Self-repairing hollow-fiber polymer composites
Introduction
The concept of using self-repairing hollow fibers in polymer composites was a major breakthrough leading to many revolutionary findings in the field of smart self-healing materials [13]. The first reports on self-healing in thermoplastics and cross-linked systems were in 1969, 1979, and 1981 [46]. Since then, many polymer composites with excellent self-healing capabilities have been developed [7].Several self-healing approaches for self-healing materials have been developed to date and there is expected to be huge demand in the fields of construction, electronics, and aviation [8,9]. The first studies reported on self-healing polymer composites were based on self-repairing hollow glass fibers (HGFs) [10,11]. The advantages of self-healing polymer composites include a reduction in the cost of maintenance and improved lifespan [12]. Several matrix materials such as polyester, polyimide, polyurethane, silicone rubber, and epoxy resins have been considered for developing self-healing composites. This is due to their low density, excellent chemical inertness, low cost, and versatility in fabrication methods. In addition, such materials exhibit better adhesion of substrates [13]. There are several key parameters in determining polymers as healing agents for self-healing applications. Some of these parameters include viscosity, gelling time, enthalpy of reaction, cross-linking temperature, stoichiometric ratio of hardeners, etc. [14]. In the past, most researchers were focused on self-healing materials based on microencapsulation of healing agents in polymeric containers. Urea formaldehyde and melamine formaldehyde are commonly used polymeric microcapsules along with catalyst/hardener dispersed in matrix [15,16]. However, polymeric microcapsules may adversely affect the mechanical property and may have limited healing capabilities. Furthermore, the polymeric microcapsules left in the polymer matrix after one-time healing may deteriorate the mechanical performance of the composites. Also, such a technique follows interfacial polymerization at the interface of healing agents and monomer solutions to produce microcapsules. This may not be suitable for very highly reactive healing agents for encapsulation due to their possible interaction with the polymerization process [17,18]. Thus, an alternate method to encapsulate healing agents that provides structural integrity as well as functional ability to the composites is preferred. HGFs are an excellent alternative for microcapsules because of their mechanical stability and thermal properties. Also, hollow fibers are able to store more healing agents without compromising the reinforcement and are easily integrated into the matrix with less restriction of the activation method and healing chemistry of healing agents [19,20]. Recently, fibers such as HGFs, halloysite nanotubes (HNTs), titanium dioxide nanotubes (TNTs), and polymeric fibers have gained a great deal of acceptance in designing the selfrepairing composites for structural applications. When damage is triggered by an external stimulus, the fiber breaks, this allows the healing agents to flow into the crack site, and it undergoes polymerization in the presence of a hardener or catalyst present at the crack site. In the following sections, self-repair polymer composites encapsulated with HGFs, hollow nanofibers, and hollow polymeric fibers are discussed.
