Cilento, Fabrizia (2021) Innovative composite materials with high graphene content. [Tesi di dottorato]

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Nanocomposites based on the biomimetic brick and mortar architecture are gathering great attention recently due to the outstanding properties of the natural analogues. Thanks to the very high in-plane orientation of nanoplatelets and the low matrix content, these materials exhibit good mechanical performance combined with excellent functional properties based on the nanoplatelets characteristics. Key feature of these materials is the presence of a regular nanostructure that consists of alternated nanoplatelet and matrix layers. This thesis addresses the study of mechanical and functional properties of nacre-like composite materials based on graphite nanoplatelets (GNPs). Particular attention is devoted to providing an insight into stress transfer mechanism in high filler content composites and describing the parameters that influence the efficiency of stress transfer. GNPs have been chosen as filler thanks to the good combination of mechanical, thermal and electrical properties, and the very low cost. This would allow the mass production of graphene-based material with remarkable properties that could give a breakthrough in the materials field and industrial applications. In particular, nacre-like GNPs/Epoxy thin films at different filler content have been prepared by a top-down manufacturing technology and their mechanical properties in tension have been experimentally evaluated. The elastic modulus has been found to exhibit a maximum of ~15 GPa between 53-67 vol% filler content and then it starts dropping at higher loadings. This is attributed to a discontinuous polymeric matrix layer, and thus to an incomplete GNP surface coverage at high filler content. As a result, the effective area for stress-transfer is considerably reduced at the expense of the reinforcement efficiency. To better understand the quality of stress transfer between the two phases, a microscopic investigation has been carried out by micro Raman spectroscopy, highlighting the poor stress transfer between the two phases at high filler content. In the light of this, a model is proposed for predicting the stress transfer characteristics in brick-and-mortar systems by paying attention to possible non-uniform matrix distribution over the nanoplatelets. It has been observed that at relatively high filler content, the elastic modulus of these systems drops after a critical concentration deviating from the expected behaviour, which dictates that the higher the filler content the higher the macroscopic elastic modulus. Thus, understanding the mechanism at the base of stress transfer in composite with brick and mortar architecture is of great importance and allows the definition of design strategies for the optimization of the mechanical properties of this class of material. The proposed analysis captures well the observed effects and paves the way for the development and further improvement of this new class of engineering materials. The material architecture of GNPs based films also contribute to the excellent thermal end electrical conductivity of the material. Also, the high anisotropy between in plane and cross-plane conductivities of GNPs is reproduced at the macroscale by the thin films. In fact, at 70 vol%, GNPs/Epoxy films exhibit in plane and cross plane thermal conductivities of 216 W/mK and 8 W/mK respectively and sheet resistance of 0.33 Ω/sq. This makes the material an excellent shield for high radiative heat flux and electromagnetic waves. Therefore, these exceptional multifunctional properties and the good structural performances of GNP/Epoxy films, can be exploited to improve those of FRP. They can be easily integrated into fibre reinforced polymers (FRP), without adding any additional steps in the fabrication process, and without compromising the weight and mechanical performances of the material. In this thesis, it has been investigated the possibility of improving fire resistance of composites by integrating on their surface protective coatings. Graphene rich films have been bonded on the heat-exposed surface of Carbon Fibres Reinforced Plastic (CFRP) laminates observing a significant reduction of the temperatures on the heated surface and of the damaged area when exposed to high power radiative heat fluxes. The behaviour of CFRP composite has also been assessed through cone calorimeter test and the effect of graphene films protection has been investigated. In addition, the reaction of CFRP composite to high power radiative heat flux have been further investigated by laser spot heating. The effect of the protective layer thickness has been tested with different laser power (25, 50, 75, 100, 150 kW/m2), simulating standard testing conditions (AC 20-135 and ISO 5660-1 Standards). Finally, damage level and residual mechanical of exposed samples have been assessed as a function of the level of protection. A significant improvement of the post-heat flexural moduli and a significant reduction of the damaged areas have been obtained in graphene films protected laminates.

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