Carbon-based polymer nanocomposites for 3D-printing

In this PhD project, novel polymer nanocomposites are developed with the aim to increase the performances of 3D-printed parts obtained by fused deposition modeling (FDM). The attention is focused on carbon-based nanomaterials incorporated into an acrylonitrile–butadiene–styrene (ABS) polymer by a solvent-free process. ABS-based nanocomposites were prepared by incorporating different kinds and amounts of graphene nanoplatelets (GNP), carbon nanotubes (CNT) and hybrid (GNP/CNT) systems. In order to understand the effect of the manufacturing process on the material’s properties, the samples were produced into two different processing routes: (i) melt compounding and compression molding, and (ii) melt compounding, following by filament extrusion, and fused deposition modelling (FDM). Several characterization techniques were employed in order to evaluate the flowablity, morphology, mechanical and functional properties of the materials. In the first part of work, ABS-graphene nanocomposites are described. Two ABS matrices having different viscosity were compared with the addition of various types of commercial graphene nanoplatelets (xGnP® M5, C300, C500, and C750 by XG Sciences) in the range 2-8 wt%. The better processability and higher stiffening effect on compression molded plates were achieved by utilizing the low viscosity ABS. The effects of GNPs on the thermal, electromagnetic shielding (EMI SE), electrical and mechanical behaviour of an ABS matrix were investigated. Melt flow index (MFI) values almost linearly decreased with all the type of GNP, especially with the highest surface area nanofiller (GNP-C750). Due to large size of graphene, nanocomposites filled with GNP-M5 showed the better properties of in electromagnetic interference shielding efficiency (EMI SE) and stiffness. Consequently, GNP-M5 were selected and incorporated at 4 wt% in ABS filaments used to feed a FDM machine to obtain specimens with various build orientations. The elastic modulus and dynamic storage moduli of 3D printed parts along three different build orientations were increased by the presence of GNP-M5 in the ABS matrix. At the same time, a decrease in both strength and strain at break was observed when GNP-M5 is added to ABS. Moreover, higher thermal stability was induced on 3D printed parts by GNP, as indicated by a reduction in both coefficient of linear thermal expansion and creep compliance. A comparison between 3D printed and compression molded parts highlighted the importance of the orientation effects induced by the FDM process. In the second part of work, the results of the investigation on ABS-carbon nanotubes nanocomposites are reported. ABS-CNT nanocomposites plate production by compression molding and their characterization was a preliminary step. Nanocomposite ABS/CNT filaments at 1-8 wt % were obtained by using direct melt compounding and extrusion. The optimal CNT content in the filaments for FDM was found to be 6 wt %; for this composite, a detailed investigation of the thermal, mechanical and electrical properties was performed. The presence of CNT in ABS filaments and 3D-printed parts resulted in a significant enhancement of the tensile modulus and strength, accompanied by a reduction of the elongation at break. As documented by dynamic mechanical thermal analysis, the stiffening effect of CNT in ABS is particularly pronounced at high temperatures. Besides, the presence of CNT in 3D-printed parts accounts for better creep and thermal dimensional stabilities of 3D-printed parts, accompanied by a reduction of the coefficient of thermal expansion. 3D-printed nanocomposite samples with 6 wt% of CNT exhibited a good electrical conductivity, even if lower than pristine composite filaments. In addition, the strain sensing capabilities of the conducting 3D-printed samples with 6 wt% of CNT with two different infill patterns (HC, and H45) were studied. Upon the strain applied, the resistance change and damage in the conductive FDM parts were detectable. Fatigue and creep loading on FDM products were also carried out. In last part of work, ABS-GNP-CNT hybrid nanocomposites are described. ABS nanocomposites plates with addition GNP-M5 and CNT at 2-8 wt% were compared. A significant higher reduction in MFI value by the addition of CNT compared to GNP was observed. The ABS/GNP nanocomposites showed the slightly higher stiffness and the creep stability compared to the ABS/CNT nanocomposites, but showed the lower tensile strength. Also, the ABS/CNT samples showed significant higher electrical properties in comparison to ABS/GNP. The total nanofiller content of CNT/GNP hybrid plates was fixed at 6 wt%. The hybrid nanocomposites showed a linear increase in modulus and strength as a function to CNT/M5 ratio. Moreover, conductive hybrid nanocomposite plates were obtained by the addition of CNT. The composition of 50:50 of CNT/GNP at 6 wt% was selected for FDM process due to the good compromise between processability and properties (e.g. mechanical and electrical). In agreement with electrical resistivity, EMI SE of 6 wt% ABS/CNT and 50:50 hybrid ABS nanocomposites resulted to be -46 dB and -31.7 dB for plate samples. EMI SE of FDM parts is about for -14 dB HC and H45 build orientation and –25 dB for PC build orientation printing from ABS/CNT nanocomposites, while parts had EMI SE about -12 dB for HC and H45 and -16 dB for PC from hybrid nanocomposites.