The preparation and characterization of double-layer microcapsules used for the self-healing of resin matrix composites

In this work, diaminodiphenyl sulfone (DDS) was used as a curing agent adsorbed at the periphery of single-layer microcapsules with Ag-80 as the healing agent within melamine-formaldehyde resin (MF). The double-layer microcapsules were made by encapsulating the particulates (the single-layer microcapsules decorated with DDS) with MF via in situ polymerization. These double-layer microcapsules simultaneously release healing agent and curing agent upon the fracture of the surrounding matrix, which increases the rate of contact between the two agents, and thus improves the healing efficiency. DDS adsorption plays an important role in the formation and final morphology of the double-layer microcapsules. When using a compound dispersant consisting of 30% Tween 80 and 70% Span 80, the particulates of dispersed DDS were homogeneous and fine. When the pH value was between 3.5 and 4.5, the dispersed DDS and MF (Ag-80) microcapsules had opposite surface charges, and tended to come together due to electrostatic interactions. The morphologies of the microcapsules were observed by scanning electron microscopy (SEM). The co-existence and stability of the healing agent and the curing agent before capsule failure were demonstrated by infrared spectrometry (FTIR). The mechanical properties of the double-layer microcapsules were investigated by nanoindentation. The results indicated that the microcapsules fractured earlier than the matrix to repair a crack, and the mechanical properties of the cured microcapsules were similar to those of the resin matrix.