Fiber-reinforced polymers (FRPs) provide excellent mechanical performance at low mass density. So, they are ideal candidates for lightweight applications and play a major role in sustainable mobility and energy. Their continuous usage increases resource efficiency and subsequently reduces emissions during the use phase [1]. With conventional recycling methods, for example, shredding, pyrolysis, and solvolysis, for FRPs [2], the part size, fiber length, and orientations are not preserved, which are considered to be important parameters for the recirculation of FRPs in high-performance applications. In order to preserve these above-mentioned parameters, a novel circular strategy for a near-complete recovery of FRPs are essential for the cleaner and sustainable reuse of FRPs. Successful recovery of glass fiber layers embedded in a thermoplastic matrix was performed by Imbert et.al [3]. These separated layers should be reconsolidated to bring them back to their original or tailored laminate structure, by which the materials circularity loop can be closed. This approach is also useful for composite applications of cutting scrap.

In this work, a novel approach with the application of power ultrasonics was investigated on a glass fiber composite embedded in a polypropylene (PP) matrix. The chosen PP-GF laminate was 4 mm in thickness, manufactured by Bond Laminates (Brilon, Germany). Rectangular specimens featuring a length of 70 mm, a width of 30 mm was extracted from the laminate. A special Fe-based cutting sonotrode was used to initiate controlled pre-cracks at 20 kHz and oscillation amplitudes of xx µm at the tip of the cutting tool. The pre-cracks were then propagated mechanically by inducing a peel loading using an in-house engineered layer peel-off setup for fiber layer separation. Microscopic examination of the separated layers revealed polymer residues and surface damages caused by the cutting tool, which indicated a cohesive failure of the matrix and a good fiber-matrix adhesion. The separated layers were reconsolidated by ultrasonic welding at 20 kHz and oscillation amplitudes in the range from 30 to 40 µm in order to close the materials circularity loop.

Microscopic analysis of the reference and the reconsolidated specimens shows that the fiber bundle arrangements were preserved with a satisfying distribution of the matrix along the reconsolidated zone. Cross-sectional microscopic investigations of the reconsolidated zone show evident of pores and a sucessful reconsolidated zone for specific process parameters. This shows that the consolidation pressure and time after the joining process play a major role in closing these pores. Furthermore, mechanical characterization (inter-laminar shear stresses, tensile strength) of these reconsolidated samples will be performed to evaluate the reconsolidation parameters.

References

[1] R. Lässig, Series production of high-strength composites, Roland Berger and VDMA Study, VDMA, 2012.

[2] S.K. Gopalraj, T. Kärki, Review on the recycling of waste carbon fiber/glass fiber-reinforced composites: fibre recovery, properties, and life-cycle analysis, SN Applied Sciences (2020).

[3] M. Imbert, P. Hahn, M. Jung, F. Balle, M. May, Mechanical laminae separation at room temperature as a high-quality recycling process for laminated composites, Material Letters 306 (2022).