Coaxial Electrospinning of Self-Healing Coatings

2010 WILEY-VCH Verlag Gm The concept of autonomic materials, such as materials which self-repair without external intervention, is a promising alternative to damage tolerant design. Such materials have the ability to repair themselves without external intervention and recover their functionalities by using the resources inherently embedded within or available to them in the local environment. This is analogous to the biological healing process in living organisms where damage to organs and tissues can be repaired by cellular activity fueled by the nutrients available in the circulatory system. An early example of a synthetic system which self-repaired was demonstrated in 2001; this system consisted of a structural bulk epoxy which contains a microencapsulated healing agent and a suspended solid-phase catalyst. Subsequently, many different types of self-healing systems have been developed including those based on microcapsule dispersions, reversible chemistries, particle segregation, microvascular networks, and hollow fibers. Self-healing coatings are a specific subfield of autonomic materials of particular commercial and scientific interest. There is substantial interest in coating systems that incorporate self-healing technologies to provide autonomic protection of the underlying substrate from environmental exposure after a damage event. Recently, Kumar et al. incorporated microencapsulated tung oil and spar varnish into a commercially available epoxy primer. When a coating containing these microcapsules was damaged, the capsule contents were released into the damaged region and some level of healing was observed. Cho et al. employed a microcapsule-based system to formulate a siloxane-based self-healing coating that demonstrated self-repair and complete protection of the underlying substrate from corrosion, even when damaged under aqueous conditions. In this work, oligomeric reactive siloxanes and a tin catalyst were individually encapsulated and incorporated into various epoxybased coating systems, which were subsequently coated onto cold-rolled steel. Damage to the coating released the siloxane and catalyst into the damaged region; the siloxane then cured, protecting the steel from corrosion. Here, we propose a new approach to self-healing polymer coating systems based on an electrospun coaxial healing agent and demonstrate the effectiveness of such an approach to add autonomic functionality to polymer coatings. Using this approach, liquid materials, such as a healing agent(s) or catalyst–solvent mixture(s), can be encapsulated into a core–shell bead-on-stringmorphology and electrospun onto a substrate. One major advantage of this process is that it utilizes purely physical forces to form the core/sheath structure, thus overcoming the rather serious limitations exhibited by methods that require a minimum emulsion stability for chemical reactions to take place and hold the capsule together. This feature makes it suitable for processing a broader variety of materials for self-healing. Another advantage of this simple one-step coaxial electrospinning encapsulation method is the inherent flexibility in controlling the diameter of the microcapsules and connecting ligaments from the microto nanometer scales. Finally, electrospinning is both a high throughput and selective area technique. Thus, the self-healing functionality can be selectively added to large area substrates in a continuous process under rather mild conditions. Figure 1a presents a schematic illustration of the system for electrospinning the self-healing coating system. It is similar to a conventional electrospinning setup except for the use of a spinneret containing two coaxial capillaries. In a typical procedure, two viscous liquids are simultaneously fed through the inner and outer capillaries, respectively. If the proper combination of liquids and operation conditions are satisfied, a layered Taylor cone can be developed and a coaxial jet can be formed when a high voltage is applied to the outer metallic capillary. The electro-hydro-dynamic forces smoothly stretch the fluid interface to generate coaxial fibers due to the electrostatic repulsion between the accumulated surface charges. A two-part healing agent system was electrospun in two steps onto various substrates, a steel substrate being the most useful experimentally. The electrospun fibers, randomly oriented on the surface, contained either part A or part B liquid polysiloxane precursors (Dow Corning 3-6575) encapsulated in a poly(vinylpyrrolidone) (PVP) sheath. As shown in Figure 2, the liquid healing agent is completely encapsulated in beads that are randomly distributed along polymer nanofibers. The as-spun beads exhibit a broad distribution from 2 to 10mm (Fig. 2a). In this study, a low viscosity, two-part polysiloxane system was chosen as for self-healing chemistry due to its physical stability over a wide temperature range ( 45–150 8C), fast cure at room temperature (5–24 h) and low viscosity (750 cP), which is important for electrospinning. The capsules were quite susceptible to mechanical damage as can be observed in Figure 2b, where an electrospun capsule mat was scribed with a razor blade. This is important; successful self-healing requires the capsules to rupture upon a damage event. To optically determine the presence of part A and part B siloxane precursors in the self-healing structure, fluorescent dyes were added to the encapsulated healing agents and imaged (Fig. 2c). This enabled visualization that appropriate quantities of the part A and part B system were electrospun, and that they were uniformly distributed across the substrate. The red (Rhodamine B) and green (Coumarin 6) regions correspond to encapsulated part A