Investigations of plasma polymerized SiOx barrier films for polymer food packaging

Abstract

Nano-scale SiOx layers were deposited on polyethylene terephthalate (PET) foils by plasma enhanced chemical vapor deposition (PECVD) in an electron cyclotron resonance (ECR) plasma process in order to enhance their barrier properties towards water vapor. Oxygen (O2) and hexamethyldisilazane (HMDSN) served as reactive gas and precursor, respectively. The effect of layer thickness and O2:HMDSN (x:1) gas mixture ratio on the water vapor transmission rate was systematically investigated. Measurements by infrared spectroscopy and scanning electron microscopy for the characterization of the chemical composition and of the surface structure of the SiOx layers, respectively, showed that both chemical composition and surface structure of the layers have a noticeable effect on their barrier properties. For low O2 content in the O2:HMDSN gas mixture ratio, organic layers were deposited. When increasing the O2 content, the growing number of inorganic compounds in the SiOx layers found by the infrared spectroscopy gave rise to a decrease in the water vapor transmission rate. A reduction of the water vapor permeation by more than a factor of 2 in comparison with the uncoated PET foil was achieved by the best performing SiOx layer. Further increase of the O2 content led to the onset of a columnar-like layer growth which showed to be causative for the water vapor permeation rising again.Finally, the barrier properties towards water vapor of 100nm thick SiOx films deposited from different O2:HMDSN gas mixtures were contrasted with their corresponding barrier properties towards O2. The minimum water vapor and O2 permeation results were found for the SiOx films plasma deposited from almost identical O2:HMDSN gas mixture ratios in the range of 25≤x≤30.