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Surface chemistry
October 15, 2007
One of the biggest secrets that a substrate has is what the surface chemistry of the film is. In general, this will be determined by the polymer that is on the surface of the film and any subsequent process steps that are applied to it such as priming or surface treatment. For instance, corona treatment of films is widely used to improve the adhesion of ink, coatings, metal etc. in the conversion process. It works by inserting oxygen molecules into the polymer at the surface of the film and the surface energy of the film surface will be a function of the amount of oxygen inserted, and the type of oxygen bond produced. But what is the real nature of the surface of the film? If the film is an oriented polypropylene (OPP) will it have the same surface chemistry as a corona treated cast polypropylene?
Well to answer this we have to know the secret of the surface for both films. In the case of the OPP film, it is very unlikely that the treated surface is a homopolymer polypropylene, rather it is likely to be a coextruded surface of copolymer or terpolymer polypropylene, or even a polyethylene. However, the cast film may indeed be a homopolymer surface or a homopolymer/copolymer blend depending on its structure and intended end use. Also, the orientation will affect the way the corona interacts with the surface of the film and even if the polymers are the same, the chemistry of the two film surfaces are likely to be different. This will be true, even if they have the same surface energy (treatment level).
If we are comparing flame and corona treatments, then the surface chemistry will be very different, on the same surface, as the mechanism of the treatment is different and we get very different results in terms of surface energy vs. levels of surface oxygen. It is possible to determine what type of treatment, corona or flame was used by a study of the surface treatment level and the amount of oxygen in the surface and we will discuss this in a further installment as well as what the treatment level really means. While it is generally felt that the higher the treatment the better, this is oftentimes not the case because we are destroying the integrity of the polymer surface when we treat to very high levels with typical treatment systems. More on this next time
Posted by Eldridge M. Mount on October 15, 2007 | Comments (0)
