Vitiello, Libera (2023) Biodegradability and structure of natural fibre composites. [Tesi di dottorato]

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Abstract

To mitigate the plastic pollution problem, biopolymers represent one of the most interesting solutions. These “green” polymers have the inherent advantage of being produced from renewable resources and, more important, they can be biodegradable or compostable. Contrary to common perception, biodegradation of such materials in the natural environment can be a very slow process, with obvious consequences in terms of their actual environmental sustainability. In the present dissertation, the pro-degradative effects of natural fibres on a host biodegradable polymer matrix are investigated to speed up biodegradation while preserving both the eco-friendliness of materials and the excellent properties featuring plastics. For such a purpose, the degradation process and environmental footprint of hygroscopic vegetal fibre-filled hydrolysable biopolymers have been deeply studied, and the design of innovative mechanical structures has been performed. From a chemical point of view, the biodegradation of polymer matrices is the result of the combined effect of water and microorganisms: the former induces hydrolysis of the polymer, while the latter digest the shortened chains and mineralizes the material. Natural fibres play a crucial role in this process, as they favour the access of water and microorganisms from the external environment, hence triggering the phenomenon. The best conditions to promote biodegradation are still debated. After the first part in which the optimal material-related parameters to speed up biodegradation have been investigated, the degradation process of the green composites has been assessed under different environmental conditions. The investigation thus focuses on the study of the degradation phenomena in the melt state, the hydrolytic degradation in water, and the biodegradation in compost. A cradle-to-gate life cycle assessment study of green composites has then been performed to evaluate their environmental footprint. The analysis was carried out by splitting the contributions of matrix and reinforcement to enable a conscious selection of the materials for realizing truly sustainable composites. In the end, the possibility of realizing mechanical metamaterials, i.e., systems whose mechanical properties are derived from their structure, has been investigated to obtain biodegradable materials with innovative mechanical behaviour.

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