Milazzo, Mauro (2022) Molecular approach for the study and development of advanced material, with high mechanical resistance, and self-healing properties. [Tesi di dottorato]

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This Ph.D. project aimed to design a thermoset-based material that exhibits self-healing properties. The thermoset resins used as matrices are commercial grades with high modulus and high chemical and thermal resistance that are already utilized in several sectors, including the automotive and construction industry. However, thermosets are usually brittle materials. As such, they are prone to cracking, causing penetration of detrimental agents inside the structure and consequent early deterioration and failure. Finding an easy way to achieve self-healing material readily applicable in the industrial process it was crucial. The synthesis and the curing of thermoset resins are characterized by complex mechanism reactions that engrave on the kinetics and the final properties of the material, depending on composition, chemistry, curing condition, presence of hardener, and additives. To address the complexity of these curable systems, research activities were first focused on the characterization of sets of some commercial resin. The thermal, structural, and thermomechanical characterization of Phenolic (phenol-formaldehyde) and Melamine (melamine-formaldehyde) based resins are reported in Chapter III. In particular, the curing behavior of a set of novolac-type phenol-formaldehyde (PF) resins and a set of melamine-based resin, containing hexamethylolmelanime or MF oligomeric resin, using hexamethylenetetramine as a hardener, has been investigated. A method based on numerical descriptors extracted from the rheological data has been proposed in order to tag the different types of resins on the basis of the curing kinetics. Self-healing tests have been performed on mixtures of a commercial fast curable phenol-formaldehyde novolac-type resins and polyamide 12 (PA12). In Chapter IV, it has been shown that the components are partially miscible and realize good adhesion at the interfaces. Viscoelastic and morphological studies have been performed on the repaired specimens in order to evaluate the efficiency of the self-healing process in the restoration. In particular, thermomechanical analysis has revealed that the mixtures with PA12 content higher than 20 %wt are inherently able to restore their integrity. Since PA12 keeps crystallizing in the cured samples before and after healing, it is suggested that the healing process is triggered by heating at temperatures higher than the melting temperature of the PA12 component through the diffusion of the melt towards the fracture and the establishment of new interactions of the amide groups with the hydroxyl groups of PF resin, followed by successive crystallization of PA12 upon cooling. It has been shown that this mechanism is general as it allows achieving efficient welding regardless of the temperature adopted for curing the mixtures and also for other types of PF resins having different curing behavior. Model mixtures of PF resin and PA12 in absence of the hardener have been also studied. Whereas investigation of the structure and thermal properties confirm the good compatibility of the two components, FTIR analysis provides direct evidence of the formation of hydrogen bonds even though the possible formation of covalent bonds at PF/PA12 interface. Finally, a simple procedure is set up, that allows using a PF/PA12 blend with high PA12 content as masterbatch, to obtain efficient self-healing materials by mixing with HAP resin or melamine-based resins. It is demonstrated that with the use of the masterbatch, the desired self-healing performances may be achieved using a lower amount of PA12 component.

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