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FLEXO Magazine : January 2009
TECHNOLOGIES & TECHNIQUES The smooth homogenous glow of atmospheric plasma treatment. How to Determine Optimal Treatment Levels for Plastic Films By Rory A. Wolf G enerally, plastic films have chemically inert and nonpo- rous surfaces with low surface tensions, causing them to be non-receptive to bonding with substrates, printing inks, coatings, and adhesives. Polyethylene and polypropylene have the lowest in surface energy of the various plastics, and are the two materials most often subjected to surface treatment to improve their bonding characteristic. Determining the optimal treatment level of plastic films is a very common objective, but is rarely achieved. This is typically because of a combination of issues, including material variations, surface treatment equipment limitations, seasonal variations in plant environmental conditions, and formulation variations with the interface (ink, coating, adhesives). Because a discussion of all these variables would lead to a convoluted set of conclusions, this article will initially make the assumption that all material, environmental and interfacial variables are controlled. Assuming such, we will focus attention on the currently available techno- logical opportunities to optimizing surface treatment techniques and their surface effects. Surface treatment can be used to optimize the bonding abil- ity of virtually all plastic materials, as well as some non-plastic materials. The two non-plastic materials most often subjected to surface treatment are foil and paper. All substrates, plastics, films, paper and foils provide a better bonding surface when they are treated at the time they are produced. This application is referred to as “post-treatment” and is used to enhance adhesion to the surface of other converting processes such as printing, adhesive lamination, etc. METHODS OF IMPROVING SURFACE TENSION The four mainstream methods by which surface treatment is accomplished are as follows: 1. Chemical priming. 2. Corona discharge. 3. Flame treatment. 4. Plasma treatment. 34 FLEXO The difference between ink presented to a treated surface and an untreated surface. JANUARY 2009 Flexographic printing results of an image where the substrate was treated on the upper portion, while the lower portion was left untreated. Chemical priming is still frequently used alone or in combina- tion with corona discharge treating. However, the primed level of treatment is not controllable in-field, and therefore will not be in this article’s purview for surface treatment optimization. Corona treatment is designed to increase the surface energy of plastic films, foils and paper in order to allow improved wet- tability and adhesion of inks, coatings and adhesives. As a result, the materials treated will demonstrate improved printing and coating quality, and stronger lamination strength. A corona treat- ing system in its simplest form can be portrayed as a capacitor, whereby voltage is applied to the top plate which, in the case of a corona treating system, would be the electrode. The dielectric portion of the capacitor would be made up of some type of roll covering, air, and substrate in the corona treating system. The final component, or bottom plate, would take the form of an www. f le xography. org FLEXIBLE PACKAGING
End of Year 2008