Gas permeability of thermoplastics at low temperature: effect of glass transition and crystallinity
Topic(s) : Experimental techniques
Co-authors :
Tristan DURAND (FRANCE), Quentin SIRVIN , Olivier DE ALMEIDA (FRANCE)Abstract :
In the current context of ecological transition, air transport is turning to hydrogen technologies. This new energy vector will help reduce greenhouse gas emissions. Depending on the type of application, hydrogen will be used and stored in different ways. For long-distance travel, companies such as Airbus have opted for the use of liquid hydrogen (LH2). This choice is motivated by the fact that in its liquid form (20K at atmospheric pressure), hydrogen can be stored with a higher energy density than in its gaseous state (>200 bar at room temperature).
Currently, due to the high volatility of hydrogen, cryogenic storage tanks for this type of liquid are mainly made of metal. The development of organic matrix composite tanks capable of storing LH2 at low temperatures is therefore required for the deployment of these technologies in the aeronautical industry. The low temperature permeation process through organic matrix composites is thus of prime importance as the main property required in a tank is its tightness (especially for hydrogen of very low steric hindrance).
This study focuses on thermoplastic resins since their toughness, processability and repairability make them promising gas barrier candidates for such applications. Indeed, thermoplastic crystallinity is a key factor of gas solubility and diffusion in polymers and composites [Klopffer et al. 2001]. Nevertheless, its effect on permeability has never been studied at very low temperature in relation with physical relaxations of polymers [Humpenöder et al. 1997, Gambaro et al. 2019]. The effects of crystallinity (Xc) and phase transitions (primary Tg and secondary Tα) were thus studied at low temperature (T < 0°C) through the analysis of different materials such as Poly(Ether-Ketone-Ketone) (PEKK), Poly(Ethylenes) High Density and Low Density (respectively HDPE and LLDPE) and Poly(VinyliDene Fluoride) (PVDF),
To achieve this, a specifically designed test facility (Figure 1) was developed to measure in-situ cryogenic gas permeability, diffusion and solubility at various temperature levels between 293K and 55K (Tests are performed with Helium 6.0 whose size is close to that of hydrogen). The results obtained on different grades of amorphous and semi-crystalline PEKKs (6002, 7002 and 8002) show a temperature-dependence according to an Arrhenius law which pre-exponential factor varies with the degree of crystallinity (Figure 2). The results also demonstrate that HDPE is more impermeable at room temperature and at low temperatures than PEKKs with a higher temperature sensitivity. The results obtained on PVDF (Figure 3) also show that polymer glass transition significantly affects permeability and induces a change in permeability thermo-dependence (a phenomenon already observed for polymers with high Tg [Komatsuka et al. 2009]). Future measurements on HDPE and LLDPE will show the effect of glass transition coupled with crystallinity.
Currently, due to the high volatility of hydrogen, cryogenic storage tanks for this type of liquid are mainly made of metal. The development of organic matrix composite tanks capable of storing LH2 at low temperatures is therefore required for the deployment of these technologies in the aeronautical industry. The low temperature permeation process through organic matrix composites is thus of prime importance as the main property required in a tank is its tightness (especially for hydrogen of very low steric hindrance).
This study focuses on thermoplastic resins since their toughness, processability and repairability make them promising gas barrier candidates for such applications. Indeed, thermoplastic crystallinity is a key factor of gas solubility and diffusion in polymers and composites [Klopffer et al. 2001]. Nevertheless, its effect on permeability has never been studied at very low temperature in relation with physical relaxations of polymers [Humpenöder et al. 1997, Gambaro et al. 2019]. The effects of crystallinity (Xc) and phase transitions (primary Tg and secondary Tα) were thus studied at low temperature (T < 0°C) through the analysis of different materials such as Poly(Ether-Ketone-Ketone) (PEKK), Poly(Ethylenes) High Density and Low Density (respectively HDPE and LLDPE) and Poly(VinyliDene Fluoride) (PVDF),
To achieve this, a specifically designed test facility (Figure 1) was developed to measure in-situ cryogenic gas permeability, diffusion and solubility at various temperature levels between 293K and 55K (Tests are performed with Helium 6.0 whose size is close to that of hydrogen). The results obtained on different grades of amorphous and semi-crystalline PEKKs (6002, 7002 and 8002) show a temperature-dependence according to an Arrhenius law which pre-exponential factor varies with the degree of crystallinity (Figure 2). The results also demonstrate that HDPE is more impermeable at room temperature and at low temperatures than PEKKs with a higher temperature sensitivity. The results obtained on PVDF (Figure 3) also show that polymer glass transition significantly affects permeability and induces a change in permeability thermo-dependence (a phenomenon already observed for polymers with high Tg [Komatsuka et al. 2009]). Future measurements on HDPE and LLDPE will show the effect of glass transition coupled with crystallinity.