Tieteestä & Tekniikasta
Specialised epoxy resins for linerless high-pressure hydrogen tanks Authors: Farzin Javanshour, Essi Sarlin, Research group of Plastics and Elastomer Technology, Tampere University Ownership of table and figures: Tampere University
I n line with the current state of global energy transition and eco- nomy, the automotive industry aims to create a synergy between climate and resource efficiency requirements and a competitive economy. Such challenges put hydrogen into perspective as a versatile and environmentally friendly energy source with a gravimetric energy density (120 MJ/kg) higher than petroleum-based fuels. However, the volumetric energy density of hydrogen is less than 10 MJ/L, which necessitates storing hydrogen as a compressed gas at a pressure range of 350-700 bar (i.e., 24-40 g/L at room temperature) for vehicle app- lications. A brief overview of hydrogen pressure vessel technologies is presented in Table 1. The current industrial standard for onboard hydrogen storage tanks is the overwrapped carbon fibre reinforced epoxy composite with polymeric liner (Type IV). The scientific and industrial ambitions are to develop linerless carbon fibre reinforced polymer (CFRP) composite vessels (Type V) with enhanced volumetric storage capacity. Producing such vessels with thin-ply CFRP materials is especially promising as they are light weight, resilient to cryogenic cycling and matrix microcracking [1, 2], and have good fatigue performance over conventional CFRP lamina- tes [3]. However, the hydrogen leak rate of linerless CFRP materials exceeds the allowable limits (roughly 0.25 % gas loss due to perme- ation) for land vehicles [2]. Therefore, it is necessary to develop spe- cialised grades of resins for filament winding, which offer a unique combination of toughness and hydrogen impermeability. In the Polymers4Hydrogen (P4H) project framework, the potential of difunctional epoxy-functionalised ionic liquid (epoxy-IL) resins was investigated as a hydrogen barrier material. Ionic liquids consist of bul- ky organic cations and anions with high solubility for small gas mole- cules, making them ideal material for gas separation. The synthesised epoxy-IL resin was cured with multifunctional amine and anhydride hardeners [4]such as gas separation membranes and polyelectrolytes. Due to the vast number of possible structures, numerous synthesis
protocols to produce monomers with different functional groups for task-specific PILs are reported in literature. A difunctional epoxy-IL resin was synthesized and cured with multifunctional amine and an- hydride hardeners and the thermal and thermomechanical properties of the networks were assessed via differential scanning calorimetry and dynamic mechanical analysis. By the selection of suitable harden- ers, the glass transition onset temperature (Tg,onset. The anhydride hardener resulted in the best glass transition temperatures (Tg) up to 127 °C, suitable for hydrogen vessels. Increasing the aromaticity of ca- tions decreased the hydrogen permeability of epoxy-IL resins to a com- parable level as high-density polyethylene (HDPE) used by industry as a hydrogen barrier liner material in Type IV vessels. Also, the size of anions was a controlling factor for both Tg and hydrogen permea- tion. However, the fracture toughness (G IC ) of epoxy-IL resins cured with anhydride hardeners (100 J/m 2 ) was inferior to the typical G IC values of aerospace grade epoxy resins (e.g., 280 J/m 2 ). Interestingly, coplymerisation of synthesised epoxy-IL with commercially availab- le diglycidyl ether of bisphenol A (DGEBA) [4]such as gas separation membranes and polyelectrolytes. Due to the vast number of possible structures, numerous synthesis protocols to produce monomers with different functional groups for task-specific PILs are reported in lite- rature. A difunctional epoxy-IL resin was synthesized and cured with multifunctional amine and anhydride hardeners and the thermal and thermomechanical properties of the networks were assessed via diffe- rential scanning calorimetry and dynamic mechanical analysis. By the selection of suitable hardeners, the glass transition onset temperature (Tg,onset resulted in a unique combination of good Tg in the range of 145170 °C and G IC of 300 J/m 2 [5]. Another milestone was achieved by developing well-dispersed graphene oxide (GO) modified epoxy-IL copolymers (Figure 1) with G IC in the range of 600700 J/m 2 and hydro- gen permeability similar to HDPE tested at 400 bar (23 °C), which is a significant development towards linerless Type V CFRP vessels [5].
Figure 1. Graphene oxide (GO) reinforced composites. A: a GO crystal with a theoretical surface area of 1315 m 2 /g and crystal thickness in the range of 100-300 nm, B: GO-Acetone dispersion with GO particle size of 13-20 µm, C: GO modified epoxy-IL copolymer, D: GO modified CFRP specimen (representing linerless vessel) for hydrogen permeability tests at 400 bar (23 °C).
10 MUOVIPLAST 1/2024
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