Strengthening Mechanism of CFRPs through Incorporation of Halloysite Nanotubes by Electrophoretic Deposition Process

Abstract

Fiber reinforced polymers (FRPs) are extensively utilized in various industries such as aerospace, aviation, automotive, marine, and civil construction, due to their exceptional stiffness-to-weight and stiffness-to-weight ratio. However, unlike isotropic materials such as metals and ceramics, the mechanical properties of FRPs in the transverse and through-thickness directions are inherently inferior to those in the primary direction due to their orthogonal anisotropic nature. The interfacial and interlaminar strength and toughness of FRPs can pose significant limitations to their performance and application breakthrough. Incorporating nano-additives has been demonstrated as an effective method for enhancing the interfacial and interlaminar properties of FRPs in recent decades. However, most studies have conventionally involved the random dispersion of nano-additives in the matrix, and a comprehensive investigation into the distribution conditions remains limited. The primary aim of this dissertation is to explore the interfacial toughening mechanism through the innovative hierarchical distribution of nanoclays, by experimental. To achieve this, various halloysite nanotubes (HNTs) with distinct structures were successfully synthesized using multiple techniques, and a hierarchical distribution was achieved by employing the electrophoresis deposition (EPD) technique. The study also delves into investigating the optimal EPD parameters and the potential agglomeration problem during the Vacuum-assisted Resin Transfer Molding (VaRTM) fabrication process, which is essential for developing an efficient and effective approach to enhancing the interfacial properties of nanocomposites. The study carefully selected the voltage range for electrophoretic deposition (EPD) to be between 6 and 12 V, which corresponds to the nanoparticle deposition working range. The carbon fabric was modified using this process to enhance its through-thickness strength, and the resulting modified fabric was incorporated into CFRP composites using vacuum-assisted resin transfer molding (VaRTM). This approach enabled precise control of several deposition parameters to achieve optimal distribution of HNTs on the carbon fabric surface. The mechanical properties of the modified CFRP composites were evaluated, and it was observed that the EPD-modified CFRPs exhibited superior mechanical properties compared to neat CFRPs. The study also found that the highest values were obtained at 0.7 wt.% and 6 V, indicating the feasibility of the EPD process and the well-dispersed morphology of HNTs, as confirmed by SEM-EDS analysis. The zeta potential plays a crucial role in determining the colloidal stability of nanoparticle suspensions and their suitability for electrophoretic deposition (EPD). The magnitude and sign of the zeta potential affect the repulsive forces between particles, which in turn determine their aggregation and sedimentation behavior. EPD typically requires a high zeta potential to ensure stable suspensions and promote uniform deposition of particles. In this investigation, the stability of the HNT dispersion was evaluated as a function of nanoparticle concentration, and the optimal dispersion range was identified through systematic experimentation. Subsequently, an HNT-reinforced composite material was synthesized by identifying the optimal range of dispersion stability and evaluating the impact strength and fracture mechanism of the interface. The study found that the HNT dispersion exhibited optimal stability in the pH range of 6.6-6.8, resulting in the highest degree of dispersion and impact strength of the composite material. The highest impact strength was achieved at a concentration of 0.7 wt.%. The interfacial dispersion of the EPD-fibers was confirmed using scanning electron microscopy and dispersive X-ray spectroscopy (SEM-EDS). The implementation of hierarchical distributed nanoclays proved effective in enhancing the interlaminar strength and toughness of FRPs by selectively reinforcing the vulnerable interfacial region. The findings and methods presented in this dissertation can be applied and cited in future research involving other nanoclays/FRPs systems.