Influence of Adjacent Defects on the Permeability of Plasmapolymer Coated Plastics

Permeation through plastics is an important factor in food and medical packaging, or in solar technology. The cause for usage of plastics is the variety of advantages, but on the other hand, the high permeability for gases or vapours has to be taken into account to reduce the mass transport. For this reason, the material is often coated with high barrier layers, e.g. SiOx-layer, that effectively reduces the permeation of gases and vapours. One of the established technologies to realise barrier coatings is the PECVD technique, in which a thin film of a few ten nanometres is generated by depositing the barrier film from the gas state to the solid state on the polymer. In theory, the permeation reduction should be much higher than typically measured. This is caused by defects in the barrier layer that arise by reasons of the film growing process during PECVD or contaminations on the polymer. A lot of research has been done to describe the mechanisms of permeation through thin films under consideration of the influence of adjacent defects. Typically a distance was defined under which an influence occur, but the influence itself was not described. In this paper, the focus is on the influence of adjacent defects. Assuming an amorphous polymer the permeation can be described by the Fick's-Law. This requires knowledge of the defect and defect-size distributions, which are predetermined by etching methods and subsequent SEM imaging. A critical defect spacing parameter and an attenuation function will be derived from the results to describe the influence of adjacent defects. The results show that a superposition principle can be applied. Depending on the number of adjacent defects and their size the permeability of an individually defect can be described. In the future, the permeation through defects will be described by a molecular dynamic approach. The output of the first step of the simulation will be a time and defect size dependent concentration function, which serves as input for here performed macroscopic simulation.