Carbon and Tungsten Disulfide Nanotubes and Fullerene-like Nanostructures in Thermoset Adhesives: A Critical Review
Thermoset adhesives such as epoxy exhibit high stiffness, strength and adhesion properties, but relatively poor toughness, elongation and conductivity. Nanofillers such as carbon (CNTs) and tungsten disulfide (WS2) nanotubes (INTs) and fullerene-like (IF) nanoparticles (NPs) possess excellent mechanical properties and also high electrical (thermal) conductivity. The quality of dispersion of the nanofillers in the thermoset adhesives has a major effect on the final properties of the nanocomposite adhesive. Dispersion techniques such as solution mixing, ultrasonication, three-roll milling, etc. have been used. Significant enhancements in stiffness, tensile strength, shear and peel strengths, fracture toughness, glass transition temperature, CLTE (coefficient of linear thermal expansion) among others, can be achieved at low CNTs concentrations (less than 1 wt.%), while at higher content of the CNTs, agglomeration reduces the toughness. Also for the WS2 nanoparticles, the lower concentrations (up to 1 wt.%) are preferable in order to improve the mechanical performance of the epoxy-based composites. Enhanced dispersion and interfacial interactions, resulting in improved mechanical performance of the nanocomposites containing CNTs, could be achieved by covalent and noncovalent modifications. The dispersion of WS2 nanostructures in the polymer medium was obtained using simple techniques like shear mixing and sonication without surface modifications. Moreover, it was found that the IF-NPs could covalently interact with the epoxy, without any surface modifications. The toughening mechanism of polymer matrices by the tubular nanofillers is mainly attributed to pull-out and crack bridging of the nanotubes. At the same time, inorganic nanocomposite adhesives exhibit promising results. IF structures of WS2 were found to exhibit crack deflection and bowing mechanisms. Electrically and thermally conductive adhesives have been obtained by adding CNTs, whereas conductive nanocomposites containing INT-WS2 have not been reported. In the case of CNTs the percolation path could be realized already at low levels (0.25 -1 wt.%). Furthermore, as the CNTs content increases, so does the conductivity of the composite material. In addition, the thermal conductivity of CNTs was found to have insignificant effect on the thermal conductivity of the nanoadhesives. Furthermore, the electrical and thermal conductive properties of most surface treated and functionalized CNTs were found to decrease, compared to untreated CNTs. The resulting nanocomposite adhesives containing carbon and tungsten disulfide nanotubes and fullerenes exhibit promising results in various applications.
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