Study on the barrier properties of polyamide resins

Barrier properties traditionally consist a topic of great interest in polymer science and technology. Focusing on the critical field of plastic food packaging, mass transport phenomena are very important as potential migration of substances from the plastic to the packaged foodstuff can be proved crucial for consumers safety. In the present doctoral thesis, a thorough study is conducted regarding the diffusion mechanisms of substances having a molecular weight close to the one of the usual commercial additives that are used in plastic food packaging. The study was focused on polyamidic matrices, polymers that have a substantial technological and scientific interest. The effect of migrant size in the diffusion process through polyamides was studied by introducing a novel homologous probe series covering a wide range of molecular weights. The study was conducted using the Fluorescence Recovey After Photobleaching method to assess the diffusion coefficient. The results highlighted the variation of the correlation between the migrant size and the diffusion coefficient as a result of the polymer matrix mobility enhancement, while an empirical relationship is given that describes this correlation in the case of polyamides. In addition, the effect of hydrogen bonding in the diffusivity of two almost identical probe molecules in polyamides was studied using the Fluorescence Recovey After Photobleaching method. The experimental results indicated the crucial role of the intermolecular interactions that lead to lower diffusion coefficients when present. Turning to the effect of polyamide matrix mobility in diffusion phenomena, the diffusion of three different substances was studied in water plasticized molecules conditioned in various relative humidity environments at a constant temperature, but also in dry polyamides stabilized in different temperatures close to the glass transition point, by conducting three layer tests. Through the experimental results it was showed for the first time the “temperature-humidity” superposition of substantially high molecular weight diffusing molecules, close to the glass transition temperature of polyamides