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 degrees 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.
This study presents an experimental - analytical investigation on the role of Y2O3-reinforced adhesive layers in governing the low-velocity impact response of titanium/carbon - basalt hybrid fibre metal laminates. A three-factor framework comprising impact mass (2-6 kg), velocity (2.5-7.5 m/s), and Y2O3 content (0-2 wt.%) was employed to establish process - structure - performance relationships. Impact responses were evaluated in terms of peak force, deformation, contact time, energy absorption, impulse, and velocity attenuation. Results indicate that impact mass is the dominant parameter, contributing up to 87.77% to peak force and 82.67% to energy absorption, reaching similar to 10107 N and similar to 38.21 J, respectively. In contrast, Y2O3 content significantly governs interfacial behaviour, contributing similar to 75.85% to contact time and similar to 32.51% to impulse, highlighting its role in energy dissipation and load transfer. Y2O3 incorporation enhances interfacial stiffness, promotes crack deflection, and limits damage propagation. Damage analysis shows a transition from localized indentation (similar to 13.20 mm(2)) to severe material removal (similar to 70.88 mm(2)), with optimized conditions exhibiting confined deformation. Statistical models demonstrate high accuracy (R-2 >94%), and multi-response optimization identifies 2 wt.% Y2O3 as optimal for balanced impact performance.
This study is primarily based on a numerical modeling framework, supported by experimental validation, to investigate the sensitivity of damage distribution and residual strength in metal - to - composite adhesively bonded joints to both the composite material system and laminate stacking sequence following low-velocity impact. To this end, three main specimen groups were considered. In the first group, the bonded joint consisted of a metal adherend and a glass fiber - reinforced polymer (GFRP) laminate with different stacking sequences, including unidirectional [0]8, [90]8, [0/45/-45/0]s, and [45/0/-45/90]s configurations. In the second group, the composite adherend was a unidirectional carbon fiber - reinforced polymer (CFRP) laminate, while in the third group a hybrid carbon/glass laminated composite with a unidirectional stacking sequence of [C/G/C/G]s was employed. The experimental validation was conducted using the baseline configuration corresponding to the unidirectional GFRP (Uni-G) specimen group. The numerical framework adopted in this study integrates a three-dimensional Hashin damage model for the composite adherends with a cohesive zone model (CZM) for the adhesive layer, enabling accurate simulation of impact-induced damage evolution within the composite plies and the adhesive layer, as well as the subsequent lap-shear loading response. For specimens subjected to an impact energy of 2.5 J, the results indicate that GFRP laminates with stacking sequences of [90]8, [45/0/-45/90]s, and [0/45/-45/0]s exhibited 57.5%, 25.4%, and 26.5% reductions in post-impact residual strength, respectively, compared to the [0]8 laminate. Moreover, under the same impact conditions, the use of a unidirectional CFRP substrate and the hybrid carbon/glass laminate composite led to 31.5% and 30.5% improvements in post-impact strength compared to the unidirectional GFRP substrate, respectively. Damage analysis results revealed that, for each impact energy level, matrix compression damage was dominant in CFRP composites and minor in multidirectional ones, while the opposite trend was observed for fiber tensile damage.
