5.1.1. Encapsulated Type Self-Healing

Corrosion inhibitors can be easily embedded into the capsules through variety of techniques. These techniques can be merged into two main categories; physical and chemical (Figure 11) [1]. There are different chemical approaches for synthesizing the microcapsules such as interfacial polymerization [110], coacervation, in situ polymerization [111,112], extrusion, and sol-gel methods. The fastest and most convenient method among them is in situ polymerization. In this approach, microcapsules containing healing agent (inhibitor) in a trapped state disperses uniformly in the matrix containing a catalyst capable of polymerizing the healing agent and fracture upon loading of the coating, releasing the low viscosity self-healing reagents to the damaged area for curing and filling of the micro cracks (Figure 11) [113,114].

Brown et al. [112] are known as a pioneer of the micro-/nanocapsules synthesis with their achievement of micro capsulation of dicyclopentadiene (DCPD) as a healing agent with urea–formaldehyde (UF) shell using in situ polymerization. Most commonly used healing agents as a core are dicyclopentadiene (DCPD), epoxy, linseed oil, tung oil, o-dichlorobenzene and dimethyl siloxane. Shell materials are mainly limited to poly(urea–formaldehyde) (PUF) and poly(melamine–formaldehyde) (PMF) or melamine modified poly(urea–formaldehyde) (MUF) [114]. Table 1 summarises recent work that has been carried out in microcapsulation based self-healing system for coating applications.

**Figure 11.** Encapsulated type self-healing through in situ polymerization technique.

Table 2 is the summary of characteristics which will be required for designing microencapsulationbased self-healing polymeric materials [115]. It is crucial to consider four steps process of achieving healing ability to obtain better functionality: storage, release, transport, and rebonding. Each of these steps depends significantly on the chemistry and properties of the healing agent system [107]. Table 2 reveals the importance to develop capsules with good compatibility with the coating matrix and considering the possibility to encapsulate and upkeep active material, and control of the permeability properties of the shell through external stimuli. Shell permeability could be changed reversibly or irreversibly by various stimuli: variation of the pH, ionic strength, temperature, ultrasonic treatment, alternating magnetic field and electromagnetic irradiation.

As a result, different responses can then be observed, such as tuneable permeability or more drastic ones like total rupture of the container shell. Also as it has been shown in this table, size of the capsules is another important parameter which should be less than 300–400 nm; capsules of larger size can reduce the protective performance of the coating [107].


**Table 1.** Summary of most recent work in microcapsule based self-healing.

**Table 2.** Characteristics required for designing microencapsulation-based self-healing polymeric materials.

