Development and optimization of bonding process for fiber-reinforced composite materials in automotive applications

The increasing use of composite materials in lightweight automotive structures requires joining technologies that are reliable, reproducible, and safe for operators. In particular, structural adhesive bonding remains a central solution for assembling composites. In addition, atmospheric plasma surface preparation (APPT) is crucial for improving adhesion quality while reducing VOC emissions and the dispersion of carbon dust associated with conventional abrasive treatments. Within this context, the present work aims to develop a robust bonding process for polymer-matrix composites, characterize the mechanical behavior to support numerical modeling, evaluate alternative adhesive solutions to determine the robustness of plasma activation, and identify an efficient, low-toxicity sealant for the project. The development of the bonding process using atmospheric pressure plasma jet treatment (APPJT) with air as the process gas for an autonomous electric vehicle built in CF-SMC and bonded with a polyurethane adhesive is organized into distinct, sequential phases. An initial surface characterization of the as-received material is performed by SEM, roughness measurements, contact angle, and XPS. The plasma-generated species are characterized as a function of process parameters by optical emission spectroscopy (OES), and a design of experiments based on lap-shear tests is used to define the process distance and speed. Finally, the analyses previously carried out on the untreated material are repeated to understand the reasons for the increase in adhesion after plasma treatment. A mechanical test campaign is also conducted, comprising SLJ and DCB tests at different bondline thicknesses and temperatures, to create a dataset suitable for simulation numerical modeling. The robustness of the activation process is confirmed by using different polyurethane adhesives on the same substrate and with the same surface preparation. Finally, the choice of sealant for the project is based on peel tests after accelerated aging, selecting the adhesive that ensures the safest working conditions for operators.