Titre de la dissertation: « Advancing dynamic non-isocyanate polyurethane chemistry for more sustainable structural composites in the frame of a circular economy ».

Promoteur: Monsieur Jean-Marie Raquez et co-promoteur: Mme Nora Aranburu (Université du Pays Basque UPV/EHU)

Résumé de la dissertation: Natural fiber-reinforced polymers are key materials for building a more sustainable future as natural fibers offer a promising low-carbon option. However, they involve the use of fossil-based matrices, hindering their environmental benefits, together with poor adhesion with cellulosic fibers and their non-recyclability. In this respect, this work aims to explore a more sustainable matrix to bring more circularity in natural fiber composites, particularly polyhydroxyurethanes (PHUs). Made from CO₂-based cyclic carbonates and polyamines, PHUs are considered to be greener/safer alternatives to conventional polyurethanes but have not been explored as a candidate for composite matrices. The hydroxyurethane moieties present along the PHU’s backbone offer unexplored opportunities that could be a breakthrough in many structural applications, particularly a special affinity between PHUs and cellulosic fibers, along with the ability of hydroxyurethane moieties to participate in reversible exchange reactions for further recycling.As PHUs have never been explored for composite applications, and are relatively new, there is a significant lack of understanding about their advantages and limitations. The relationship between the macromolecular structure, the processability, and the final properties of PHUs was herein evaluated to establish, to a sufficient extent, a comprehensive overview of these emerging thermosets. This allowed us to create an optimized PHU matrix for flax-fiber composites with excellent mechanical performances, 20-30% better than epoxy-based benchmark composites. These exceptional properties are ascribed to the existence of strong hydrogen bonds between PHU and fiber as shown through atomic force microscopy and rationalized by atomistic simulation. These PHU composites also demonstrated self-welding capabilities due to the reversible nature of hydroxyurethane moieties.To implement these PHUs for large-scale composite and industrial processes, we developed an approach relying on a synergetic copolymerization, which significantly increased dynamic performance and enabled the production of high-performance, re-shapable fiber composites derived from natural fibers as well as carbon fibers. This approach also allowed for efficient fiber recovery.Further investigating this copolymerization strategy, the processability was investigated deeply to provide a better understanding of the systems and efficiently apply it to resin transfer molding with controlled conditions and fast curing. While PHUs were not easily processable, these hybrids can now be infused and cured in an efficient protocol. The dynamic matrices allow the reshaping and welding of post-cure materials and the recovery of the fibers, even using natural fibers.Starting from an unexplored pathway, PHU chemistry was explored, developed, and enhanced to enter a new field, composites. Through the implementation of PHUs in composites, particularly NF ones, substantial advantages might be expected in terms of sustainability, possibilities, properties, and economic benefits. Finally, to objectively state the sustainability of our developed materials, an LCA study was conducted to establish the playground of a sustainable discussion and draw future perspectives for implementing PHU chemistry in the realm of composites for structural applications.