Enhancing Interfacial Adhesion and Electrical Conductivity of Steel/Polymer Composites via Molecular Adhesion

The demand for steel/polymer composites (SPCs) is increasing in various industrial fields due to the need for weight reduction during the energy crisis. However, the development of SPCs faces a major challenge in achieving strong interfacial adhesion between heterogeneous materials like steel and polymer. Conventional physical adhesion methods, which rely on surface roughness, have drawbacks such as decreased durability and reduced interfacial adhesion under external conditions like temperature, humidity, and stress. Therefore, it is necessary to explore novel approaches to improve the adhesion of SPCs in practical applications.<br/>In this study, we controlled the surface chemistry to enhance the chemical and physical interactions at the steel/polymer interface to improve interfacial adhesion. The materials used in the study include electro-galvanized steel, polyethylene (PE), acrylonitrile-butadiene-styrene (ABS), and Nylon 6, which are commonly employed in the automotive industry for interior components. We employed an organosilane coupling reaction to anchor primary amines on the steel surface, and introduced maleic anhydrides (MAs) onto the polymer film surface using a photo-initiated grafting method. The presence of amines on the steel plate and the amount of MA introduced onto the polymer surface were confirmed and quantitatively analyzed. The adhesive properties were evaluated using a lap shear test. The adhesion strength of PE increased from 0 to 11.5 MPa, from 1.5 to 11.1 MPa for the case of ABS, and from 12.7 to 14.1 MPa for Nylon 6. Notably, the adhesion strength of Nylon 6 was maintained at 89.0% even after 7 days of aging, in spite of the detrimental effect of moisture absorption on interfacial adhesion. This highlights the robust and sustainable nature of molecular adhesion, which can withstand changes in external conditions such as moisture.<br/>Ensuring the in-process applicability is crucial for expanding the industrial application potential of SPCs. One important aspect in this regard is the feasibility of spot welding which is widely used in automated automobile manufacturing processes. In order to achieve the spot welding, we employed a conductive core layer. In order to achieve high conductivity, a network of conductive fillers was created within the resin. We prepared a polyketone composite by mechanofusion, where conductive fillers such as graphene nanoparticles were adsorbed onto the surface. The composite was then hot pressed to develop a conductive plastic with exceptionally high electrical conductivity of 207.5 S/cm. However, this high electrical conductivity was not sufficient to achieve an effective spot welding. We prepared conductive structure composite where steel fiber meshes incorporating into the plastic core layer. To ensure the steel fiber mesh was embedded in the plastic without creating voids, an organosilane-based treatment was performed at the surface between the steel fiber mesh and the plastic via molecular bonding. Through this approach, we successfully produced a polymer/steel fiber mesh composite core layer that was void-free and capable of spot welding<br/>In conclusion, this study successfully enhanced the interfacial adhesion in SPCs by improving the chemical and physical interactions at the steel/polymer interface. By employing molecular adhesion via precise control at the molecular level, the adhesion strength was significantly increased, and durability was improved, even in the presence of external factors such as moisture. Furthermore, for industrial application, we fabricated conductive SPCs which enabled spot welding by developing a conductive core layer and utilizing a steel fiber mesh. These findings is believed to contribute to the development of practical and effective SPCs, addressing the demand for weight reduction in various industries, particularly the automotive sector.