Sorption and transport of gases and organic vapors in poly(ethylene terephthalate)
Poly(ethylene terephthalate) (PET) is one of the most widely used barrier polymers for food and beverage packaging. The sorption and transport of organic molecules in PET are important considerations in flavor scalping, carryover, contamination, and therefore, container shelf life. However, literature data on transport properties of large flavor and aroma compounds in PET are limited due to their extremely low vapor pressures and long experimental timescales. The goal of this research project is to develop a systematic database of sorption and transport properties of large organic compounds in PET by overcoming these technical challenges. Ultra-thin (0.9 µm thick) biaxially oriented PET films were used to overcome the drawback of long experimental timescales. Several model flavor compounds (i.e., low molecular weight analogs of common flavor compounds) were selected from various classes of non-polar and polar organic compounds to study the effect of penetrant size, shape, and thermodynamic properties on solubility and diffusivity in PET. For example, n-butane, i-butane, n-pentane, and i-pentane were selected to study the effect of chain length and branching in non-polar alkane hydrocarbons; acetone, methyl ethyl ketone (MEK), methyl n-propyl ketone (MnPK), and methyl i-propyl ketone (MiPK) from the family of ketones, and methyl acetate and ethyl acetate from the family of esters, were also studied to provide a similar series of data for polar organic model flavor compounds. To overcome the drawback of extremely low vapor pressures of large organic compounds, a new experimental sorption technique was developed. It is estimated that penetrants with vapor pressures as low as 10-7 mmHg might be studied using this technique. It could be useful in industrial applications involving polymer membranebased gas/vapor separations and food packaging. The solubility and diffusion coefficients of toluene vapor in PET were obtained using this technique. Interestingly, the solubility difference between n-pentane and i-pentane is 10 times larger in glassy PET than in rubbery low-density polyethylene. Among the ketones and esters studied, both solubility and diffusivity decrease in the following order, acetone
MEK > MnPK > MiPK and methyl acetate > ethyl acetate. The diffusion coefficients of MiPK and i-pentane are among the lowest ever reported for PET. Surprisingly, diffusion coefficients of acetone and methyl acetate, and MEK and ethyl acetate are quite similar despite the esters being somewhat larger than ketones. For all penetrants studied, solubility and diffusion coefficients correlate well with penetrant critical temperature and critical volume, respectively. By extending these correlations, it might be possible to predict the sorption and transport properties of large flavor compounds in PET with reasonable accuracy.