The Effect of Tungsten Sulfide Fullerene-Like Nanoparticles on the Toughness of Epoxy Adhesives

In this work the effect of inorganic fullerene-like (closed cages) nanoparticles of tungsten disulfide (IF-WS2) on the mechanical properties and especially on the toughness of epoxy resins, was studied. The epoxy resin used was the well-known DGEBA (di-glycidyl ether of bis-phenol A) cured with polyamidoamine. The epoxy/IF-WS2 nanocomposites were prepared by applying a high shear mixing to obtain a uniform dispersion and homogeneous distribution of the IF nanoparticles in the epoxy matrix. Two mixing procedures were used - a low shear of short duration and high shear with a long mixing time. The resulting epoxy nanocomposites were first characterized for their shear and peel strength using appropriate bonded joints. The experimental results demonstrate that enhanced shear strengths and shear moduli were achieved, together with a significant increase in the peel strengths at low concentrations of the IF-WS2 nanoparticles (more than 100% increase at 0.5 wt% IF-WS2). Above the threshold value of 0.5% IF-WS2 the peel strength decreased sharply. The fractured surfaces of the bonded joints were examined by transmission and scanning electron microscopy in order to characterize the fracture mechanisms and analyze the dispersion level of the nanoparticles within the polymer. The electron micrographs indicated that the presence of the nanoparticles in the epoxy matrix induced fracture mechanisms which were different from those observed in the pristine epoxy phase. These mechanisms included: crack deflection; crack bowing; and crack pinning. Evidence for a chemical interaction between the nanoparticles and the epoxy were obtained by infrared measurements in the attenuated total transmittance mode. The data suggests the formation of new carbon-oxygen-sulfur bonds, which are most likely due to the reaction of the outermost sulfur layer of the IF nanoparticles with the reactive epoxy groups. The observed simultaneous increase in both shear and peel strengths at very low IF-WS2 concentrations, found in this work, could lead to the development of high performance adhesives and to new types of structural and ballistic fiber nanocomposites. (C) Koninklijke Brill NV, Leiden, 2010