Mantovani, Federica (2008) Studio di nanocomposti a matrice epossidica per impieghi industriali nel settore di Coating. Analisi delle relazioni tra caratteristiche morfologiche, propriet√†¬† reologiche e prestazioni. [Ph.D. thesis]
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The PhD work was focused on epoxy matrices nanocomposite systems. Their preparation methods were optimised according to the matrix, filler and organic modifier characteristics.
The nanocomposites were studied on the rheological and morphological point of view, with the aim of looking for the relationships correlating structural characteristics of the materials in their fluid phase before crosslinking and their behaviour under flow and small deformation as well.
A variety of rheological behaviours was observed, according to the dispersion and intercalation/exfoliation features of the polymer hosted inside the nanofiller lamellae, for both phyllosilicates and metal hydroxides. In more detail, prepolymer nanocomposites containing high aspect ratio lamellae exhibit strong deviations from Newtonian behaviour: such a kind of rheological response was typically observed in the case of structures either exfoliated or intercalated with large interplanar distances and few layers per tactoid. Such deviations occur for sufficiently high dispersed phase concentrations, exceeding a critical value that can be interpreted as the achievement of the percolation threshold. It moves to progressively smaller concentration values with increasing aspect ratio of the lamellar particles dispersed throughout the polymeric matrix. These systems become more and more structured with increasing disperse phase concentration tending to exhibit a solid-like behaviour, as it was highlighted by the oscillatory tests carried out under linear conditions. In particular, the storage modulus G' tends to becomes almost independent of frequency in the low frequency region, as it happens for some thermoplastic nanocomposites described in literature. Moreover, these structured systems exhibit complex rheological properties which are strongly dependent on the previous mechanical history. Indeed, high shear conditions can lead to substantial orientation and/or alignment of nanofiller lamellae in the flow direction, and such shear-induced structural effects result into partially reversible responses.
The dependence of the viscosity on the shear stress or, equivalently, on the shear rate, were compared with different models, having different complexity and fitting capability. They were also used for implementing the duct flow simulations and for estimating the relevant macroscopic parameters, in view of possible technological applications.
As it happens for the rheological responses, also the dielectrical performances of nanocomposites are related to their morphology, as well as to the individual characteristics of the components. In some cases neat transitions are noticed in correspondence of the percolation threshold.
The characterisation carried out on intercalated nanocomposites prepared with mica nanofillers in alternate current, with increasing voltage, demonstrated dielectrical losses reduction and, particularly, a minor occurrence of partial discharges. Conversely, the performances exhibited by exfoliated nanocomposites resulted poorer than those of the polymeric matrix, in terms of volumic and surface resistivities, dielectrical losses and polarization effects.
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