Mechanisms of permeation of helium, hydrogen, oxygen, and water vapor through silicate-based composite barrier coating layers

When considering coating of flexible films for the packaging of sensitive products, a common goal is to meet all gas barrier requirements in a single process step. One way to achieve this is to improve the barrier performance of polymeric coating layers by incorporating silicate particles. In order to tailor the gas barrier performance of the coatings, understanding the permeation mechanisms through these composite coating layers is required. In this study, polyethylene terephthalate films were coated with composite lacquers comprising montmorillonite particles and a polymer matrix. The compatibility of montmorillonite with polymer matrices of polypropylene, polyacrylate, polycarboxylic acid, and polyvinyl alcohol was tested. The permeation behavior of helium, hydrogen, oxygen, and water vapor in these coatings was investigated. For a composite coating layer comprising montmorillonite and polyvinyl alcohol at a mixing ratio of 1:1 by weight, barrier improvement factors of Image 1 compared with the pure polymer coating were found for helium, hydrogen, and oxygen, respectively. It was shown that the permeability coefficients of composite coating layers decrease with increasing permeant kinetic diameter. A comparison of calculated and measured permeability values indicated that the integration of montmorillonite leads to a tortuous permeation path and changes in the free volume and crystallinity of the polymer matrix. The permeation mechanism for water vapor turned out to be completely different from that for non-polar helium, hydrogen, and oxygen and is determined by the so-called polar path effect.