Effect of aging on the performance of solvent-based self-healing materials
Solvent healing of thermoset polymer matrices was suggested as an efficient and cost-effective alternative to other encapsulated self-healing systems. However, self-healing efficiency is not guaranteed over time. We report here on the autonomic self-healing of under-cured epoxy with ethyl phenylacetate-filled microcapsules under different aging conditions. We carried out quasi-static fracture tests and observed a decrease of healing efficiency from 77% for fresh samples down to 13% for samples aged at room temperature for 77 days. DSC results of pure epoxy matrix curing showed that the residual heat of reaction only decreased from 161 J g(-1) to about 135 J g(-1); this decrease was largely due to water absorption and could only partly explain the large decrease in healing efficiency. Furthermore, FTIR spectra did not indicate a change in conversion as observed by monitoring the associated oxirane peak in the pure epoxy matrix over time, confirming the above findings. While the microencapsulated solvent has good shelf life properties, evidence was found that solvent diffusion into the epoxy matrix prior to cracking was responsible for the reduced healing capability. DSC and FTIR results of aged self-healing epoxy confirmed the presence of the EPA solvent in the matrix even though no damage was introduced to the microcapsules. Therefore two phenomena responsible for the decreased solvent-healing capability in under-cured epoxy were identified: ambient moisture uptake and premature solvent diffusion through the microcapsule walls.