Co-authors​ :

 Jesse SAVOLAINEN , Pekka LAURIKAINEN , Essi SARLIN (FINLAND) 

Valorisation of recycled fibres for second-generation composites increases in importance as the technical readiness level of composite recycling technologies increases. Pyrolysis, fluidized bed and chemical recycling technologies have been successfully demonstrated for the extraction of reinforcing fibres, but the quality and the characteristics of the extracted fibres vary between different technologies and process parameters used. To avoid downcycling, the recycling technology should maintain the mechanical properties of the fibres as well as possible. Equally important is, that the load transfer capability of the fibre-matrix interface is ensured before application of the new resin. This can be accomplished by ensuring that the recycling technology produces fibre surface characteristics that are compatible with the targeted matrix material or by applying an additional surface treatment step with coupling agents, film formers and/or binder chemicals. Naturally, the cost effectiveness of a method can be best ensured if the number of process steps and associated use of energy and chemicals are minimized. In this study, the focus was on the surface treatments of thermochemically recycled glass fibres for nonwoven second-generation epoxy matrix composites. The fibre-matrix interfacial shear strength and the properties of the non-wovens were in the core of the experimental part. The addressed research questions were the following: (i) Can sufficient composite properties be achieved by applying only the binder without the sizing? How does (ii) the chemical formulation or (iii) the molecular weight of the binder affect the composite properties? (iv) How does the solid content of the binder polymer affect the final composite properties? The methodology of the study focused on interfacial fibre-matrix adhesion studies with the microbond method using the automated high-throughput microbond tester, FIBRObond, and with dynamic mechanical analysis. The nonwovens were analysed before matrix application by measuring their bending resistance and by thermogravimetric analysis to quantify the solid content of the binder polymers in the nonwovens. Additionally, scanning electron microscopy was used to visualize both the nonwoven and second-generation composite morphologies. The results imply that the sizing step prior to binder application improves especially the consistency of the composite quality decreasing the scatter of the results. Within the selected binder chemistries, polyurethane based treatment resulted in the highest quality nonwoven and most consistent composite behaviour, whereas the epoxy-based binder resulted in the highest interfacial strength. Increasing the solid content and the molar mass of the binder decreased the interfacial strength which was assumed to be due to a weaker interfacial layer generated by the high local concentration of comparably small molecular weight binder components at the fibre-matrix interphase.