Characterization of solvent-filled polyurethane/urea–formaldehyde core–shell composites

•An analytical model was used to extract Young's moduli from core–shell structures.•The thickness of UF shells was independent of capsule diameter.•PU shell thicknesses scaled with capsule diameter until a saturation thickness.•Compared to UF shells, PU/UF shells had a five-fold lower Young's modulus.•PU shells resisted well due to increased thickness and elongation at break.

Encapsulation of liquid phases is a crucial step in many self-healing material systems where a healing agent has to be protected during processing and then released during a damage event. In this work, the mechanical properties of polyurethane (PU) reinforced urea–formaldehyde (UF) shells are characterized. It was found that shell thickness is both a function of PU content in the core phase and of the microcapsule diameter. Furthermore, a saturation thickness was found for high PU contents or high capsule diameters and this phenomenon had direct implications on the bursting force under compression of single microcapsules. With help of an analytical model, the Young's modulus of the hybrid PU/UF was determined and in general, PU-reinforced shells had a lower modulus but higher ductility in terms of elongation at break, leading to more resistant microcapsules overall.