An experimental study of hydrogen sorption and permeation in high-performance polyamides

Semicrystalline polyamides (PAs) are optimal materials to develop high-pressure resistant liners for type IV hydrogen storage tanks due to a favorable combination of barrier performance, mechanical resistance, and lightness. However, experimental data on hydrogen transport in PAs are incomplete or inconsistent, and usually do not report separately the contributions of solubility and diffusivity, hence limiting a deep understanding of the permeation mechanism and its dependence on the material structure. Moreover, recent developments have led to the design of modified polyamides which could better serve the high-pressure storage applications. In this work, the hydrogen barrier performance of Polyamide 6 (PA6), Polyamide 11 (PA11) and an impact-modified PA 6 (PA6-I), was evaluated and the results obtained with different techniques and on different samples compared. Permeation measurements were performed in constant-volume and constant-pressure apparatuses at different temperatures and pressures, on different samples of each material. Sorption measurements were carried out into a differential sorption system. Results from the permeation and sorption devices were compared against each other and with literature data, allowing to understand the effect of various factors. The H2 2 solubility in PA is mostly affected by density, as a lower free volume of the amorphous phase leads to a lower gas uptake. On the other hand, diffusivity and, consequently, permeability, are also strongly affected by the morphology of the crystal phase, which depends on the production protocol. In most of the cases inspected, the discrepancy between data from different experimental techniques or literature works can be explained by the different crystal morphology of the samples used in the test. Temperature enhances diffusivity, permeability and solubility, while the pressure reduces the permeability, as it lowers the free volume, and increases the activation energy of permeation. An estimation of the minimum thickness required to meet high-pressure storage technical guidelines was provided for the case of PA6-I.