Fabrication and characterization of ultra high performance cnts/pei nanocomposite filaments for additive manufacturing

Additive manufacturing (AM) or three-dimensional (3D) printing by another name produces a diverse range of geometrical shapes by layer by layer deposition while eliminating post-processes such as cutting and trimming. Fused deposition method (FDM) has been most widely used 3D printing technology that works with the principle of layer by layer deposition of guiding melted thermoplastic filaments. So far, due to their low cost and easy processability acrylonitrile butadiene styrene (ABS), polylactic acid (PLA) and polyamide (PA) have been most used thermoplastic polymers. However, they are not strong and durable enough to tolerate high loads particularly in structural components. Besides, due to the intrinsically restricted mechanical response of these printed polymer structures, there is significant demand to fabricate high performance structures by using 3D printers. Thus, high performance thermoplastic materials such as polyetherimide (PEI) and polyether ether ketone (PEEK) have been found promising as competitors to strong and conductive but relatively dense metals and metal alloys in aerospace applications. Even though, there are still processing challenges such as high melting temperatures and the need of good control on ambient temperature, 3D printing technique offers many advantages in the fabrication of structural polymer based components, including high precision, cost effectiveness and customized complex 3D geometries. Another asset to utilize strong but light weight polymer filaments in 3D printers is their ability to tune their micro structures by incorporating nanomaterials. Among many nanomaterials, carbon nanotubes (CNTs) have been a substantial area of research over last two decades due to nanotubes' high aspect ratio, low mass density and superior electrical, mechanical and thermal properties. Novel advanced materials produced from CNTs blended with amorphous high performance PEI polymers, provide new capabilities and new skills in aerospace applications. Not only engineered materials but also the use of 3D printing technology facilitates chemically and thermally resistant and conductive prototype production with complex geometries. More importantly, 3D printed objects fabricated by using conductive filler reinforced filaments have ability to conduct electrons. All these efforts on nanocomposite filaments addresses the possibilities to replace metals and metal alloys. This thesis attempts to explore CNTs reinforced PEI nanocomposite conductive filament fabrication by melt processing and their later use in AM process. First, customized twin-screw and single-screw extruders were designed to process high performance polymers including PEI and PEEK % CNTs/PEI nanocomposite filaments were fabricated by using consecutive single and twin-screw extrusion processes. Prior to single-screw extrusion, CNTs and PEI granules were directly fed through twin-screw extruder to produce CNTs/PEI blended pellets. To ensure homogeneous CNTs dispersion in PEI matrix that dictates the electrical conductivity and mechanical performance of nanocomposite, twin-screw extrusion was repeated for five times and then nanocomposite strands were granulized to be later fed into the single extruder to produce CNTs/PEI nanocomposite filaments. CNTs reinforced nanocomposite filaments were fabricated with diameter of 1.70 mm, which was specifically aimed for additive manufacturing. Thermal characterization of both neat PEI and CNTs/PEI samples was performed by Differential Scanning Calorimeter (DSC) which pointed out that the introduction of CNTs did not change thermal transition of PEI. Thermogravimetric analysis (TGA) revealed that CNTs caused slight shift in decomposition temperature compared to neat PEI. Morphological studies of both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that CNTs were homogeneously dispersed and distributed in PEI matrix. TEM micrographs clearly pointed out that CNTs were also oriented along extrusion direction without having any curly conformation or agglomerated CNTs clusters due to flow induced orientation. This effect positively contributed both to tensile properties and, conductivity by reducing percolation threshold of CNTs/PEI filaments. Volume conductivity measurements by four point probe method showed that CNTs presence in the PEI matrix significantly increased the conductivity. For 7 wt % CNTs/PEI filaments, surface conductivity was achieved up to 2.57x10^{-1} S/cm. Percolation threshold of CNTs/PEI nanocomposite system was estimated between 0.1 and 0.25 wt %, which was remarkably lower than reference studies in literature. Moreover, specific stiffness showed gradual increase up to 20 % with CNTs addition where specific tensile strength of nanocomposite filaments with 7 wt % CNTs reinforced improved about 10 % compared to neat PEI specimen. Overall, this thesis traces the effect of CNTs on electrical and mechanical properties of CNTs/PEI 3D printer filaments and also highlights the process and material requirements to work with structural polymers in melt processes.