances when the sealant surface can accidentally be treated. This is known as backside treat and can be caused by a number of factors, all of which relate to air getting behind the web as it goes through the treater.
As stated above, the process of corona treatment relies on the conversion of oxygen in the air gap between the treater electrode and the film, creating ozone which oxides the film surface. If there is also air behind the film (between the film and the backup or ground roller), that air too is oxidized creating ozone behind the web, which then oxidizes the backside of the film. This oxidation of the backside can be in small areas or across the entire web depending on how much the web is lifted off the backup roller. Mechanical steps can be taken to reduce the potential for this air gap and thus reduce the potential for backside treat.
Should the areas of backside treat fall across a seal (e.g. in a pouch side seal), the treatment will again interfere with sealability and a weak seal will be formed. This can result in small areas where leaks can occur, as the sealed film tends to pull easily away from itself. The problem can become worsened with higher levels of backside treat. Thus in most cases backside treat is totally unacceptable. It can of course occur during the film manufacturing process when the film is initially treated or during the converting process if the film is retreated to “clean up” the surface to enhance lamination or ink bonds. Proper measures must be taken to assure it is not present and operators should always be testing for its potential appearance.