Effect of the direct injection plasma (DIP) treatment on the bonding strength of adhesive bonded joints

The adhesive bonding of thermoplastic materials requires, attributable to the low surface energy, a surface activation. The atmospheric pressure (AP) plasma treatment is a common method to improve the surface characteristics. An entirely new process type is the direct injection plasma (DIP) treatment, which combines the injection molding and the surface activation to a full-inline process. During the cooling phase of the injection molding cycle, a pre-defined area of the molded part can be activated by sending a plasma afterglow through a channel. In preliminary publications, it could be shown that the surface activation leads, dependent on the process parameters, to high surface energies up to a treatment length of more than 500 mm. The aim of the current investigations was to examine the influence factors for high bonding strengths after surface activation. Therefore, in this work, the bonding strengths of DIP-treated surfaces were investigated and compared with the surface energy and the XPS-spectrum along with the treatment length. Based on the DIN standard 1465, the lap shear test method was used to measure the bond strength, which, caused by the application-related test specimen, had to be adjusted for the investigations. Furthermore, for the investigation, two different substrate materials and adhesives were used. With PP, a nonpolar material and, with PA6, a polar material for the substrates were used. As adhesive systems, a polyurethane (PUR) and an epoxy resin system were used, both as multi-component adhesives.