Isostatic and Quasi-Isostatic Methods for Determining the Permeability of Organic Vapors Through Barrier Membranes

Two test methods have been developed which provide quantitative and reliable values for the rate of diffusion of organic vapors through polymer membranes. Method I is based on a quasi-isostatic procedure and utilizes gas chromatography analysis for quantifying the amount of organic vapor that has permeated through the membrane. The permeability of a number of permeant/polymer membrane combinations was determined by this method, with studies being carried out at constant temperature and with varying permeant concentrations. The effect of permeant concentration on the permeability rate, P, permeability constant, ¯P, the apparent diffusion coefficient, D_a, and the limiting diffusion coefficient, D₀, was determined for the diffusion of toluene vapor through oriented polypropylene, Saran, and a Saran-coated oriented polypropylene structure.

Method II involved an isostatic procedure, in which a constant and low concentration of permeant is flowed through the upper cell chamber of a permeability cell. Simultaneously, nitrogen carrier gas (N₂) of a known flow rate is continually passed through the lower cell chamber of the permeability cell and is conveyed to a gas chromatograph by means of a computer-aided gas sampling valve.

The methods developed allow determination of the diffusion of organic aroma constituents through barrier membranes under defined conditions including (1) temperature, (2) permeant concentration, (3) membrane thickness, (4) polymer morphology (percent crystallinity), and, (5) polymer thermal mechanical history (that is, degree orientation), and they will have significant utility in evaluating plastic packaging material for food packaging, particularly as plastics are showing increased usefulness as packaging for fresh and processed food products.