Adherence: influence of fillers composition and polymer network structuring in aerospace epoxy primers

This study investigates the influence of pigment dispersion on the structural and adhesive properties of epoxy-based aerospace primers. Epoxy-amine systems are widely used in high-performance coatings due to their thermal and mechanical stability. However, their interaction with inorganic surfaces, particularly metal oxides, can significantly alter the polymer network formation. In this contribution, we aimed to explore two main aspects: (1) the chemical reactivity of amine functions with dispersed fillers, and (2) the physical impact of pigment concentration on the thermoset structure. Initial tests using Differential Scanning Calorimetry (DSC) and FT-IR NIR spectroscopy revealed that certain pigments react with amine groups, leading to incomplete cross-linking and reduced glass transition temperatures (Tg). Films formulated with non-reactive pigments maintained Tg values comparable to the pure polymer matrix, while reactive pigments induced a Tg drop of up to 16°C. Further analysis showed that, above a critical pigment volume concentration (CVPC), the nature of the pigments becomes secondary while physical effects such as steric hindrance and internal stresses dominate. Adhesion tests using three-point bending on aluminum substrates confirmed that reactive pigments reduce the film adherence, especially at low CPV/CVPC ratios. At higher concentrations, adhesion differences reduce, aligning with thermal data. Our findings suggest that pigment reactivity and concentration jointly influence film performance, with implications for optimizing primer formulations. This work highlighted two main domains which will have to be further explored, namely mechanical property characterization and residual stress analysis, to better understand the durability of coatings and adhesive joints.