*5.1.1 Self-healing based on defect-filling effect*

In this type of healing mechanism, the stored polymerizable healing agent releases due to generating a defect caused by mechanical damage. These materials can polymerize into a film by reacting with hardener or even with moisture or oxygen in the environment and can fill the defect. One of the classic examples of this type of healing mechanism was reported by White and Sottos [32] in 2001. In their proposed system, healing was accomplished by incorporating microencapsulated dicyclopentadiene in the PUF shell within an epoxy matrix. The average healing efficiency was reported 60% of the original fracture load. This method was further developed in the following years to be used for various metal substrates. **Table 2** listed several autonomous self-healing coatings based on defect-filling effects for different applications.


#### **Table 2.**

*Several autonomous self-healing coatings based on defect-filling effects for different applications.*

#### *5.1.2 Self-healing based on corrosion inhibitors*

The embedding of corrosion inhibitors in coatings is another mechanism of autonomous self-healing systems. Some of the most widely used corrosion inhibitors in autonomous self-healing coating are nitrites, phosphates, vanadates, molybdates, tungstates, borates, mercaptobenzothiazole, benzotriazole, imidazoline, 8-hydroxyquinoline, and aliphatic amines [18]. In this type of self-healing coatings, the healing process is accomplished by anodic dissolution and cathodic reactions which leads to corrosion inhibition. The conceptual design of inhibitor-based coatings is simple and this can be achieved by adding corrosion inhibitors directly to a polymeric matrix. In the early studies, the addition of doping nitrates and phosphates of cerium into organic coatings was proposed to prevent the corrosion of zinc, galvanized steel, and aluminum alloy [53–56]. This method is subject to problems such as poor compatibility between the organic coating resins and particle agglomeration. However, recently, the approach of encapsulating inhibitor agents has received more attention because in this way inhibitors can be released in a stable and controlled manner [57]. **Table 3** listed a number of self-healing based on corrosion inhibitors for different applications.

#### **5.2 Non-autonomous healing mechanisms**

In non-autonomous systems, healing effects accomplish by an external stimuli, such as heat and light, which trigger the chemical reactions or physical transitions necessary for bond formation or molecular chain movement. In fact, this type of coating are healed by recovering the intrinsic chemical bonds and/or physical configurations of the polymer networks in the coating matrices. The external stimulus provides the activation energy required for bond breakage/reformation. For example a heat stimulus can enhance the reactions of the broken bonds by bringing them


#### **Table 3.**

*A number of self-healing based on corrosion inhibitors for different application.*

closer together. The most common light stimulus are sunlight, near infrared (NIR) light, and UV light. But heat sources can be artificially applied (e.g., by a heat gun) or generated from the service environments (e.g., sunlight, abrasion) [18]. Nonautonomous healing polymers have vast applications in healthcare, aerospace, construction and electronics industries. Thus, a comprehensive review of these materials beyond the scope of this chapter and only some systems that are more closely related to protective coatings are discussed in the following section.

### *5.2.1 Non-autonomous self-healing based on dynamic bonds*

The dynamic bonds refer to reversible break and reform bonds which allow a continuous modification of the constitution by reorganization and exchange of building blocks [64]. For instance, at a certain temperature thermally reversible bonds can decompose, which allows the polymer chains to flow to the defect and re-crosslink to repair the defect [65]. Diels-Alder (DA) reaction is one of the most prominent examples of this healing system. A recent work by Chuo et al. [66] demonstrated the possibility of a tetra-functional furan-capped aniline trimer, a trifunctional maleimide, and a trifunctional to prevent metal corrosion. The corrosion current density in the polarization curve was used to evaluate the corrosion protection efficiency of the scratched coating. The results of a cycle test showed that the proposed system could recover protection efficiency from 79.8% to 99.2%. Some other researchers reported self-healing effects by using light stimuli. For example, UV-sensitive self-healing polymers have been developed based on reversible photo-crosslinking reactions [67], or near-infrared light-triggered self-healing ability was used for biocomposites [68]. Light-responsive self-healing polymers have several major advantages over thermally induced self-healing systems, e.g., the healing can be triggered instantaneously, remotely, and on demand. In fact, unlike heat-stimuli systems where all surfaces are exposed, in light-responsive systems, the light stimulus can be applied exactly to the damaged area, which leads to a reduction in side reactions and degradation in the intact coating during the healing process [18].

#### *5.2.2 Non-autonomous self-healing based on shape memory polymers (SMPs)*

Shape memory materials are a kind of smart materials that can recover their original shape from a deformed state by applying external stimuli such as heat or light. Both polymers and alloys can perform shape memory behaviors with different mechanisms. In shape memory alloys (SMAs), such as NiTi-based, Cu-based (CuAlNi and CuZnAl), and Fe-based alloys, the shape memory effects are governed by the phase transformation among twinned martensite, detwinned martensite, and austenite [69]. In shape memory polymers (SMPs), the shape memory effects are usually due to the viscoelastic transformation of polymer chains when cycled through a thermal transition temperature, such as a glass transition temperature (Tg) or a melting temperature (Tm) [70]. From the engineering point of view, tailoring the properties is much easier for polymers than metals/alloys. Polymers have traditionally lower material prices and processing costs [71]. Furthermore, SMPs have lower weight, easy fabrication methods, and higher elasticity. Hence, they have been proposed for biomedical [72], aerospace [73], and many other applications.
