Evaluation of hydronitrogen plasma treatment to improve the bonding process of PCM composite material in automotive: A case study

Atmospheric Pressure Plasma Treatment (APPT) technology, using hydronitrogen (95% N2 and 5% H2 composition) as the process gas, was used in this study to treat Prepreg Compression Molding (PCM) substrates to increase the lap shear strength of joints bonded with an epoxy adhesive. A crucial factor for plasma treatment is the torch working speed, which influences the treatment time of the substrate. In particular, it was found that the speed of 100 mm/s increased the lap shear strength by about three times compared to that of the untreated joints. Therefore, the mechanisms underlying the improvements in the lap shear strength of the bonded PCM joints were investigated. It was found that the surface roughness was not the main reason for the increase in the lap shear strength but the surface free energy (SFE). After plasma treatment, there is a significant increase in SFE which indicates a chemical change in the substrate surface. These functional groups introduced by APPT, especially those containing oxygen or amino groups, can covalently bind with the adhesive forming strong bonds and leading to the increase in the lap shear strength of the bonded joints.