**1. Introduction**

In recent decades, carbon fiber-reinforced polymers (CFRPs) have been extensively utilized in various applications of the aerospace industry, transportation, infrastructure [1–3], energy, sport industries, defense, medical sector, and electronics [4,5]. The physical properties of CFRPs have led to increased use of these materials as replacements for more conventional options, including steel, aluminum, alloys, etc. These physical properties include low density and lightweight, superior strength to weight ratios and elastic modulus [6–8], stiffness, low expansion or shrinkage, thermal stability [9–12], electrical conductivity, superior resistance to corrosion and chemical attack, and exceptional endurance of physico–chemical properties [13–16]. Given these properties, they have been heavily utilized in the manufacture of automotive parts to reduce weight and improve fuel efficiency [17]. CFRPs' composites are engineered materials that compose of carbon fibers (CFs) acting as reinforcing matrix materials within versed thermosetting polymer to form structures. [18]. The fibers act as load-carrying elements for their orientation and to resist environmental damage [11,19].

**Citation:** Chin, K.-Y.; Shiue, A.; Wu, Y.-J.; Chang, S.-M.; Li, Y.-F.; Shen, M.-Y.; Leggett, G. Studies on Recycling Silane Controllable Recovered Carbon Fiber from Waste CFRP. *Sustainability* **2022**, *14*, 700. https://doi.org/10.3390/su14020700

Academic Editor: Antonio Caggiano

Received: 28 October 2021 Accepted: 6 January 2022 Published: 9 January 2022

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CFRPs' composite materials are differentiated by silane with different properties, suitable for a variety of strength requirements. In response to increased demand to reduce CO2 emissions through weight saving measures and production related energy costs, the use of CFRP products and applications has a high potential to grow significantly over the coming years. Carbon fibers were first commercially produced in the late 1960s. They are now manufactured around the world. It is predicted that by 2025, revenues associated with use of CFRPs will exceed 25 billion dollars per year [20,21]. Recent annual growth rates of more than 10% underline these forecasts [22]. Given that the service life of CFRPs is approximately 50 years, as end of life is reached, this will bring about a considerable increase in CFRPs waste generation and present a significant challenge for the disposal and recycling of CFRPs based components [6,23].

With increasing global focus on a circular economy model at domestic and foreign scale, the problems of recycling and waste reduction are becoming more and more prominent, with industry increasingly considering the impact of growth on finite resources. The presence of carbon fibers in composite materials is a challenge when it comes to recycling and separating the fibrous material from epoxy resins, plastics, and other materials. In recent years, industries have speedily adopted these materials. However, it has displayed no proper awareness and consideration of their disposal and possible recycling. The main treatment methods for waste CFRPs have been landfill disposal and incineration [9,24]. Economic viability, new legislations, limited landfill capacity, and fuel incineration, and ecological aspects are driving towards recycling and processing for CFRPs waste [25,26]. Thus, the treatment methodology of CFRP waste is becoming a more critical issue, with industry currently lacking in the skills and methodologies to respond effectively. Support is required to provide industry with tools that can be implemented effectively and economically.

Currently, several technologies have been developed for recovering CFs from CFRPs with excellent recycling yield and maintenance of critical physical properties. There are three main approaches:


Microwave pyrolysis is a relatively new approach, designed to replace conventional pyrolysis. Heating by microwave pyrolysis has the advantage that the rate of thermal transfer is increased, with reduced energy consumption [26,51] and heat loss reduction [52,53]. This process can recycle CFRPs, with resulting fibers maintaining desirable mechanical

properties [49,54–58] These studies are focused on removing resin and maintaining mechanical strength to ge<sup>t</sup> high quality rCF, without considering the issue of residual silane on the recovered carbon fibers. However, the manufacturing process of commercial CFRPs involves adding an appropriate amount of silane [59,60]. Silanes are often used as a coupling agent, possessing differing functional groups at each molecular end. For example, trialkoxysilane can bond with inorganic substances. Alternatively, there are organofunctional groups (methacrylate, epoxy, etc.). These silanes can enhance the compatibility between carbon fibers and the resin matrix, improving the interfacial bonding strength of CFRPs. Therefore, if silane could be saved during the fiber recovery process it would reduce the need for the addition of further silane coupling agen<sup>t</sup> when using rCF to manufacture new materials, reducing overall costs and waste [61]. Furthermore, Li et al. [62] compare the dispersion of sizing of carbon fiber after removed silane treatment and nontreatment, with the product intended to be added to portland cement. The results show the carbon fiber of removed silane can be efficiently dispersed to form carbon-fiber-reinforced cement, with this composite cement exhibiting higher compressive strength. The residue of silane could be a disadvantage when considering recovered carbon fibers for reinforced inorganic applications.

In this study, the residual silane content of rCF surface can be controlled by the temperature of microwave pyrolysis. The influence of the reaction temperature on the degradation of silicon on the carbon fiber surface was studied. The recovered carbon fibers were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Energy-dispersive X-ray spectroscopy (EDS).

#### **2. Experimental Methods**
