## 4 Microencapsulation Techniques
### Chemical Process
#### 4.1.3 Microencapsulation by Interfacial Polycondensation  
The microencapsulation of phase change materials by interfacial polycondensation or polyaddition consists of their solubilization in the dispersed phase containing solvent and multifunctional monomers before emulsification in the continuous phase containing emulsifiers and stabilizers (Figure 7). The primary shell is formed through the precipitation of the insoluble polymeric materials on the droplet's surface after adding a complementary monomer in the continuous medium. In most cases, the membrane growth in the organic side and is achieved by crosslinking reactions. The aggregation of particles during encapsulation is one of the main drawbacks of this method. To overcome this shortcoming, many parameters should be studied, such as stirring speed during the formation of droplets and during the addition of the second monomer, time and temperature of encapsulation step, and the influence of catalysts. It was reported that the polymerization process could be improved by increasing the temperature and time of encapsulation and adding catalysts. Moreover, the decreasing stirring rate before the addition of the second monomer suppresses the aggregation of particles. However, it was also necessary to deposit the initial coating layers at a higher stirring speed to avoid coalescence and the formation of unstable microcapsules [122]. Polyurea is commonly synthesized in encapsulation by interfacial polycondensation due to its excellent mechanical properties [123] but poor thermal stability and compactness [124; 125]. Cyclohexane, as a solvent, is employed to promote the diffusion of monomers from the oily phase to the interface. However, a high proportion of solvent in the dispersed phase decreases the PCM's loading content in microcapsules and the encapsulation efficiency. Thus, [PERSON] et al. preconized using a small volume of cyclohex  
Figure 7: Schematic representation of the microencapsulation process by interfacial polycondensation. ane and a weight ratio PCM to cyclohexane about 6 to ensure the formation of the shell material [122]. The double-layered shell polyurea/polyurethane was employed in the encapsulation of _n_-octadecane by [PERSON] et al. to improve the thermal stability and compactness of the microcapsules [126]. The formation of the shell requires the use of polyisocyanate, aromatic and aliphatic monomers. The use of methylene diphenyl diisocyanate (MDI) or toluene diisocyanate (TDI) leads to the formation of an undesirable phenylamine compound [127], whereas this is not obtained with isophoronediiscocyanate (IPDI) [128]. The use of a solvent allows adapting the physicochemical properties of the dispersed phase to ensure the formation of a stable emulsion and optimize the chemical reaction at the interface. Nevertheless, in the frame of development of the microencapsulation process without organic solvent, some studies were realized to adjust the formulation of the PCMs to ensure the total miscibility of the isocyanate monomer in this phase. Therefore, the PCMs formulation was based on fatty acid. For instance, butyl stearate was used as co-PCMs to paraffin to allow the miscibility of TDI [129] or IPDI [130] for the formation of polyurea/polyurethane double shell or polyurea microcapsules. Furthermore, using aliphatic isocyanate allows controlling the polycondensation rate, obtaining a uniform and compact microcapsule shell. [PERSON] et al. used ethyl palmitate to solubilize IPDI and tetraethylorthosilicate (TEOS), at 30 \({}^{\circ}\)C, as monomer or precursor for the shell formation through interfacial polycondensation [131]. The presence of TEOS led to the formation of a polyurea-polyurethane-SiO\({}_{2}\) shell, allowing to enhance the thermal stability of the mPCMs. [PERSON] et al. studied the encapsulation of fatty acid ester as PCM material to justify that the solvent-free process was more straightforward and effective [132]. The formation of polyurea membranes by interfacial polycondensation is not limited to the use of amine monomers.