Presented by Dr. Wolfgang Decker, VAST FILMS, Ltd.
Selectively Metallized Films have been used in various applications. Most everybody has seen the mass mailed envelopes, using a metallized PET or oPP film with a clear window for the address. And most everybody has already devoured a food item, that was heated in a microwave using a susceptor laminated to a paper board, whereas the susceptor had a pattern to it.
Most of these patterns were created by pattern etching. There are two different
approaches for this, and both start out with a fully metallized film. The first technology prints a protective coat over the areas that are supposed to stay metallized. In a subsequent step the film then is exposed to a caustic bath, that etches away the aluminum that is still exposed. The film then is washed in order to remove the caustics. The second method prints a concentrated caustic solution right onto the areas that need to be etched. The caustics then dissolve the aluminum, and like in the first step, the film is then washed and cleaned before further processing. Either method represents an additional production step, and both methods have to deal with the leftover: Caustics with dissolved aluminum,
which needs to be disposed of according to EPA rules.
An alternative to this method has been around for some time now, but has to date not found widespread application: In Chamber Selective Metallization. Here the pattern is created by preventing the aluminum from depositing in the areas that are supposed to stay clean. This is achieved by applying a thin film of oil onto the areas that are not supposed to be metallized. The thin layer of oil fulfills two functions: 1. It creates an area with low surface energy, which prevents the aluminum from attaching, and 2. when exposed to the heat of the evaporator, it also starts evaporating itself, creating a gas stream in the opposed direction to the aluminum, further preventing the metal from depositing. Although this technology has been around for some time the only major application so far is the use in patterned capacitor films. One of the reasons for this might be the type of oil
being used so far, which is not fully FDA compliant. Since there typically is a small residue of oil left on the film, the concern with the majority of the applications that aim towards food packaging is the exposure of the packaged food to the pattern oil. Finding the right oil is not a simple task, as it needs to fulfill several requirements: Besides being FDA compliant, preferably not only for food contact, but even as a food additive, it needs to be vacuum compliant, needs to have the right viscosity under the conditions under which it is being used, and it needs to have just the right evaporation behavior that it only leaves a minor residue on the pattern metallized film.
In term of the residue there was, initially, major concern about the compatibility with further processing, especially with lamination. It was found, however, that the oil has either no or only minimum impact on several types of adhesives. Full compatibility with specific adhesives however should be tested prior.
The main advantage of in chamber patterning is the reduction in production cost, as it completely eliminates one process step. In addition, it eliminates the environmental issues with the leftover caustic solutions.
It was also found that much finer print resolutions can be achieved with in-chamber metallization. Print resolutions up to 300 lpi are achievable, and create numerous opportunities for creative and decorative applications.
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