We begin this article by presenting a model for gas permeation through defects in barrier coatings, which is based on a very simple geometrical approach. This model allows us to evaluate permeation through a single- or through multiple circular defects in the coating, and also in special cases of size distributions and noncircular geometries. The model agrees well with published results based on complex computer simulations, and it has proven very useful in the analysis of our permeation measurements for barrier-coated plastic films. We then present correlations between measured O2 transmission rate (OTR) values and the number densities and size distributions of defects in SiO2 and SiN barrier coatings on polyester (PET), results which we compare with published data for aluminized PET. We also show the temperature dependence of OTR and H2O vapor transmission rate for SiO2 and SiN coatings deposited on one and on both sides of PET films. The apparent activation energies evaluated from these measurements support the model of defect-dominated mass transport, but suggest different mechanisms for oxygen and water vapor transmission: Excellent agreement is found between measured and calculated OTR values; these, and the apparent activation energy for OTR through single or double side SiO2 and SiN coatings on PET films, confirm that the observed residual OTR is indeed controlled by the presence of (pinhole) defects in the coatings.

Transparent barrier coatings on polymers are receiving much attention in industry, for pharmaceutical, food and beverage packaging applications. Plasma-enhanced chemical vapor deposition (PECVD) is among several competing techniques which can produce thin layers of inorganic glassy barrier materials. In this article we describe the performance of silicon compounds (SiO2 and Si3N4) on 13 μm polyethylene terephthalate (PET) substrates, the barrier coatings being deposited in a dual-frequency (microwave/radio frequency) pilot-scale PECVD reactor for continuously moving flexible webs up to 30 cm in width. The volatile silicon compound used for SiO2 deposition is HMDSO (C6H18Si2O), while SiH4 serves to deposit Si3N4. Coating thicknesses, d, in the range 8 nm⩽d⩽200 nm, are measured using a variety of techniques, namely stylus profilometry, continuous wavelength optical interferometry, x-ray fluorescence, variable angle spectroscopic ellipsometry, and transmission electron microscopy, while film compositions are determined by x-ray photoelectron spectroscopy. Oxygen transmission (OTR) and water vapor transmission (WVTR) measurements are carried out with MOCON “Oxtran” and “Permatran-W” instruments, respectively. As also reported by other workers, we typically find OTR values of about 0.5 scc/m2 day and WVTR about 0.3 g/m2 day, for barrier thicknesses exceeding a “critical” value (dc, about 15 nm), but the minimum permeation values depend upon the concentration of defect sites in the coating (mostly related to substrate microroughness). In order to confirm this correlation, we have developed a technique combining reactive ion etching through the PET, followed by optical and transmission electron microscopies, to characterize the types and number densities of coating defects. On the basis of these, we find good agreement between measured and calculated values of OTR.

No abstract

Diamond-like carbon films, doped or undoped with silver nanoparticles, coating the base of PMMA-based dentures could be an alternative procedure for preventing candidosis in denture users.

No abstract