Experimental and numerical investigations of multifunctional properties of MWCNT-based hybrid nanocomposites

Abstract

Although carbon nanotubes (CNTs) have displayed great potential for enhancement of multifunctional properties of a polymer matrix, still incorporation of CNTs with the polymer matrix requires further improvement in terms of synthesis, processing, functionalization, etc. This study involves the in-depth investigation of epoxy-based multiwall carbon nanotube (MWCNT) nanocomposites containing cluster free uniform dispersion of different types of MWCNTs (MWCNTs and MWCNT/ZrO2) in “bisphenol A” based epoxy matrix using an innovative ultrasonic dual mixing (UDM) technique. In this study, the optimization of UDM process parameters for neat epoxy (EPOFINE 1564) in terms of tensile strength and the effect of unprocessed and UDM processed epoxy on matrix morphology, tensile strength, and viscoelastic properties are examined. This study investigates the effect of MWCNTs contents, surface modification of MWCNTs, and their dispersion on thermo-mechanical, adhesive, and viscoelastic properties of epoxy nanocomposites. The surface decoration of MWCNTs by ZrO2 nanoparticles formed a new hybrid structure of nanofillers (MWCNT/ZrO2 hybrid nanofillers) that provide strong interfacial interaction of MWCNTs with the epoxy matrix. The morphology of the newly developed MWCNT/ZrO2 hybrid nanofillers have been studied using transmission electron microscopy (TEM). The chemical structures, purity, morphology, and physical characteristics of the MWCNTs and MWCNT/ZrO2 hybrid nanofillers have been studied using Field Emission Scanning Electron Microscopy (FESEM), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), and X-Ray Diffraction (XRD) analysis respectively. The strong adhesive and coating behaviour of epoxy-based nanocomposites is studied by performing the lap shear strength test and anti-corrosion test on the mild steel surface. The superior lap shear adhesive joints are prepared on mechanically treated mild steel substrates. The anti-corrosion behaviour of epoxy-based nanocomposites is studied on mild steel surface in 3.5% NaCl solution by using Gamry Interface 1000 Potentiostat instrument. The thermal stability of epoxy-based nanocomposites is studied by performing the thermogravimetric analysis (TGA) test. The mechanical and viscoelastic properties of epoxy-based nanocomposites are studied by performing the tensile test, 3-P notch bending test and dynamic mechanical analysis. The toughening mechanisms and fracture behaviour of epoxy-based nanocomposites are investigated by examining the fracture surfaces of the tensile test and 3-P notch bending (3-P SENB) test using FESEM images. The obtained fracture surfaces assist in understanding the structure-property relationships of the resulting epoxy nanocomposites. The FESEM images revealed that the homogeneous dispersion and strong interfacial interaction of MWCNTs are primarily depended upon the MWCNTs contents and their surface modification in the epoxy matrix. An appreciable breaking of clusters of MWCNTs at relatively higher loading (1.0 wt.%) is observed by decorating ZrO2 nanoparticles on the surface of MWCNTs. The incorporation of MWCNTs (0.50 wt.%) and MWCNT/ZrO2 hybrid nanofillers (1.0 wt.%) into the epoxy matrix via UDM technique results in the maximum enhancement in the value of glass transition temperature (Tg) of the resulting epoxy nanocomposites. However, a further increase of MWCNTs content results in the reduction of Tg, that is mainly due to the presence of a large amount of clusters of MWCNTs. These clusters of MWCNTs act as a barrier to the cross-linking density of the epoxy matrix. The incorporation of MWCNTs (0.50 wt.%) and MWCNT/ZrO2 hybrid nanofillers (1.0 wt.%) via UDM techniques results in enhancement of thermal stability of the resulting epoxy nanocomposites. The homogenous dispersion of CNTs in the base matrix offers a large number of obstacles to the heat flow. While, at higher loading of CNTs content beyond a critical limit, a detrimental effect on the thermal stability of the epoxy nanocomposites arises due to the agglomeration of CNTs with their non-uniform distribution, causing relatively less restriction to the heat flow. The superior anti-corrosion performance of MWCNT/ZrO2 hybrid epoxy nanocomposites (MNCs) having 1.0 wt.% of hybrid nanofillers arises due to superior dispersion and high aspect ratio of MWCNTs in the epoxy matrix. The homogeneous and cluster free dispersion of CNTs hindered and enlarged the diffusion path of O2 and H2O molecules in the epoxy matrix. The delay in the rate of O2 and H2O molecules for corrosion on mild steel surface improves the corrosion protection of nanocomposite coated mild steel. A significant improvement in the tensile properties at the optimum content of MWCNTs (0.50 wt.%), and MWCNT/ZrO2 hybrid nanofillers (1.0 wt.%) is observed in the epoxy-based nanocomposites prepared by the UDM technique. The fracture surfaces of resulting nanocomposites displayed the combination of various toughening mechanisms such as CNTs pull-out, plastic void growth, plastic deformation, crack deflection and crack bridging, that are primarily responsible for the enhancement in mechanical properties. At the same loading of nanofillers, the nanocomposites prepared by UDM technique showed a significant enhancement in fracture toughness. The lap shear joint strength of the UDM processed nanocomposites on mild steel substrate is significantly enhanced by the incorporation of MWCNTs (0.50 wt.%), and MWCNT/ZrO2 hybrid nanofillers (1.0 wt.%). The increase in the lap shear strength is attributed to the change in the mode of joint failure from an interfacial failure for neat epoxy adhesive to a mixed-mode cohesive-interfacial failure for epoxy nanocomposite. The UDM assisted MWCNT/ZrO2 hybrid epoxy nanocomposites showed an overall better performance compared to other MWCNT-epoxy nanocomposites on equal loading of the nanofiller. The ZrO2 nanoparticles on the surface of MWCNT act as a spacer and prevent the re-agglomeration of MWCNTs. The synergistic effect of MWCNTs and ZrO2 nanoparticles leads to the enhancement in the overall performance of MWCNT/ZrO2 hybrid epoxy nanocomposites. Apart from the experimental investigations, the mechanical and fracture properties of MWCNT/ZrO2 hybrid epoxy nanocomposites (MNCs) are also investigated using numerical techniques. The tensile and fracture properties of MNCs are studied by performing the tensile test and 3-point (3-P) single edge notch bending test via FE simulations. Firstly, the mechanics of materials (MOM) and finite element (FE) model are developed to study the micromechanical behaviour and the effective elastic properties of two- and three-phase epoxy nanocomposites. The obtained effective elastic properties are infused in the FE model to study the tensile and fracture properties of MNCs. Finally, the obtained numerical results are found to be in good agreement with the experimental results with a maximum discrepancy of ~11.24 and 6.39% in tensile and mode I fracture test at 1.0 wt.% loading of hybrid nanofillers. Keywords: Carbon nanotubes; Epoxy resins; Fracture toughness; Mechanical properties; Thermal properties; Ultrasonic dual mixing, Micromechanics; Mechanics of materials; Finite element method; Nanocomposites.