**4. Self-healing materials**

To better understand the basis of self-healing materials, it would be better to define self-healing meaning first. Self-healing materials can heal (recover/repair) internal damages automatically and autonomously. Definitely, it is a truly amazing characteristic that can fill not only the cracks but also small pinholes. The different types of materials including polymers, metals, and ceramics can have the ability to self-healing with their own self-healing mechanisms [19]. In this chapter, the focus is on self-healing polymers and their mechanisms. Different strategies can be considered to design self-healing polymers, such as the release of healing agents, reversible cross-links, and miscellaneous technologies, including electrohydrodynamics, shape memory effect, and nanoparticle migration. In this section, the different strategies are discussed.

#### **4.1 Release of healing agents**

The release of healing agents is one of the most covenant strategies for designing self-healing materials. This is accomplished by loading one or more healing agents, including monomers, dyes, catalysts, and hardeners in a container such as microcapsules, hollow fibers, and microvascular. The containers are then embedded into a polymeric matrix, which can release the healing agents by stimulation, such as physical damage or even pH alteration [20]. **Figure 7** depicts a schematic of the healing process using the release of healing agents from containers.

#### *4.1.1 Microencapsulation*

In the encapsulation process, self-healing microcapsules are synthesized by loading a self-healing agent (solids, liquid, or gases) in an inert shell which in turn isolates and protects them from the external environments. Therefore, self-healing microcapsules are made of two parts, the core (healing agent) and the shell, which may vary in shape (spherical or irregular shape) and size (from nano to micro). **Figure 8** shows a schematic of a microcapsule structure. Generally, microcapsules have a diameter of 3–800 μm, and consist of 10–90 wt% core materials [23]. Different types of materials can be used as the healing agent and shell. Core materials or healing agents are selected based on the application of healing material such as stabilization against environmental degradation, improvement of longtime efficiency, maintenance of non-toxicity of degradation products, and easy handling through solidification of liquid core [24]. Some of the most common materials used in self-healing coatings are cerium nitrate,

#### **Figure 7.**

 *A schematic of healing process using release of healing agents from containers [ 21 ].* 

 **Figure 8.**  *A schematic of a microcapsule structure [ 22 ].* 

dodecylamine, polyethyleneimine, linseed oil, and sodium alginate. The shell material can be various from traditional organic polymers to novel inorganic materials. Usually, core materials are loaded in poly(Urea formaldehyde) (PUF), poly(melamine formaldehyde), cellulose nanofibers (CNF), halloysite nanotubes, etc. **Table 1** listed some conventional materials used in self-healing coating for different substrates.

 Interfacial polymerization and in-situ polymerization are the most common methods for the synthesis of microcapsules, which are defined as follows [ 34 ]:

**Interfacial polymerization:** interfacial polymerization is an encapsulation procedure that mainly developed in the late 1960s [ 35 ]. This procedure consists of four main steps and two sets of monomers. One monomer is soluble in the oil phase and the other one is soluble in the water phase. At first monomer A (soluble in the water phase) is dissolved in the water phase. Secondly, monomer B (soluble in the oil phase) is dissolved in the oil phase. Then the oil phase is introduced into the water phase and emulsification is carried


#### **Table 1.**

*Some conventional materials used in self-healing coating for different substrates.*

out under constant stirring. Finally, the polymerization reaction takes place through a chemical reaction between the monomers A and B which is initiated by changes in pH (acids or bases) and can be accelerated by the use of catalysts. The polymerization reaction leads to the form of a polymer film at the interface of monomers and the polymer formed is deposited around the drops which leads to encapsulation [35]. The encapsulation efficiency is enhanced when a low-molecular-weight shell material is used, due to the higher mobility of the small molecules. However, this reduces the shell's strength [36]. In addition, stirring speed is manipulated to control the size of the microcapsules. The higher the speed of the stirrer, the smaller the microcapsules are formed.

**In-situ polymerization:** encapsulation via in-situ polymerization technique is very similar to interfacial polymerization. The difference is that there are no reactants in the core material in *in situ* polymerization. In fact, in this process, polymerization takes place in the continuous phase, rather than in the interface between the continuous phase and the core material. In-situ polymerization also includes four steps. First, the core material is dispersed in the water phase. In the next step, the shell material is introduced into the water phase. Typically, this includes monomers that react in continuous phase and form a polymer. Then the pH is reduced by the addition of acid to initiate the polymerization reaction. Finally, the polymer film formed covers droplets. The size of the microcapsules is controlled by the agitator speed, so that the higher the speed of the stirrer, the smaller the microcapsules are achieved [37].

#### *4.1.2 Hollow fiber embedment*

The use of hollow fiber as a container for healing agents is another interesting method as agents can restore up to 97% of their initial flexural strength. In this method, the healing mechanism is similar to that of microcapsule-based methods with the difference that the healing agent is stored in hollow tubes or fibers until they are ruptured by damage [38]. **Figure 9** depicts a schematic of the self-healing concept using hollow fibers. Recently, fibers such as hollow glass fibers, hollow site nanotubes, titanium dioxide nanotubes, and polymeric fibers have been used as a container for self-healing applications [38]. In the hollow fibers approach, a healing agent and a curing agent (i.e., epoxy resin and its hardener) are loaded in separate hollow fibers that react together when both fibers are broken due to external damage.

**Figure 9.**

 *A schematic of self-healing concept using hollow fibers [ 34 ].* 

One of the most remarkable advantages of this self-healing method is releasing a large amount of healing agent because of the large size of containers that can be suitable for filling cracks or large holes. However, this can also be a limitation. Because the rupture of hollow fibers depletes the healing agent contained within it which leads to a limitation on the number of times that a damaged region can be healed. On the other hand, this self-healing system faces a significant drawback which is the development of a practical technique for filling the hollow fibers with the liquid healing agent [ 39 ].
