Recently, there has been an increasing movement in the aerospace industry to increase the use of thermoplastic composites. Moreover, interest in fusion bonding and adhesive bonding is increasing because it can reduce cracking, unlike mechanical bonding. In addition, interest in the oven welding process increases in bonding such thermoplastic composite materials by applying fusion bonding and adhesive bonding. However, there is a lack of research on the adhesion performance when using a thermoplastic polymer as an adhesive for bonding using a thermoplastic composite material as a substrate material. In addition, research about the effect of the interphase on an adhesion properties is insufficient, even though adhesive bonding generate interphase through chemical bonding between a substrate and adhesive. And it is unknown whether applying the vacuum bag only(VBO) method to the oven welding process is appropriate.
Therefore, lap shear strength test and fracture surfaces observation through digital optical microscope(Digital OM) and scanning electron microscope(SEM) were performed after bonding by varying the pressing force of carbonfiber/polyetherketoneketone(CF/PEKK) fusion bonding and adhesive bonding using polyetheretherketone(PEEK) and polyetherimide(PEI) as adhesive materials in order to evaluate whether the application of the VBO method is appropriate. In addition, lap shear strength tests were performed at room temperature, 100 and 160 and analysis using Fourier transform infrared spectroscopy(FT-IR) and differential scanning calorimetry(DSC) and fracture surface observation using digital OM and SEM were conducted to understand the adhesion properties of the bonding of PEEK and PEI adhesives having ether and ketone groups. Moreover, the conclusion is as follows.
Applying vacuum pressure by the VBO method was inappropriate when bonding CF/PEKK through oven welding process, as the pressing force was too high. In addition, the PEEK and PEI adhesive bonded CF/PEKK showed higher adhesive strength than the fusion bonded CF/PEKK because the increase of molecular formation of ether, ketone and imide constituting the interphase during the adhesive bonding enhance the adhesive strength. Furthermore, at high temperatures, the tendency of the change in strength differed depending on the type of bonding, which was judged to be due to the formation of adhesive layer, respectively different glass transition temperature, and the increased ductility with temperature increase.
Therefore, this experiment shows the effect of adhesive application having similar chemical structure with thermoplastic of substrate material on the adhesive strength enhancement. In addition, it was suggested that it is essential to consider the glass transition temperature of the adhesive when using the structure to which adhesive bonding is applied in a high temperature environment.