Carbon fiber reinforced resin matrix composites (CarbonFiberReinforcedPolymer, hereinafter referred to as CFRP) have the advantages of light weight, high strength, high specific modulus, fatigue resistance, corrosion resistance, designability and good molding processability. It has been widely used in building reinforcement, transportation and other fields. CFRP mostly uses thermosetting polymers (epoxy resin, unsaturated polyester, phenolic resin, etc.) as the matrix resin. After curing and molding, it forms a three-dimensional cross-linked network structure, which cannot be remolded or processed, and it is difficult to handle.
As the application of CFRP becomes more and more extensive, more and more CFRP is discarded, and the recycling of waste has become a new problem faced by the industry and society.CFRP will generate about 30% of the corner waste during the preparation process, and some aircraft of the first generation using carbon fiber will soon reach a service period of 25 to 30 years, and more and more aircraft will be scrapped, resulting in a large amount of CFRP waste. All CFRP in discarded wind turbine blades, trains, road vehicles, boats, sporting goods reaches end of life.
At present, the annual output of global tow carbon fiber has exceeded 27,000 tons. Faced with multiple pressures, it is imperative to develop low-cost, green waste carbon fiber composite materials recycling and reuse technologies.
Recycling method of waste CFRP
Among the sources of waste CFRP, one is the waste generated in the production process, such as corner waste, expired prepreg, etc., and the other is the waste whose life has reached the service life. The composition of waste from different sources is also different. The waste usually contains impurities such as paper, thermoplastic resin, adhesive, metal, etc., which further increases the difficulty of its recycling.
The recovery methods of waste CFRP mainly include physical recovery, energy recovery and chemical recovery. The physical recycling method is mainly to crush the waste CFRP into particles or grind it into powder to be used as filler directly or added to paving materials and cement. This method has a simple treatment method and low cost, but most of the obtained is low-value regeneration. It is not suitable for CFRP product which containing high-value carbon fiber.
Energy recovery is a method of incinerating organic matter in CFRP waste and utilizing its energy. The recovery method is simple in process, but CFRP waste will release toxic gases during the incineration process, causing secondary pollution.
The chemical recycling method can not only obtain high-value carbon fiber, but also recover the resin as a material or energy, and is the most suitable method for processing waste carbon fiber composite materials. Chemical recovery methods mainly include pyrolysis and solvent decomposition.
Chemical recycling method of waste CFRP
1.Pyrolysis
Pyrolysis is a method of using high temperature to decompose resin in composite materials into small organic molecules to recover carbon fibers. According to the reaction atmosphere, reactor and heating method, it is divided into thermal cracking, fluidized bed, vacuum cracking and microwave cracking.The thermal cracking method does not use chemical reagents and is easy to carry out industrial scale-up. It is also the only method in the world to realize the commercial operation of CFRP recovery. The fluidized bed method means that oxygen or air is introduced into the fluidized bed reactor, and the reaction temperature is controlled at 450~550°C to decompose the resin. The recovered carbon fiber flows under the action of a cyclone and is separated from the metal, and the gas generated by the decomposition of the resin The energy expended during the operation of the fluidized bed is replenished as fuel.Pickering’s team at the University of Nottingham, UK, has done extensive work on recycling waste fiber composites in fluidized bed reactors. The temperature in the fluidized bed reactor is more uniform and easier to control than thermal cracking. It can process waste CFRP with more pollutants. At the same time, there is no carbon residue on the surface of the recovered fiber, but the fiber length and mechanical properties are seriously lost.
2.Solvent decomposition method
The solvent method refers to the use of the combined action of the solvent and heat to break the cross-linking bonds in the polymer and decompose it into low molecular weight polymers or small organic molecules that are dissolved in the solvent, thereby separating the resin matrix and the reinforcement. According to different reaction conditions and reagents used, solvent methods can be divided into nitric acid decomposition method, hydrogenation decomposition method, super/subcritical fluid decomposition method, atmospheric solvent decomposition method and molten salt method.
The nitric acid method utilizes the strong oxidizing properties and strong acidity of nitric acid, which can decompose CFRP at a low temperature below 100 °C, and the surface of the obtained carbon fiber is clean and free of carbon deposits, and the mechanical properties of the carbon fiber are not lost.it is relatively high, and some nitrogen oxide gas will be produced at the same time.The super/subcritical fluid method refers to the method of decomposing the resin matrix of CFRP waste by using the diffusion and dissolving ability of the solvent in the super/subcritical state, thereby obtaining clean carbon fibers.This way can well preserve the mechanical properties of the original carbon fiber. The normal pressure decomposition method refers to the use of a solvent to degrade the resin matrix in the composite material under normal pressure conditions to make it a soluble substance, so that the components in the composite material can be easily separated, recycled and reused. It avoids the problem of frequent operation of high-pressure reaction, the process is simple, and is conducive to further industrialization.
Challenges of Recycling Waste CFRP
The commercialization of CFRP recycling still faces many problems.
1.Difficulty in collecting, classifying and disposing of waste
The source and composition of waste CFRP are complex. If it is not classified and processed directly, the product quality will be uncontrollable. Therefore, it is necessary to strengthen the cooperation between waste generators and recycling manufacturers.
2.Quality control of recycled carbon fiber
The source of carbon fiber in waste is complex, and it is necessary to establish corresponding classification and evaluation methods, conduct correct cost and performance evaluation of recycled carbon fiber, and determine the suitable market.
3.Reuse of recycled carbon fiber
Recycled carbon fibers are mostly chopped fibers, and their electrical and thermal conductivity properties are used as fillers for thermoplastic resins, which is one of its applications, but the added value is relatively low. It is necessary to research and develop products with higher added value for recycled carbon fiber to improve the economic benefits of the entire recycling process.
Why CFRP is recycled and not fiberglass
Compared with glass fiber, carbon fiber has higher economic value. CFRP contains a lot of expensive carbon fiber. Although it is economical and economical to directly landfill it, the expensive carbon fiber is wasted. The chemical recycling process of waste CFRP is therefore more commercially attractive.
Usually fiberglass is just landfill disposal…