Since White et al. first reported a demonstration of self-healing functionality in a polymer system1 it has become a growing field of research and technology development with a diverse set of technologies adding new functionality to polymeric materials. This article provides an overview of the classes of technologies that have been developed and their ongoing and/or potential uses in the design of coating systems.
Research on healing in polymeric materials goes back to the late 1970s and early 1980s when Wool and O’Connor extensively studied semicrystalline polymers, amorphous glassy polymers, block co-polymers, elastomers and fiber-reinforced composites at temperatures ranging from 50 ºC below glass transition temperature (Tg) up to 100 ºC. In these studies, healing after damage in the form of a crack in the polymer was facilitated by heating above the glass transition temperature.2-4 Wool later provided a framework describing the stages of healing,5 which when applied to his earlier work suggests heating above Tg provided the energy for (1) molecular rearrangement on opposing crack faces, (2) approach of the crack faces towards each other, (3) wetting, which describes polymer chains on opposing crack faces coming in contact with each other, (4) diffusion through discrete volumes occupied by polymer chains on opposing crack faces, and (5) diffusion to an equilibrium distance and randomization. A schematic of these stages is provided in Figure 1. From the late 1980s into the 1990s, researchers began using solvents to initiate the stages of healing described above by swelling the polymers.5 It became clear that whether through the application of heat or a solvent, mobility within a polymeric material was essential to initiate a healing response.