Mechanical and Dynamic Behavior of Additively Manufactured Polymer Nanocomposites

Abstract

Rapid production of high quality components without any additional tools is the key to cost reduction in industrial applications. The present work deals with the additive manufacturing of polymer nanocomposite and their graded variants using fused filament fabrication (FFF) technology based 3D printing process. High Density Polyethylene (HDPE) is used as a matrix, and functionalized MWCNTs are used as a filler material in this work. The development of nanocomposite (NC) with lightweight functionalized MWCNTs serves the purpose of reduction in weight with enhanced properties. The functionalized MWCNTs (0.5%, 1%, 3%, and 5% by weight) are blended with the HDPE to develop functionalized MWCNTs/HDPE NCs in the form of pellets, which are further characterized through scanning electron microscopy (SEM), melt flow index (MFI), and thermogravimetric analysis (TGA). Then, these developed NCs are utilized to extrude NC filaments for 3D printing. The extruded NC filaments are characterized for quality and printability, which are further utilized in 3D printing of NC and functionally graded nanocomposite (FGNC) samples. The 3D printed NCs and FGNCs are comprehensively characterized for prints quality through various techniques such as SEM, rheology, XRD, and density, and finally subjected to the tensile, flexural, compression, hardness, impact, mechanical buckling, free vibration, and thermal buckling. Also, the waste NC filament is investigated for its recycling potential. The SEM analysis revealed the uniform distribution of the functionalized MWCNTs in the HDPE in the developed nanocomposites, extruded nanocomposite filaments, and the 3D printed samples, confirming the suitability of the processing parameters used in the blending, extrusion, and 3D printing. It also revealed the seamless and strong layer bonding in the 3D printed NCs and FGNCs. The rheology findings also confirm the uniform distribution of the fillers qualitatively. It is observed that complex viscosity (η*) for the NCs slightly increases as the functionalized MWCNTs content increases up to 0.5%, and then dramatically increases for 5% (almost two orders) at the low frequency by maintaining a steeper graph, confirming the good dispersion of the MWCNTs in the HDPE matrix. The complex viscosity (η*), storage modulus (E'), and loss modulus (E") of the 3D printed NCs and FGNCs increase while the damping factor (𝑇𝑎𝑛𝛿) decreases as the functionalized reinforcement content rises. The MFI of the developed NCs (H0.5-H5) showed a decreasing trend with increase in the MWCNTs content, confirming the inclusion of the fillers in the matrix. The MFI decreases in the 14.52%-51.60% range for the H0.5- H5 NCs compared to the HDPE. The thermal stability of the developed NCs increases with the addition of the filler. The highest thermal degradation temperature is observed for H5 NC (479.07 ºC). The density of the filaments and the specimens decreases with the increase in the MWCNTs content. The highest density reduction of 16.06% and 15.58%, respectively, is observed in the H5 filament and the respective H5 print compared to the pure HDPE. The density of the FGNCs was also found to decrease with layer gradation. The FGNC-2 exhibited the highest weight-saving potential of 12%. 𝑇𝑀𝑒𝑙𝑡 of the filaments and the specimens increases with an increase in the MWCNT content, while 𝑇𝐶𝑟𝑦𝑠𝑡 increases up to 0.5% and 1% for the filaments and the specimens, respectively, then decreases. 𝛼𝐶𝑟𝑦𝑠𝑡 is higher for the filaments and the specimens than the neat HDPE. The XRD results also revealed the higher crystallinity of the NCs than the pure HDPE. A tensile study of the extruded filaments, NC, and FGNC prints revealed that the modulus and strength increase with the addition of the MWCNTs. The highest tensile modulus and strength are observed for the H5 NC filament, which is 105% and 30% higher than the pure HDPE filament. For the printed NC specimens, the modulus increases by 15.89%, 19.41%, 44.87%, and 81.43% in the H0.5, H1, H3, and H5 NC specimens, respectively, compared to the pure HDPE specimens. The highest modulus is registered for the H5 NC specimen (~81.5%). Further, it is noted that the tensile modulus of the printed specimens is higher than the respective filaments. The tensile modulus of the printed H-H5 NC specimens increased by 15.42%, 13.97%, 13.96%, 15.47%, and 2.38% compared to the respective filaments. The highest tensile strength (UTS) is registered for the H5 (16.6 MPa), 12.16% higher than the pure HDPE. It is noted that the H5 NC specimen has the highest modulus and strength among all the NC specimens, 81.43%, and 12.16%, respectively, higher than the HDPE. Further, the FGNCs are also tested for tensile. It is seen that their modulus and strength increase with the layer gradation. The enhancements in the moduli are 55.15% and 90.41% for the FGNC-1 and the FGNC-2 compared to the pure HDPE, respectively. Moreover, the FGNCs exhibited higher moduli than their respective homogeneous NCs, showing the potential to replace the NCs. The FGNCs showed higher strength than the respective homogeneous NCs, 23.75% and 37.12%, respectively, higher strength than the HDPE are noted for the FGNC-1 and FGNC-2. The tensile properties of the NCs are compared with the existing composites, where it is found that the 3D printed NC sample exhibited the highest tensile strength compared to the other fillers reinforced HDPE composites. The printed NCs and FGNCs are also investigated for the flexural responses. It is observed that the flexural modulus and strength of the NC specimens increase with the MWCNTs increase. The highest modulus and strength, 24.71% and 22.23% are noted for the H5 NC print compared to the neat HDPE. It is noticed that the specific strength and modulus of the printed NC specimens increase with the filler loadings. The H5 specimen showed the highest specific modulus and strength, 47.62% and 44.73% higher than the pure HDPE. For the FGNCs, the flexural moduli and strength are also