Mechanical and curing behavior of tetra-functional epoxy reinforced with nano-fillers
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Synopsis Tetra-functional epoxy resins are used as a matrix for aerospace composites owing to their excellent mechanical, adhesive, corrosion resistance and thermal properties. Although epoxy is a high-performance thermosetting polymer, it is brittle due to its highly cross-linked network. This limitation must be addressed to make epoxy fit for aerospace applications. Plenty of potential filler materials such as organic and inorganic nano-particles, CNTs, two-dimensional nanomaterials such as graphene and its derivatives have been studied as reinforcing agents for epoxy composites. The high aspect ratio and active surface area of these nano-fillers contribute to the improved mechanical performance of the epoxy composites. Moreover, since these nano-fillers have active surface chemistry, it is essential to understand their effect on the epoxy-hardener curing reaction. Chapter 1 of this thesis briefly introduces the types and grades of epoxy resins and curing agents and the various stages of the curing reaction. The literature on the effect of nano-fillers on the curing behaviour of epoxy composites and the literature addressing the mechanical properties of epoxy composites for low loadings of nano-fillers was reviewed. It was found that studies on inorganic two-dimensional nano-fillers as toughening agents for epoxy composites are limited and have scope. Also, the curing behaviour of tetra-functional epoxy in the presence of nano-fillers is not very widely reported. Chapter 2 of the thesis lists all the materials, characterization methods and protocols used in this work. The models and equations used for the evaluation of curing kinetics have been explained in detail. The standard testing procedures used to calculate the mechanical properties of the epoxy composites have been explained in detail in this chapter. This thesis contains three works on the curing and mechanical behaviour of tetra-functional epoxy composites reinforced with nano-fillers. In the first work (chapter 3), the effect of GO and p-phenylenediamine modified GO on the curing mechanism of epoxy composites was studied using an isoconversional approach. The filler chemistry and loading level effect on the curing activation energy was studied using the Starink method. The amine groups present on functionalized GO acted as a secondary hardener and accelerated the curing by lowering the activation energy of the curing reaction (Figure 1). The autocatalytic reaction mechanism represented by the truncated Sestak Berggren model was used to calculate the kinetic parameters of the curing reaction. The calculated and the experimental data showed a good fit, thereby validating the model and proving the reliability of the calculated parameters of the curing reaction. Figure 1: Dependence of activation energy (Ea) on conversion (α) for neat epoxy and composite samples In the second work (chapter 4), a core-shell nano-particle consisting of polystyrene core and GO shell was synthesized using an in-situ emulsion polymerization technique. GO sheets have a high aspect ratio and a large active surface area, and they tend to restack or aggregate. Hybrid nano-fillers prevent them from restacking and enhance the interaction area of the GO sheets with the epoxy matrix. The aqueous GO dispersion was the water phase, and the styrene monomer was the oil phase. The GO sheets acted as a secondary surfactant and stabilized the emulsion by adhering to the monomer molecules and finally self-assembled as core-shell nano-particles due to strong π-π interactions between the GO sheets and PS nano-particles. Figure 2 shows the core-shell nano-particles where the GO sheets enveloped the PS particles, and the wrinkles and folds of the GO sheets were visible. Figure 2: TEM image of PS-GO core-shell nano-particles showing PS particles enveloped by GO sheets It was observed that a higher degree of exfoliation of GO was achieved due to the core-shell morphology, which in turn contributed to an improved filler-matrix interaction. The mechanical properties of PS-GO/epoxy composites were superior to GO/epoxy composites and neat epoxy. The fracture toughness of 1.0 wt.% PS-GO/epoxy composites was 28% higher than neat epoxy. The major toughening mechanisms causing improvement in fracture toughness were crack-deflection, void formation and improved filler-matrix interaction. The compressive strength of the 0.1 wt.% PS-GO/epoxy composites was 25% higher than neat epoxy. The thermomechanical properties of the PS-GO/epoxy composites were also better than neat epoxy, meaning that the thermal performance of the epoxy was not harmed during the process of improving the mechanical properties. The third work (chapter 5) of the thesis delves into the possibilities of using inorganic two-dimensional fillers as toughening agents for epoxy. In this regard, MXenes have become increasingly popular in the last few years. Ti3C2 nano-sheets were chosen as the nano-filler for the third work. Further exfoliation of the nano-sheets was achieved by functionalization using polyethene-imine (PEI) (Figure 3) to promote uniform filler dispersion and strong filler-matrix interaction in the composite. Figure 3: TEM image of PEI- Ti3C2Tx nano-sheets (a), HRTEM image of PEI- Ti3C2Tx showing exfoliation of sheets up to two layers, SAED pattern of the PEI- Ti3C2Tx (b inset) showing a hexagonal crystal structure PEI functionalized Ti3C2 nano-sheets lowered the curing activation energy due to the abundant amine groups present on their surface. The fracture and compressive properties also showed remarkable improvements over neat epoxy and epoxy reinforced with blank Ti3C2 sheets. Compared to neat epoxy, the fracture toughness and compressive strength improved by 70% and 40% for 0.5 wt.% PEI- Ti3C2Tx/epoxy composites, respectively. The fracture toughness enhancement resulted from the toughening mechanisms such as crack deflection and blunting and strong filler-matrix interaction. The dynamic mechanical properties of PEI- Ti3C2Tx /epoxy composites were also better than neat epoxy due to uniformly dispersed filler aiding a favourable filler-matrix interaction. In a nutshell, this thesis attempted to investigate the effect of low nano-filler loading (≤ 1 wt.%) on the epoxy-hardener curing reaction and the mechanical properties of the epoxy composites. The reasons behind improved mechanical properties and the predominant toughening mechanisms contributing to the enhanced fracture toughness were discussed in detail. Chapter 6 of the thesis sums up all the findings and outcomes of the investigations carried out in this work and offers perspectives into the future scope of this research area.