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What composite materials will be used in Type IV hydrogen storage tanks?
1. The current hydrogen storage bottles can be divided into four categories:
All-metal gas cylinder type I (not on-board) Metal Liner Fiber Circumferentially Winding Gas Cylinder Type II (Not Vehicle-mounted) Metal liner fiber fully wound gas cylinder type III (car-mounted) Plastic liner fiber fully wound gas cylinder type IV (car-mounted)
As we all know, hydrogen is a flammable and explosive gas, and the atomic diameter is very small. It is very likely to penetrate into metal materials, or to deteriorate the metal, resulting in hydrogen embrittlement, resulting in safety hazards such as combustion and explosion. The liner of the IV bottle is made of polymer material, which can not only avoid the problem of hydrogen embrittlement, but also meet the requirements of lightweight. Therefore, the IV bottle with a plastic liner has also become the next focus of hydrogen storage bottle research and development.
2. Type IV hydrogen storage bottle structure
Except for the metal bottle valve seat, the bottle body of the IV type hydrogen storage bottle is all made of non-metallic materials
The innermost gas barrier layer: in direct contact with hydrogen, with a thickness of about 2~3mm, it is an olefin plastic polymer that acts as a barrier to hydrogen;
Middle pressure-resistant layer: Carbon Fiber Reinforced Composites (fibra de carbono venta + epoxy resin), the thickness of the layer is the thickest. On the premise of ensuring the pressure resistance level, the thickness of this layer should be reduced as much as possible to improve the hydrogen storage efficiency;
The outermost protective layer: Glass Fiber Reinforced Composites (gam fiberglass + epoxy resin), with a thickness of about 2~3mm. Know the major categories of materials, and what non-metallic materials can be used in the production and application of type IV hydrogen storage bottles? It must be a topic that everyone is curious about. Next, let's take a look at it from the application point of view:
3. Hydrogen barrier material
The liner material of the hydrogen storage bottle is the key to the safety of hydrogen barrier
The barrier properties of EVOH copolymer to oxygen, water vapor and carbon dioxide are far beyond other polymer materials. Although PVDC polyvinylidene chloride has excellent barrier properties, it is not suitable for use in gas storage tank liner that is in direct contact with hydrogen because the residual trace chlorine gas will react with hydrogen gas.
EVOH has both the gas barrier properties of PVA and the processability of PE. By adding EVOH, the liner of the hydrogen storage bottle is elastic even at low temperature, ensuring excellent barrier properties. The high gas barrier properties of EVOH are mainly due to high crystallinity and hydrogen bonding of hydroxyl groups. When the ethylene content decreases, the gas barrier property increases; when the ethylene content exceeds 50 mol%, the gas barrier property is seriously impaired.
It should be noted that the gas barrier property decreases with the increase of temperature. For example, when the temperature increases from 20°C to 35°C, the oxygen transmission rate increases by more than 3 times, and when the relative humidity is greater than 30%, water molecules and EVOH in the The effect of the hydroxyl group will lead to a significant decrease in gas barrier properties.
The hydrogen is compressed to 70Mpa and stored in the storage tank. It is necessary to ensure that the strength of the main body can withstand high pressure. When the hydrogen in the storage tank is consumed, the main body also shrinks as the pressure decreases. The high pressure environment and repeated inflation and deflation will cause the material fatigue. Using glass chopped strands as raw materials, high strength and high elastic modulus are achieved through the firing technology of nano-scale finely controlled fiber structure, and nano-alloys of composite materials are prepared by means of pre-impregnation process.
Fiberglassing used as storage tank material is not only impermeable, but also has excellent heat and chemical resistance, as well as good dimensional stability, strength and wear resistance. The carbon fiber bundle consists of 10,000 to 50,000 carbon fibers with an average diameter of 5 to 8 μm. The weight ratio of the epoxy resin layer and the Carbon fiber layer is 20-30:70-80.
Between the skeleton carbon material and the adhesive, there is not only physical adhesion, but also chemical bonding. Aromatic epoxy resins provide higher strength than aliphatic epoxy resins such as bisphenol A.However, high viscosity will make the dipping operation difficult, and it is difficult to impregnate the interior uniformly, so epoxy resin or solvent with a low degree of polymerization is used to adjust the viscosity during dipping. The disadvantage is that the dipping must be repeated to control the volume shrinkage rate. large fluctuations.
5. Epoxy composites
The viscosity of epoxy resin used for hydrogen storage tank is 4000~8000cP at 25℃, and the solid particle size is less than 10~25μm. The heat resistant fiberglass in the outermost fiber reinforced resin layer has low thermal conductivity, so it should be ensured that the equivalent ratio of epoxy resin to curing agent is 1:1, and the inorganic ferrite particles that can absorb microwaves should be uniformly dispersed in order to cure sufficient, or the fibers are reinforced by heating after irradiating microwaves.
In the process of epoxy curing by heat, the polyolefin liner will be softened. In order to protect the structure of the liner, the maximum curing temperature should be controlled to be more than 10°C lower than the softening point of the liner. Under the action of high pressure, hydrogen penetrates into the material and gradually diffuses to the interface of the composite material, and the repeated charging and unloading of hydrogen forms a pressure difference, which eventually leads to the yielding and blistering of the liner material and the tearing of the interface. An improved approach is to add clay to the composite.