# Recycled Carbon Fibers with Improved Physical Properties Recovered from CFRP by Nitric Acid
## 1 Introduction  
Various global efforts to achieve carbon-neutral societies are now accelerating. In automobile industries, vehicle electrification is urgently needed; however, to proceed with the electrification of the vehicles, reducing the weight of vehicle bodies is essential due to the weight of the batteries and motors.  
Carbon-fiber-reinforced plastics (CFRPs) are receiving much significant attention as lightweight and high-strength materials. Recent years have seen the new use of CFRPs in vehicle frames, and global demand for automotive CFRPs is forecast to double between 2020 and 2030 [1]. In a wide range of other industries, CFRP applications and its global market size are expected to continue growing in the future.  
However, the expanding use and growing global market for CFRPs means the generation of large volumes of used CFRPs waste in the future [2]. No suitable method of processing scrapped CFRP has currently been established [3], and the majority of solutions require disposal in a landfill. Therefore, it is important to establish recycling methods and plan for the effective use of resources, prior to the generation of large volumes of scrapped CFRPs. This is particularly important because of the large amount of energy necessary for manufacturing virgin carbon fibers.  
Much research into CFRP recycling has been carried out in recent years, such as recycling methods that use thermal decomposition and chemical decomposition. The thermal decomposition method decomposes the resin at 400-800 \({}^{\circ}\)C, negatively resulting in the loss of the interfacial shear strength between the carbon fibers and the resin [4],and decreased fiber strength [5]. In addition, the resin cannot be recycled because it will be completely decomposed and exhausted in the form of CO\({}_{2}\). The supercritical and subcritical fluid methods, which are chemical decomposition methods, decompose the resin at 100 \({}^{\circ}\)C-300 \({}^{\circ}\)C and 10-20 MPa [6; 7]. According to this method, since functional groups are formed on the carbon fiber surface during the recycling process, it is possible to reduce the deterioration in the fiber quality compared to thermal decomposition methods [8]. However, similarly to thermal decomposition methods, the resin cannot be recycled, since the majority of the resin will be removed in the form of gas components, and a small amount of charred substance will be produced as a residue.  
For these reasons, we have focused on a chemical decomposition method that uses nitric acid [9; 10; 11; 12; 13; 14; 15]. The nitric acid recycling method could decompose epoxy resin while immersing the CFRP sample into nitric acid under relatively mild conditions. This method not only decomposes epoxy resin but also allows the decomposed epoxy resin to be chemically recycled [12; 13]. This method was used on a CFRP prepreg material (sheet-shaped intermediate material impregnated with semi-cured epoxy resin), and successfully recovered the carbon fibers [14]. Furthermore, based on our previous report [15], this method was also successful in recovering both the carbon fibers and the decomposed resin component from CFRP automobile parts, which are expected to be generated in large volumes in the future.  
To apply these recycled carbon fibers to CFRP products and achieve closed-loop recycling, it is highly important to understand the physical properties of the recycled carbon fibers, for recycled CFRP products to perform in a stable manner. This study attempted to identify the mechanisms behind the expression of the physical properties of recycled carbon fibers and also conducted a study concerning the future expansion of this nitric acid recycling method.