Abstract
The effect of temperature and relative humidity on the permea tion of ethyl acetate vapor through silica deposited polyethylene terephthalate film (SiOx PET) and composite structures was studied by a quasi-isostatic proce dure. For comparison, the permeation of ethyl acetate vapor through an ethyl ene/vinyl alcohol copolymer (EVOH), and an EVOH laminate structure was also determined. Permeability studies were carried out at 50, 65 and 76 ° C with a constant vapor concentration of 300 μg/cc (mass/volume).
Results of permeation studies at 65 ° C showed that the transmission profile curve of the EVOH film followed typical Fickian behavior, while the SiOx PET film exhibited atypical behavior during the initial transport period. For the first 24 hours, permeation through SiOx PET was not detectable. Following this initial induction period, a significant increase in the transmission rate was observed; and after 96 hours, mass transport was at steady state. At steady state, the rate of permeation through the SiOx PET was lower than that of the EVOH film.
SiOx PET film showed relatively high temperature dependence (Ep = 22.1 kcal/mole) and a low transmission rate. PET also showed relatively high tem perature dependence (23.2 kcal/mole). Based on the activation energy, the EVOH film exhibited very significant temperature dependency (Ep = 48.8 kcal/mole).
In over 500 hours of continuous testing, there was no measurable permeation of ethyl acetate vapor through the SiOx PET and EVOH films at 22°C, 56% RH. However, at 87% RH, the EVOH film exhibited a permeation rate of 3.14 x 10-11 kg/m 2·sec for ethyl acetate vapor, while the SiOx PET film was not influenced by sorbed water vapor and showed no measurable permeation of the organic permeant.
Analysis by optical microscopy on the SiOx surface showed no effect of temper ature on the structural integrity of the SiOx coating, as evidenced by the absence of cracks or crazing of the SiO x coating. While the functional role of the silica coating is not fully understood, the following mechanism is proposed. For the silica deposited PET film, the barrier properties of the structure are governed by defects, non-homogeneity of both the surface coating thickness and composition, or preferential diffusion paths-all of which can contribute to the observed permeability rates. The permeability of the polymer layers imme diately adjacent to the silica coating are thus critical in determining the effect of such defects.
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