Surface modification of carbon fiber by grafting gallic acid as a bridging model

• Main Author: Sara Quake
• Other Authors: Aravind Dasari
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/147643
• Institution: Nanyang Technological University

Carbon fiber (CF) is often used in composites as it has superior high strength-to-weight ratio and temperature resistance. However, its smooth surface limits its wettability with polar matrices and results in poor interfacial interaction in CF reinforced composites. Thence, it is important to modify the fiber surface to maximise their full potential. In previous works, methods such as acid and dry treatment were reported to graft OH groups on the surface, but these treatments were also reported to lower the mechanical properties of the fiber or were difficult to conduct. In this report, although the fiber was not chemically grafted with hydroxyl (OH) groups, poly (vinyl alcohol) (PVA) was physically coated on them to simulate the modified fibers. This report focuses on using gallic acid (GA) as a bridging molecule to increase OH functionality between the modified CF and matrix. Two main methods of grafting GA onto the modified CF were attempted through esterification with activated gallic acid chloride and Steglich esterification with protected gallic acid. Protection of OH groups was required to esterificate and further epoxidize the GA ‘bridge’. Acetylation was selected due to the availability of chemical reagents and simplicity. But deacetylation or the deprotection could not be conducted as it potentially removes the ester bond formed at the fiber-matrix interface and breaks off the GA ‘bridge’. Alternatively, to deprotection and then epoxidation of GA, epoxidation of PVA coating on CF (PVA_CFE) was conducted to emulate a similar epoxidized chemical structure. Then, these chemical structures were characterized by Fourier-transform infrared spectroscopy and high performance liquid chromatography whilst their thermal properties were studied with differential scanning calorimetry and thermogravimetric analysis. To evaluate the interfacial fractures of the modified fibers-epoxy resin composites, an Izod impact test was performed to evaluate impact strength and optical microscope was used to observe the fracture. However, the 1%PVA_CFE composites had 18% lower impact strength as compared to untreated CF-epoxy composite. It is likely that the functionalised PVA coating on CF was removed and it formed a dense crack network at the interface. The lack of proper degassing the epoxy resin and small sample size could also improperly represent these observations. To evaluate GA as a bridging molecule more qualitatively, chemically modifying CF and using other protection methods such as silyl ethers are recommended. A higher fiber loading (~0.4wt%), better degassing and higher sample size should be taken to better represent the findings.