Influence of pore spacing in barrier coatings on the mass transport through plastics—a simulative and experimental approach

Abstract

In many applications, the use of plastics is only made possible by functionalizing them with coatings for barrier against gases or flavors. These coatings are often produced in low-pressure processes such as plasma enhanced chemical vapor deposition or physical vapor deposition processes. Mass transport through these systems takes place mainly through pores in the nanoand micrometer range. A lot of research has been done to describe the mechanisms of permeation through these thin films under consideration of pores and their influence to each other on the permeation. Until now, all investigations to describe the influence of neighboring pores have been mathematical or simulative. In this work the barrier coating is substituted by a stainless steel foils with 10 000 micro pores (d = 1.5 µm) at different spacings. The micro drillings by means of ultrashort lasers proved to be extremely reproducible and well suited for the application. Real transmission rates are compared and correlated with simulated data to gain a better understanding of the permeation processes. The comparison shows a good agreement of the data regarding the curve progression but most simulation models underestimate the influence of adjacent pores. Real measurements of oxygen transmission through polyethylene terephthalate with porous barrier coatings show an influence up to a pore spacing of 100 µm or 133 times the pore radius, which results in a 66%-100% larger spacings than stated in other simulations in literature.

On the role of defects in modelling approaches for thin film gas barrier coatings on polymer substrates: I. Model development

“…They used the ILT to estimate D and S coefficients of each layer. Recognizing the defective properties of the AlO x barrier coating, they estimated an effective D. For this, they applied an area fraction model that assumes the defects to be filled with polymer and factors in the influence of defect spacing as described elsewhere [20,22,25]. Using WVTR measurements of single-barrier systems to calculate multi-barrier systems with their model, they found that defects were the dominant diffusion pathway.…”

“…They found that the relation of defect area and resulting flow rate is not linear, which could be partially explained by the significant flow reduction due to defect cluster formation. In a following paper, Wilski et al [25] validated the simulation model with an experimental approach, emulating a porosity with defined pore sizes and distances by precise laser drilling of a stainless-steel foil that subsequently was bonded with a PET film. The measurements showed good agreement with the simulation data, confirming the mitigating influence on mass flow of adjacent pores.…”

“…In order to obtain information about nanoscale defects in the coating, variable energy positron annihilation lifetime spectroscopy (VEPALS) can be conducted. It has already been successfully applied on SiO x coatings [33,34], yielding valuable insights on their structure. The method is described in detail elsewhere [35].…”

“…This change in lifetime is measured and evaluated, resulting in depth-resolved information about defect size (within the scope of this work, all defects are assumed as spherical) and relative intensity. Table 1 shows the evaluated VEPALS data from Wilski [33], which we used to develop an algorithm to set up volume elements of coating with a corresponding defect allocation pattern.…”

“…In our algorithm, defect overlap is limited to the mean radius of two adjacent pores to reduce distortion of Table 1. Processed VEPALS data from Wilski [33] for five SiOx coatings with varying thickness. For each measurement, the detected defects were assigned to two groups.…”