Application of microencapsulated unsaturated polyester toward temperature-triggered healing in epoxy composites

In this article, we demonstrate the potential of encapsulated unsaturated polyester resin toward introduction of temperature-triggered healing functionality in a representative cycloaliphatic epoxy matrix. Unsaturated polyester resin was encapsulated in poly(urea–formaldehyde) shell by dispersion polymerization technique which resulted in the formation of free-flowing microcapsules (diameter ∼130 ± 49 µm) with a core content 58 ± 4%. Calorimetric studies confirmed the chemical activity of the encapsulated unsaturated polyester resin, which spontaneously polymerized in the presence of a free radical initiator, 2,2′-azobis(2-methylpropionitrile), at temperature as low as 80°C. Temperature-triggered healing of epoxy-microcapsule composites was performed at 110°C and the healing efficiency was quantified as the ratio of impact strength of healed and virgin specimens. The healing efficiency was found to increase with the increasing amount of microcapsule in the formulation and reached a maximum (100 ± 2%) at 20% (w/w) loading. Fractographic analysis of the surface revealed the flow pattern of chemically active resin from the ruptured microcapsules, which subsequently cured in the presence of 2,2′-azobis(2-methylpropionitrile) pre-dispersed in the matrix.