Bifulco, Aurelio (2020) Surface modifications of natural fibers and synthesis of inorganic nanoparticles for tailoring of the interphase and the flame retardancy of green composites. [Tesi di dottorato]

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This thesis wants to show some applications which exploit sol-gel methodologies and its advantages to solve industrial and technological problems inherent the use of polymer based bio-composites. These composites can show severe limitations due to the easy flammability of the polymer matrix; this behavior can significantly restrict the application fields of these materials, especially when the possibility of the use of the composites is strictly related to specific regulatory fire tests that have to be passed, hence ensuring public safety (e.g., in the aerospace industry). Additional limitations descend from the mechanical properties of the above mentioned bio-composites, which may be due to a low interfacial adhesion between the filler (e.g., natural fibers) and the polymer matrix. Sol-gel methodologies can improve the fire behavior and the mechanical properties of bio-composites through the in-situ synthesis of ceramic domains in the polymer network and the tailoring of the interphase between filler and matrix. The main results obtained can be summarized as follow: - In this thesis, hemp fabrics have been modified with a new, inexpensive, simple and eco-friendly waterglass treatment, hence improving their fire behavior when utilized as reinforcing agent fillers in epoxy-based composites. A silica-based coating, able to protect the fabrics from heat sources, has been obtained. Moreover, when properly treated with the above-mentioned waterglass solution, hemp fabric easily gives silica-based coated fibers of diameters from tens of microns to tens of nanometers with the aid of a low power mixer. The silica-based coated fibers can be easily functionalized with a coupling agent and then dispersed in epoxy resin. Micrographs of the composites show a tendency to give web-like structure formed by fibrils and microfibrils continuously interconnected, from which particularly good mechanical properties may be expected to descend. - In this thesis, new hybrid silica/epoxy composites cured with a cycloaliphatic amine have been prepared by using an in-situ sol–gel process. The experimental results suggest that the new hybrid structures consist of very fine silica nanoparticles, homogeneously dispersed in an epoxy/silica hybrid network. The presence of the silica domains in the hybrid organic–inorganic networks prevents melt dripping phenomena in vertical flame spread tests. In addition, as assessed by forced-combustion tests, the inorganic domains act as a thermal protective layer, hence improving the heat and smoke-related parameters. Finally, the efficiency of the protective ceramic layer decreases after the non-isothermal post curing treatment carried out on the hybrid systems. The above-mentioned in-situ sol-gel process. has been modified by adding two phosphorous based flame retardants and eventually melamine just before the addition of the hardener. The use of the two phosphorous-based flame retardants allow to achieve self-extinguishing capacity for the studied epoxy system with only 2 wt. % of P-loading. Additionally, the presence of only 2% silica guarantees the absence of dripping phenomena during the burning of the epoxy resin. The formation of an intumescent char is observed when the two flame retardants are used in combination with melamine, hence a remarkable increase of the post-combustion residues is recorded. The flame retardancy of the epoxy composites has been improved by adding very low amount of phosphorous and silica. As a future prospective, the above sol-gel functionalized hemp fibers may be used as reinforcement in in-situ silica/polymer bio-composites added with green phosphorous-based flame retardants. This may allow to combine excellent performances in terms of fire behavior with improved mechanical properties.

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