재활용 탄소섬유의 회수·개질 및 복합재 제조 공정에 따른 복합재 물리적 특성 = Physical properties of composites as a function of recycled carbon fiber recovery, modification, and composite manufacturing processes

다국어 초록 (Multilingual Abstract)

The growing global demand for carbon fiber-reinforced plastic (CFRP) has intensified the need for sustainable recycling and upcycling technologies to address increasing composite waste and resource depletion. This study systematically investigated the recovery, modification, and reuse of recycled carbon fibers (rCFs) for high-value composite applications.
First, rCFs were recovered from waste CFRP using three distinct recycling methods: mechanical shredding, pyrolysis, and chemical dissolution. The surface morphology and chemical structure of the recovered fibers were analyzed using field-emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS), while the tensile, flexural, and thermal properties of rCF/PA66 composites were evaluated. The rCF/PA66 composites exhibited over 80% of the mechanical performance of commercial CF/PA66 composites, and the chemically recycled rCFs showed the highest interfacial compatibility due to the PA6-based sizing layer on the fiber surface.
Next, the effect of multiple recycling cycles on the microstructural and mechanical degradation of rCFs was examined. Thermogravimetric analysis (TGA) identified 550°C as the optimal pyrolysis temperature for removing the epoxy matrix while minimizing fiber damage. Repeated pyrolysis caused gradual reductions in fiber diameter and tensile strength, providing insight into the evolution of fiber structure and performance during successive recycling processes.
To improve fiber dispersion and interfacial bonding, a silane-assisted surface treatment was introduced into the conventional wet-laid nonwoven fabrication process using rCFs. FE-SEM, Fourier transform infrared (FT-IR), and XPS analyses confirmed the formation of a siloxane network on the fiber surface. When the optimal silane concentration was applied, the tensile strength of the resulting composites increased by approximately 64% compared to untreated rCFs, owing to improved molecular chain entanglement and interfacial adhesion within the matrix.
Finally, a masterbatch-chip-based extrusion and injection molding process was developed to enhance the processability and mechanical uniformity of rCF-based thermoplastic composites. Composites with rCF contents ranging from 20 wt.% to 40 wt.% were prepared using both conventional and masterbatch-based approaches. The masterbatch process enabled accurate quantitative control of fiber content, resulting in superior mechanical, thermal, and electrical properties compared with conventionally produced rCF/PA6 composites.
Overall, this study demonstrates an integrated approach encompassing recycling, structural recovery, surface modification, and compounding technologies to improve the performance, processability, and sustainability of recycled carbon fiber composites. The results provide a scientific foundation for expanding the upcycling potential of rCFs in lightweight and environmentally responsible composite applications.

목차 (Table of Contents)

• CONTENTS I
• List of Figures IV
• List of Tables VIII
• Abstract IX
• Chapter 1. Introduction and background 1
• 1.1 Background 1
• 1.2 What is carbon fiber 3
• 1.3 Carbon fiber recycling 5
• 1.4 Carbon fiber upcycling 8
• Chapter 2. Comparative analysis of carbon fiber-reinforced plastics: evaluating recycled carbon fibers as substitutes for commercial grades 10
• 2.1 Objectives 10
• 2.2 Experimental 14
• 2.2.1 Materials 14
• 2.2.2 CF/PA66 and rCF/PA66 composite manufacturing process 15
• 2.2.3 Characteristics 20
• 2.3 Results and discussion 23
• 2.3.1 Surface morphology and chemical composition analysis of cCFs and rCFs recovered via different recycling methods 23
• 2.3.2 Evaluation of density and void content of cCF/PA66 and rCF/PA66 composites 28
• 2.3.3 Mechanical, thermal, and interfacial characterization of cCF/PA66 and rCF/PA66 composites 31
• 2.4. Conclusions 35
• Chapter 3. Pyrolysis-based carbon fiber recycling: Structural and Mechanical Property Changes in Repeated Processes 37
• 3.1 Objectives 37
• 3.2 Experimental 41
• 3.2.1 Materials 41
• 3.2.2 CFRP Pyrolysis 42
• 3.2.3 Characterization 44
• 3.3 Results and discussion 48
• 3.3.1 Thermal decomposition behavior and surface characteristics over pyrolysis durations 48
• 3.3.2 Mechanical properties of recycled carbon fibers based on pyrolysis durations 52
• 3.3.3 Thermal decomposition behavior and surface characteristics based on pyrolysis recycling cycles 55
• 3.3.4 Mechanical properties of recycled carbon fibers based on pyrolysis recycling cycles 58
• 3.3.5 Structural evolution of recycled carbon fibers as a function of pyrolysis recycling cycles 61
• 3.4 Conclusions 67
• Chapter 4. Facile Enhancement of Mechanical Interfacial Strength of Recycled Carbon Fiber Web-Reinforced Polypropylene Composites via a Single-Step Silane Modification Process 69
• 4.1 Objectives 69
• 4.2 Experimental 73
• 4.2.1 Materials 73
• 4.2.2 Preparation of silane-treated nonwoven fabrics 74
• 4.2.3 Fabrication of nonwoven composites via hot-press molding 76
• 4.2.4 Characterization 79
• 4.3 Results and discussion 81
• 4.3.1 Characteristics of the functionalized rCF surface 81
• 4.3.2 Microstructural and Interfacial Characterization of Composites 89
• 4.4 Conclusions 95
• Chapter 5. Facile method to improve the workability of the extrusion process using a masterbatch made from rCF and advanced to physical properties of rCFRTP manufactured through the injection process 97
• 5.1 Objectives 97
• 5.2 Experimental 102
• 5.2.1 Materials 102
• 5.2.2 M-rCF manufacturing process method 103
• 5.2.3 rCF/PA6 and M-rCF/PA6 composite manufacturing process method 105
• 5.2.4 Characterization 108
• 5.3 Results and discussion 112
• 5.3.1 Thermal and structural evaluation of composites 112
• 5.3.2 Physical properties of composites 117
• 5.4 Conclusions 122
• Chapter 6. Summary 124
• References 128
• Acknowledgement 146
• 요약(국문초록) 148