Preparation Process And Gas Barrier Mechanism Of Graphene Reinforced Polyethylene Composites

Hydrogen energy is the mainstream technical route of domestic energy technology innovation,and hydrogen storage tank technology is one of the four key technologies for its development.To improve the hydrogen storage density,lightweight Type IV hydrogen storage tanks with polymer liners have become a key development direction in China,and the gas barrier performance of polymer liners is the core indicator that restricts the widespread use of Type IV tanks.Based on the urgent need to improve the gas barrier properties of polymer liner materials for hydrogen storage tanks,this paper focused on the research work in four aspects:the epitaxial crystal modification of low-defect graphene flakes（GFs）,the preparation and gas barrier mechanism of unoriented GFs reinforced composites,the microstructure orientation treatment and gas barrier mechanism of composites,and the effect of mechanical and thermal properties of composites on gas barrier properties.Finally,the dispersion problem of low-defect GFs in the polymer matrix was emphatically solved,the orientation arrangement of GFs and molecular chain in composites was realized,the gas barrier mechanism of unoriented composites and oriented composites was revealed,and the“continuous weak interfaces”mechanism and gas diffusion model suitable for GFs reinforced composites were proposed.Thus,it lays a theoretical and experimental foundation for the application of GFs reinforced polymer in the liners of hydrogen storage tanks.In this paper,low-defect GFs were efficiently prepared via an ultrasonic-assisted electrochemical exfoliation method,and high density polyethylene（HDPE）was used as a modifier to achieve controllable functionalization of GFs by precisely inducing the epitaxial crystallization of HDPE molecular chains along the surface of GFs.The test results showed that the average lateral size of GFs is 2.79μm,and the average thickness of GFs is about 4.28 nm.Therefore,the aspect ratio of GFs can reach 650.Moreover,the C/O ratio of GFs is 13.1,and the D-band to G-band intensity ratio of GFs is only 0.57.This indicated that GFs have fewer oxygen-containing groups and defect content.Benefiting from the high structural integrity of GFs,the epitaxial crystallization of HDPE molecular chains on the surface of GFs was effectively induced by the controlled solution crystallization method.Through comparative experiments,the optimal modification conditions of GFs were summarized:the crystallization temperature is 90-95℃,the optimum GFs:HDPE mass ratio is4:1-1:1,and the crystallization time should greater than 30 min.Under this modified condition,a layer of rod-like HDPE crystals with orientation characteristics will be formed on the surface of GFs,thereby weakening the interaction between GFs flakes,improving the compatibility with HDPE matrix,and facilitates the dispersion of GFs.The unoriented GFs/HDPE composite pellets were prepared,the microscopic“continuous weak interfaces”mechanism of GFs affecting the gas barrier properties of the composites was proposed,and a gas diffusion model based on this mechanism was established.The research results showed that GFs can effectively improve the gas barrier properties of HDPE,but there is an optimal concentration value.When the GFs content was 0.5 wt%,the helium leak rate of the composite reached the lowest value,which was 1 order of magnitude lower than that of pure HDPE.However,once the GFs concentration exceeded 0.5 wt%,the GFs could connect to adjacent sheets to form continuous interconnected networks,and the cohesive energy of the interfaces between the network structures and the HDPE matrix is as low as 0.1448 J·m^-2.Therefore,“continuous weak interfaces”were formed in the composites,which provided an effective continuous diffusion path for gas molecules,thereby reducing the gas barrier efficiency of the composites.The oriented GFs/HDPE composites were prepared by the induced orientation effect of the tensile-shear coupled flow,and the mechanism of the orientation induced by the coupled flow was analyzed by deriving the melt rate equation of the composites.The research results indicated that the GFs and HDPE molecular chain were highly oriented,the crystallinity of the composites was increased by 11.4%,and the fractional free volume of composites was reduced from 6.52 vol%to 5.43 vol%.Furthermore,it was found that GFs can be combined with HDPE crystals to form super large impermeable areas with aspect ratios up to 3500.As a consequence of the synergistic effects of the above four aspects,the helium leak rate of the oriented composites with 0.5 wt%GFs was reduced by 2 orders of magnitude compared with unoriented pure HDPE.The tensile properties,thermal properties and bonding performance of GFs/HDPE composites were studied,the influence of mechanical and thermal properties on gas barrier performance was clarified,and the microscopic mechanism of GFs concentration and orientation of composites affecting the properties of the composites was analyzed.The test results showed that the orientation of GFs and polymer molecular chains can significantly improve the tensile strength and thermal diffusivity of the composites.The addition of GFs can significantly improve the bonding strength of composites to stainless steel,and the optimal GFs concentration is 0.5 wt%.The improvement of bonding strength is attributed to the increase of mechanical interlocking between GFs and stainless steel and the strong covalent interaction between GFs and nickel in stainless steel.