noted to increase with the layer grading. The highest flexural strength and moduli are exhibited by the FGNC-2, which is 28.57% and 26.83% higher than the pure HDPE. The FGNC-2 also exhibited the highest specific strength and modulus, 46.16% and 44.14%, respectively, higher than the pure HDPE. The flexural behavior of the NCs and the FGNCs was also studied numerically, and the experimental findings were found to match well with the FEA results. The compression, hardness, and impact studies are also conducted on the printed NCs and FGNCs, which revealed an increasing trend of compressive modulus and yield strength with the increased MWCNTs content. The highest compressive modulus and yield strength are found for the H5 NC, which are, respectively, 44.89% and 9.28% higher than the pure HDPE. In the FGNCs, the layer gradation also notes a similar increasing trend. The FGNC-1 and FGNC-2 exhibited 35.75% and 61.14% higher modulus than the pure HDPE with the highest of the FGNC-2. The yield strengths of the FGNC-1 and the FGNC-2 are 8.89% and 11.56% higher than the pure HDPE with the maximum FGNC-2. The hardness and the impact strength of the NCs and FGNCs also increase with the filler content. The highest hardness and impact strength are noted for the H5 NC among all the NCs, while in the case of the FGNCs, are for the FGNC-2. The FGNC-2 exhibited the highest hardness and impact strength, respectively 76.80% and 119.99% higher than the pure HDPE. Mechanical, thermal, and dynamic loading causes instability, leading to the failure of the structure. Therefore, the printed NCs and FGNCs are also investigated for mechanical buckling, free vibration, and thermal buckling behavior. Results revealed that the NCs and the FGNCs buckling strength increases with the increased MWCNTs content. The highest critical buckling load (𝑃𝑐𝑟) is noted for the H5 NC among the NCs, which is 79.03% (by DTM) and 79.13% (by MBC method) compared to the HDPE. The 𝑃𝑐𝑟 of the FGNC-1 and the FGNC-2, calculated from DTM and MBC methods, are 54.38% and 91.34% higher than the pure HDPE. It is noted that the H5 NC and FGNC-2 displayed the highest buckling strength among the 3D printed NCs and FGNCs. The 𝑃𝑐𝑟, calculated from DTM, MBC, and VCT methods, exhibited good agreement. The natural frequency of the NCs and the FGNCs increases with the MWCNTs loading while decreases with increasing compressive load. Their damping factor showed a decreasing trend with the filler loading while observed to be increasing with a rise in the compressive load. The experimental and numerical findings are observed to be in very good agreement. The property chart reveals the superior performance of the H5 NC and the FGNC-2 compared to thermosetting composites. Thermal buckling studies of the NCs and the FGNCs under various heating conditions revealed that the critical buckling temperature (𝑇𝑐𝑟) and the deflection due to heating is very sensitive to heating type. The 𝑇𝑐𝑟 is noted as the highest in case-3 and the lowest in case-2. The maximum deflection is observed in case-2, while no significant difference is observed in case-1 and case-3. The 𝑇𝑐𝑟 of the NCs and FGNCs increases, while the deflection decreases due to the addition of the MWCNTs and their gradation. The highest 𝑇𝑐𝑟 is observed for the H5 NC (11.16% higher than the pure HDPE) and FGNC-2 (19.06% higher than the pure HDPE). The lowest deflection is observed for the H5 NC (53.33% lower than the pure HDPE) and the FGNC-2 (73.34% lower than the pure HDPE), indicating enhanced thermal stability. It is seen that the H5 NC exhibited the superior performance among all the NCs, while the FGNC-2 showed the best performance between the FGNC-1 and FGNC-2. Moreover, the FGNCs exhibited the superior performance to all the homogeneously reinforced NCs. In addition to comprehensive studies on the 3D printed NCs and FGNCs, this work also considered the study on the recycling potential of the waste polymers, which is increasing in the environment and causing many hazardous environmental problems. In this study, the waste extruded nanocomposite filaments that get wasted during filament extrusion are collected and examined for recycling potential. The waste filaments of the functionalized MWCNTs/HDPE NCs (H1 composition) are recycled to obtain the useful NC filaments for utilization in 3D printing. These NC filaments are further tested to check their potential for 3D printing. The samples are 3D printed using the recycled filaments, and subjected to density, XRD, and tensile tests to examine their recycling potential. The recycled filaments and the respective prints showed enhanced density, crystallinity, and tensile properties with respect to the extrusion cycle. After third extrusion cycle, no enhancement in the properties is found. The tensile strength and modulus of 1x, 2x, and 3x prints are 63.82%, 67.11%, and 67.76%, and 45.63%, 55.34%, and 97.81%, respectively, higher than the W/UR print. The highest tensile strength and modulus are observed for 3x print, which is 67.76% and 97.81%, respectively, higher than the W/UR print. This study revealed that the H1 NC filament can be recycled (extruded) three times to achieve property enhancement. The H1-3x print exhibited superior tensile strength and modulus among all the recycled prints. Through such an approach, the environmental pollution due to the plastic waste generated by 3D printing industries can be substantially lowered, in addition to expanding the filament material options for the FFF community. A similar route can also be explored for the other NCs (0.5%, 1%, 3%, and 5 wt.%), increasing application areas across various engineering fields such as structural, marine, aerospace, and automobile. The efforts presented in this research exhibited the potential of the 3D printed NCs and the FGNCs to be utilized as integrated and jointless functional components in a wide spectrum of engineering applications such as marine, automobile, aerospace, construction, defense, electronics, and naval in addition to increasing the novel materials options for 3D printing industries.