Abstract
Having worked intensely on plasma-assisted barrier coating (plasma enhanced chemical vapor deposition, PECVD) of polymers, a technique already used commercially to enhance shelf-life performance of PET bottles, for the last two decades, it seemed to be obvious to investigate the transfer of this existing technique to novel beverage materials such as PLA and PP. As a result of their high oxygen and carbon dioxide permeability the market potential of PP and PLA is still restricted in the beverage packaging sector.
On the one hand, the use of plasma coatings to improve the barrier properties of these materials shows advantages. For example, these coatings are less likely to interfere with PLA-bottle biodegradability. On the other hand, materials such as PP and PLA put higher demands on plasma coating processes due to their different surface properties that make it much more difficult to establish an adequate adhesion between coating and bulk material and their increased sensitivity to high temperature.
By using a pre-treatment specific to the materials, we succeeded in applying effective barrier coatings by plasma polymerization using both hydrocarbons as well as silicon organic monomers. For PLA the pre-treatment process can be integrated directly into the plasma polymerization process, whereas for PP it was necessary to use a LF-plasma in an external facility to enable a good coating adhesion.
The barrier properties against oxygen could be increased by a factor of about 12 for PP and 4 for PLA. Hydrocarbon-based coatings proved to be more effective compared to HMDSO-based coatings. These first evaluations show that it is possible to raise the barrier performance of PP and PLA bottles to match the levels obtained with uncoated monolayer PET types. Our current research concentrates on improving the processes in order to increase the barrier performance and the efficiency of the process.
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