Reactivity and Release of Isocyanates from Microcapsules Used in Self-Healing Materials

Synthetic self-healing polymeric materials that can recover from physical or chemical damage autonomously have been explored in the past 20 years with the aim of reducing the environmental burden of polymers while exceeding the capabilities of current synthetic polymers. These smart materials contain an embedded healing agent that is exposed when the material is damaged and reacts with the outside environment to solidify in place, covering the damaged area and restoring the polymer’s properties. In this study, we investigated the reaction and release of a diisocyanate healing agent which is designed for underwater applications. Our results show that micellar and encapsulated isophorone diisocyanate (∼100 μm diameter) remain unreacted in water for at least 8 h. While this reaction rate indicates a potential for isocyanates to be released into the environment, no isocyanate was detected in flow through experiments with crushed microcapsules. Instead, the results suggest that polyurea, the product of the isocyanate and water reaction, can be released in the form of microparticles that are less than 25 μm in diameter. This study shows that the assumption of fast reaction kinetics and minimal chance of release into the environment of healing agent chemicals needs to be reconsidered.