Implementing recyclable bio- and CO2-sourced synergetic dynamic matrices via precise control of curing and properties for natural fiber composites within industrially relevant resin transfer molding

Natural Fiber Composites (NFCs) offer significant environmental benefits. However, the use of thermoset matrices presents major challenges since they are oil-based, non-recyclable, and require energy-intensive curing processes, which diminishes their sustainability. Various proposed alternatives, particularly sustainable, dynamic matrices, have limited industrial applicability as they often lack the appropriate curing kinetics and viscosity needed for conventional composite processing techniques, such as Resin Transfer Molding (RTM). CO2-derived polyhydroxyurethanes (PHUs) show promise as NFC matrices but are hampered by poor processability and complex workup procedures, confining them mainly to laboratory settings. This study explores a synergetic copolymerization strategy that combines epoxy and PHU to allow fine-tuned polymerization kinetics and, thus, suitability for the RTM process. We demonstrate a synergetic catalytic effect that accelerates curing compared to each neat component. The formulation maintains a low viscosity (<5 Pa.s) at room temperature while curing within 30 minutes at 80°C—unattainable conditions with pure PHUs. The RTM-made composite achieves a fiber volume fraction of 58-60% and a porosity below 1%, making it ideal for high-quality NFCs. Moreover, the catalyst-free dynamic matrix allows the reshaping after curing, and flax fibers can be easily separated without toxic reagents from the polymer under mild conditions (60°C for 2 hours) and reused, retaining properties similar to those of virgin yarns. This strategy could broaden the application of PHU chemistry in sustainable NFC manufacturing while preserving both natural and fossil feedstock.