Material information
Carbon Fiber is a kind of high-strength, high-modulus, soft, and machinable fiber material with excellent physical and chemical properties. Carbon Fiber Reinforced Polymer/Plastic, CFRP has been widely used in energy facilities, aerospace, military equipment, rail transit, and sporting goods. Global demand for carbon fiber reached 107,000 tons in 2020 and continues to grow. Guangzhou Saio's "2020 Global Carbon Fiber Composites Market Report" predicted that the global carbon fiber demand will break 200,000 tons in 2025.
With the increasing application of CFRP, more and more waste is generated in the process of processing, use, and waste. For example, wind power technology has been rapidly promoted in recent years, but the service life of the blades is only 20-30 years. By 2030, the discarded blades of CFRP wind power will reach tens of thousands of tons per year. In the civil aviation sector, nearly 8,500 aircraft worldwide will be retired by 2025, and the recycling of CFRP waste from their bodies is a challenge. With the improvement of people's environmental awareness, the relevant environmental laws and regulations of CFRP waste disposal are becoming more and more strict. How to efficiently recycle carbon fiber for recycling has become a hot issue at present.
The resin used in processing CFRP is divided into thermoplastic resin and thermosetting resin. The recovery method of thermoplastic resin matrix composites is simple, but the viscosity of the thermoplastic resin in the melting process is high, the adhesion of resin on the fiber surface is poor, and the interface property of composite material is poor, which affects the performance of products. At present, the CFRP matrix is a mainly thermosetting resin, such as unsaturated resin, phenolic resin, and epoxy resin. After curing, the thermosetting resin will form a three-dimensional network cross-linked structure, which can improve the physical properties of CFRP compared with thermoplastic resin.
CFRP recovery technology
CFRP can not be degraded naturally, incineration or landfill is the early treatment method, but CFRP waste incineration will produce a large number of toxic and harmful gases, and affect the natural environment, at the same time, landfill incineration waste residue will also cause soil secondary pollution; Landfill treatment of CFRP waste will pollute the soil and occupy a large number of land resources. This paper mainly introduces the application of the mechanical recovery method, heat recovery method, and solvent recovery method in CFRP recovery, and expounds on the challenges and prospects of each method in practical application.
(1) Mechanical recovery method. The mechanical recovery method is to roll and grind CFRP waste under the action of mechanical force so that carbon fiber is stripped from the resin matrix, resin particles and short carbon fiber can be obtained after treatment. The millimeter scale cut fiber can be used as building filler, and the micron scale cut fiber can be used as the blend filler of sheet molding plastic, block molding plastic, or thermoplastic. The mechanical recovery method has the advantages of a simple process and low investment cost. It does not produce new environmental pollution while recovering the fiber and resin. However, the mechanical force causes damage to the fiber during the separation process of resin and fiber, and the fiber performance retention rate is low.
(2) Heat recovery method. According to the different process routes, the heat recovery method can be divided into the high-temperature thermal decomposition method, fluidized bed thermal decomposition method, and microwave thermal decomposition method. The principle is to decompose the resin into small molecular compounds under the action of heat energy.
① High-temperature thermal decomposition method. Firstly, CFRP waste is made into fragments under the action of mechanical force. The fragments are heated to 600±200℃ under the inert gas atmosphere, and the resin is decomposed into low molecular pyrolysis oil and pyrolysis gas under the condition of no oxygen. The pyrolysis gas is mainly composed of carbon dioxide, hydrogen, and methane. Then inject an appropriate amount of oxygen into the system, so that low molecular combustible combustion, combustion generated heat continues to provide heat energy for the system. The oxygen in the incoming system needs precise quantitative control. Excessive oxygen intake will increase the risk of the explosion of the system. Meanwhile, it will also cause peroxidation of recycled carbon fiber and decrease the mechanical properties of the fiber. The oxygen intake is too low, and the residual resin and pyrolytic oil on the fiber surface can not be removed completely, which affects the fiber finish. The temperature of high-temperature pyrolysis depends on the type of resin. Generally, the polyester resin can decompose at a lower temperature, while epoxy resin needs to decompose at a higher temperature. Because of the simple operation and high recovery, the high-temperature thermal decomposition method has been applied in industry. After the treatment of high-temperature thermal decomposition, the smooth surface of the short carbon fiber can be obtained, but after the treatment of the fiber will occur different degrees of oxidation, and the surface of the fiber occasionally carbon deposition, affecting the mechanical properties of the fiber.
② Fluidized bed thermal decomposition method. The process of CFRP recovery by fluidized bed thermal decomposition is shown in Figure 1. The composite material is imported from the waste material and added to the fluidized bed. The resin in the composite material is decomposed at high temperatures in the hot air flow field, and the decomposed pyrolysis gas is continued as the heat energy of the system through combustion. After thermal decomposition, carbon fiber and resin particles are recovered in the cyclone separator. The thermal flow takes the recovered fiber to the fiber tank, while the refractory material is left at the bottom of the fluidized bed. The CFRP processed by fluidized bed pyrolysis is generally 2 ~ 3cm² in size, which can be continuously fed to the fluidized bed to achieve continuous production, and the short carbon fiber can be obtained by recycling. The friction between the inner wall of the cyclone separator and the gas-bound gravel in the fluidized bed and the fiber will cause some mechanical damage, so the tensile strength of the fiber after this treatment will be reduced by about 1/4.
③ Microwave thermal decomposition method. CFRP was placed in a microwave irradiation field, and the resin was heated by microwave to degrade into small molecular compounds. The microwave thermal decomposition method can effectively shorten the time required for carbon fiber recovery, the number of equipment is relatively small, and the process operation is simple.
(3) solvent recovery method. The solvent recovery method refers to the resin in CFRP waste in the solvent declining into soluble substances, through the degradation and dissolution of the resin to achieve the separation of fiber and resin, carbon fiber after washing, and drying to recover fiber. Solvent recovery methods are generally divided into ordinary solvent methods under normal pressure and supercritical solvent method under high pressure.
① Common solvent method. The common solvent method uses nitric acid and alcohol as the reaction solvent of degradation resin under atmospheric pressure, which is simple in operation and low in equipment input cost. The recovered fiber basically maintains the original fiber length and can be used again as long fiber in composite materials. However, the degradation time of resin in the solvent is longer, and the waste solvent treatment after use is difficult, which increases the recovery cost and easily causes environmental pollution. According to the different forming processes of the composite material, the resin used is different, and the process used is different.
② Supercritical solvent method. When the temperature and pressure of a substance exceed a certain critical temperature and pressure, the special state of high compressibility, high solubility, high permeability, high diffusivity, low density, and low viscosity is called "supercritical state", and the solvent in this state is called "supercritical solvent". The CFRP waste resin was degraded by a supercritical solvent with high solubility and high permeability of polymer materials, and the purpose of carbon fiber recovery was realized. Using this recycling method, the fiber surface is smooth, the fiber maintains the original length, the fiber performance is high, the recycling process does not produce new pollution, green environmental protection. However, the application of this method requires a large investment in equipment and harsh process conditions, and it is still in the laboratory stage temporarily and has not been transformed into an industrial one.
Expectation
With the progress of carbon fiber production technology, carbon fiber production increases at the same time the production cost is gradually reduced, its application in various fields will continue to increase, and CFRP recovery and reuse have become a prominent problem restricting the extensive application of carbon fiber.
