Creep behavior investigation of 3d printed polyetherimide parts with carbon black reinforcement via experimental analysis and modeling

Abstract

Additive manufacturing (AM) technologies, also known as 3D printing technologies, have garnered significant attention across various sectors due to their distinct production methodology compared to conventional manufacturing methods. AM technologies based on a principle of building geometries layer by layer, offering advantages such as minimum waste material, the ability to produce complex geometries in a single process, no need for assembly, and the elimination of initial investment costs. Initially introduced for rapid prototyping, AM has evolved with technological advancements, leading to more precise product dimensions, faster part production, and a broader range of raw materials. Consequently, it has found applications in aerospace, automotive, and medical sectors. Fused Filament Fabrication (FFF) technology is the most widely used AM technology for polymeric part production. Its popularity stems from nearly zero waste production, accessibility and ease of use of FFF printers, and relatively low production costs compared to other AM technologies. Despite its many advantages, FFF-produced parts often exhibit lower mechanical performance compared to their conventionally manufactured counterparts, and the relatively slow production speeds limit their use in high-strength demanding fields. In recent years, researchers have focused on using high-performance thermoplastics in FFF applications to overcome these limitations. High-performance thermoplastics, such as polyetherimide (PEI), maintain their mechanical strength at high temperatures and exhibit greater chemical and thermal stability compared to commonly used thermoplastics like polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). PEI filaments, with a high glass transition temperature (Tg), high tensile strength, and thermal stability, are suitable for use as load-bearing functional components. Additionally, PEI's flame retardancy, low smoke density, and low toxicity make it an ideal material for aerospace applications. Its low coefficient of thermal expansion and amorphous structure further enhance its dimensional stability, making it suitable for FFF. The corporation of various reinforcement materials to PEI filaments to create composite materials has been shown to enhance the performance of FFF-produced parts for advanced technological applications. Carbon-based reinforcements like carbon nanotubes (CNT), graphene, carbon black (CB), and carbon fiber (CF) improve the mechanical properties of neat thermoplastic filaments and impart additional features such as electrical and thermal conductivity, making them multifunctional. These developments have increased the importance of composite materials in high-performance applications and brought new opportunities for their employment. This thesis investigated the potential of overcoming the mechanical weaknesses of FFF-produced parts compared to their conventionally manufactured counterparts by reinforcing PEI with CB particles. The study systematically investigates the recovery of mechanical strength lost due to the layered structure inherent in AM by adding varying amounts of CB. Samples were characterized using various techniques, with a focus on their creep behavior, and compared to conventionally produced samples. Additionally, the creep behavior of the samples was further analyzed through mathematical modeling. First of all, composite filaments consisting of neat PEI and varying concentrations of CB (5%, 10%, and 20% by weight (wt)) were prepared using a co-rotating twin-screw extruder. Creep test samples were produced from these filaments using a 3D printer specifically designed for printing high-performance thermoplastics. To compare the mechanical properties of FFF-produced samples with those produced conventionally, neat PEI creep test samples were also manufactured using the hot-press technique. The thermal, rheological, morphological, and mechanical characterizations of the composite filaments were conducted to evaluate the CB reinforcement effects. Thermogravimetric analysis (TGA) revealed that PEI experienced no significant weight loss up to 500 ◦C. The initial thermal degradation temperatures of all samples were calculated, showing that CB did not significantly affect this temperature. Rheological measurements indicated that the storage moduli and complex viscosities of all filaments increased with CB content, reaching a rheological percolation threshold around 10 wt% CB. Scanning electron microscope (SEM) analysis confirmed adequate dispersion of CB within the PEI matrix. Tensile tests showed that 5 wt% CB reinforcement resulted in the highest tensile strength, with strength decreasing slightly at 10 wt% and reducing significantly at 20 wt% due to brittleness. Creep tests conducted between 30 ◦C and 190 ◦C, in 20 ◦C increments, with a 3 MPa load applied for 10 minutes, demonstrated that increasing CB content improved the creep resistance of the composites. The 5 wt% CB/PEI samples exhibited the least creep deformation and similar behavior to conventionally produced neat PEI samples, indicating that the mechanical strength lost in FFF-produced samples could be recovered with CB reinforcement. Finally, experimental creep data were simulated using viscoelastic models. The Burgers and Findley power law models successfully modeled the behavior, with the Findley power law showing better alignment with experimental results. The generalized Kelvin-Voigt model, despite fitting the curves well, provided inconsistent parameter values. The study concluded with the creation of time-temperature superposition principle (TTSP) master curves, where manual shifting using Python provided the most reasonable results. This thesis aims to enhance the mechanical properties of high-performance composite materials produced via FFF, thereby increasing their potential use in advanced technological applications. The findings contribute significantly to the broader industrial application of FFF-based 3D printers by optimizing the mechanical strength and creep resistance of PEI and CB-reinforced composites.