A novel approach to rapidly process vitrimer composites using power ultrasonics

Abstract Fibre-reinforced polymers (FRPs) play a key role in lightweight structural design but are often processed using high pressure, temperature, and time. FRPs frequently use a thermoset matrix that cures into an insoluble, non-meltable polymer network. Driven by recyclability, processing advantages, and performance, there is a growing global trend toward thermoplastic FRPs. Vitrimers, which feature dynamic covalent networks, offer a transformative alternative by enabling reshaping, repair, and recycling after curing, making them highly attractive for sustainable composite manufacturing. However, vitrimer composites are highly desired, but are challenging to be processed, due to covalent bonds within the vitrimer polymer network. This feasibility study presents a novel ultrasonic moulding process that was combined with ultrasonic joining and that enables the rapid fabrication of three-dimensional carbon fibre-reinforced vitrimer (CF-Vitrimer) structures. By superimposing ultrasonic vibrations with moderate forces, CF-vitrimer prepregs were processed within seconds. A specially designed sonotrode served as a mould to apply oscillations and forces into stacked prepreg layers, impregnating the fabric from the surface with the vitrimer matrix and simultaneously forming it. Compared to established CF-reinforced composite manufacturing and processing techniques, the main advantages of this process lie in its high speed, low processing forces, and cost-effective tooling. Using a 20 kHz ultrasonic system, CF-Vitrimer prepreg plies were moulded and joined in 5 s per operation. The process required only 2500 N of force (equivalent to a pressure of 1.5 MPa), up to 30 times lower than conventional thermoforming, and is about six times faster than advanced infrared-heating assisted techniques. Typical defects in CF composite processing include pores, wrinkles, delamination, fibre misalignments or breakage, and thermal degradation of the matrix, all of which significantly reduce mechanical performance. Therefore, the microstructural characteristics and mechanical behaviour in terms of strength and stiffness were used as indicators of the quality of the produced structures. The mechanical performance and microstructural properties of the resulting parts were found to correlate strongly with the processing parameters. Durable CF-vitrimer half-shells with measurable mechanical strength and less than 2% void content were obtained only when the energy input exceeded a certain threshold, defined by the combined effect of amplitude (~ 25 μm), applied force (~ 2500 N), and process time (5 s). Particularly remarkable are the exceptionally short processing times and the nearly defect-free consolidation of multiple prepreg layers, even though the polymer matrix was initially only distributed on the surface. This makes ultrasonic forming a promising alternative to established methods. Beyond further investigations into FRP quality and the development of forming tools for larger and thicker structures, this approach holds significant potential for high-volume production of lightweight CFRP components, enabling widespread applications in aerospace, automotive, and renewable energy technologies.