EPOXY BASED CNT NANOCOMPOSITES WITH ADVANCED PHYSICAL AND MECHANICAL PROPERTIES

Abstract

This dissertation involves the in-depth investigation of multiwall carbon nanotube (MWCNT)-epoxy nanocomposites containing cluster free uniform dispersion of different types of MWCNTs (MWCNTs, MWCNT/APTES and MWCNT/TiO2) in „bisphenol A‟ based epoxy matrix prepared by innovative ultrasonic dual mixing (UDM) technique. In this study, MWCNTs is synthesized by chemical vapour deposition method using in house developed chemical vapour deposition rector. This study investigates the effect of processing technique parameters, surface modification of MWCNTs and MWCNTs content as well as their dispersion scenario in the epoxy matrix on mechanical properties, thermal properties, fracture properties and viscoelastic properties of epoxy nanocomposites. In order to improve interfacial interaction of MWCNTs with epoxy matrix, surface modification of MWCNTs has been done by attaching 3-Aminopropyl triethoxysilane (APTES) on the surface of MWCNTs (MWCNT/APTES). To avoid the degradation in atomic structure arrangement of carbon atoms in MWCNTs by critical chemical oxidation process during functionalization of MWCNTs, the surface modification of MWCNTs has been also done by decorating its surface with TiO2 nanoparticles and formed a new hybrid structure of nano filler (MWCNT/TiO2 hybrid filler). The morphology of newly formed MWCNT/TiO2 hybrid nano filler and MWCNT/APTES has been studied using transmission electron microscopy (TEM). Further the prospect of using such epoxy nanocomposites as adhesive and as coating material in preparation of superior adhesive joints of mild steel have been explored by studying the mechanical and fracture properties of lap joint of mild steel sheet and anti-corrosion properties for mild steel. Toughening mechanisms of epoxy nanocomposites has been investigated by examining the fracture surfaces of tensile test, 3 point bend test and single lap joint test specimens using field emission scanning electron microscopy (FESEM) in order to understand the structure-property relationships of the epoxy nanocomposites. The FESEM reveals the homogeneous distribution of individual MWCNT largely depending upon MWCNTs content and surface morphology of MWCNTs in the epoxy matrix. An appreciable breaking of clusters of MWCNTs at relatively higher loading has been marked by decorating TiO2 nanoparticles on the surface of MWCNTs. The Fourier transform infra-red (FTIR), Raman, TEM and FESEM analysis of MWCNTs reveals that the decoration of TiO2 nanoparticles on MWCNTs leads to no degradation in the arrangement of carbon atoms in the form of defects. The incorporation of optimum content of all three types of CNTs such as MWCNTs (0.75 wt.%), MWCNT/APTES (0.75 wt.%) and MWCNT/TiO2 (1.0 wt.%) into the ii epoxy via UDM technique results in the maximum enhancement in the Tg of the epoxy nanocomposites upto ~ 87, 91 and 95 oC respectively from the Tg ~ 78 oC of neat epoxy. However, further increase of CNTs content in each case results in reduction of the Tg due to significant agglomeration of CNTs in the form of cluster. The decrease in Tg attributed to the hindrance offered by the large amount of clustered CNTs to the cross linking density of the base matrix. The incorporation of optimum content of all three types of CNTs via UDM techniques results in enhancement of thermal stability of the resulting epoxy nanocomposites. When the CNTs are dispersed uniformly, they act as large number of obstacles to the heat flow but, at higher level of CNTs content beyond critical limit, a detrimental effect on thermal stability of the epoxy nanocomposites arises out of agglomeration of CNTs with their non-uniform distribution in the epoxy matrix causing relatively less restriction to the heat flow through it. A significant enhancement in tensile properties at optimum content of all three types of CNTs has been observed in the epoxy nanocomposites prepared by the UDM techniques. In case of UDM processed MWCNT-epoxy, MWCNT/APTES-epoxy and MWCNT/TiO2-epoxy nanocomposites, combination of various toughening mechanisms such as CNTs pull-out, plastic void growth, plastic deformation, crack deflection and crack bridging are found responsible for enhancement in mechanical properties. At optimum content of all three types of CNTs, the nanocomposites prepared by UDM techniques have shown a significant enhancement in fracture toughness and fracture energy. The lap shear joint strength and joint toughness of the UDM processed nanocomposites enhance significantly at the optimum content of all three types of CNTs. The increase in the lap shear strength and lap joint toughness is attributed to the change in mode of joint failure from an interfacial failure for neat epoxy adhesive to a mixed mode cohesive-interfacial failure for epoxy nanocomposite. The reinforcement of optimum level of all types of CNTs in epoxy by UDM process shifts the failure mode from cohesive to mixed mode failure, predominantly include adhesive failure. The superior anti-corrosion performance of CNT-epoxy nanocomposites containing optimum contents of CNTs arise due to superior dispersion of CNTs in epoxy matrix by UDM process and high aspect ratio of CNT. The homogeneous and cluster free dispersion of CNTs hindered and enlarged the diffusion path of O2 and H2O molecules in the epoxy matrix. This delay of O2 and H2O molecules for corrosion on mild steel surface improves the corrosion protection of nanocomposite for mild steel. The TiO2 nanoparticles decorated MWCNTs epoxy nanocomposite shows an oval all better performance compared to other types MWCNTs epoxy nanocomposite on equal loading of filler. The TiO2 nanoparticles on the surface of MWCNT act as spacer and prevent the re-agglomeration of MWCNTs. This TiO2 assisted dispersion of MWCNTs leads to enhancement in the performance of MWCNT/TiO2-epoxy nanocomposite.