Systematic variation of inorganic additives and their impact on interfacial delamination processes of automotive coating systems

Inorganic fillers incorporated in adhesives for the automotive industry enable both functional and mechanical tuning of adhesive properties. While integral effects of additives are well understood, the individual effects of single components and how they may lead to synergies as well as antagonistic effects remain unclear. Here, we systematically vary the concentration of CaO, CaCO3 and hydrophobic fumed SiO2 to understand individual and combined effects. For this purpose, the inorganic additives were mixed into a model formulation without inorganic additives and compared with a close to real application one-component epoxy adhesive, which also contained these fillers. The water uptake of the filled adhesives was measured with EIS and the results were compared with findings from cataplasma tests under atmospheric conditions. These results indicated that hydrophobic SiO2 can result in accelerated water uptake, although the opposite would be expected due to the more hydrophobic surface coating. In addition, blistering was observed on all galvanized steel (ZE) surfaces, while filiform corrosion was observed on all hot-dip galvanized Zn-Al-Mg steels (ZM). Cataplasma tests in oxygen-depleted and oxygen-rich atmospheres allowed to identify interfacial delamination processes. Our results showed that CaO can cause osmotic blistering and accelerate the diffusion of water into the adhesive due to a local increase in pH near the hydrated CaO particles, while CaCO3 reduces water diffusion. However, coating defects may occur due to the addition of CaCO3, leading to increased cathodic blistering. A strategy for improving beneficial and synergistic effects is discussed.