## 4 Microencapsulation Techniques
### Chemical Process
#### 4.1.2 Microencapsulation by In Situ Polymerization
The microcapsules using PUF present a higher thermal storage capacity, but they were easily broken at around 95 \({}^{\circ}\)C due to their thin shell. In addition, mPCMs/PMF present low-capacity storage. The higher encapsulation ratio was attributed to the mPCMs with PMUF and was about 53%. The mean diameter and the dispersion stability were also affected by the type of surfactant and the use of cosurfactants (Tween(r) 40 and Tween(r) 80). [PERSON] et al. investigated the influence of surfactants on the synthesis of mPCMs [113]. SMA, Tween(r) 20, and polyethylene glycol p-isocotylphenyl ether (OP-10) were used as emulsifiers agents. They indicated that SMA as an anionic emulsifier is more suitable for preparing MUF mPCMs, leading to regular spherical mPCMs. In addition, the phase change enthalpy increases with the increase in the SMA amount. Methyl/butanol etherified MF prepolymer was identified as an excellent modifier providing a relatively simple and effective way to prepare mPCMs. _n_-dodecanol was encapsulated by [PERSON] et al. using a mixture of methanol-modified MF (MMF) and traditional MF as shell materials using SMA as an emulsifier [114]. The influences of operating parameters such as PCMs/shell ratio, MF/MMF ratio, amount of surfactant, rate, and agitation time were investigated. They concluded that more stable spherical microcapsules were obtained with minor deformation within a more extended growth period due to the increasing amount of self-polymerized nanoparticles. Hence, complete in situ shell formation was achieved within 60 min under 70 \({}^{\circ}\)C. They observed that the size distribution of mPCMs depends on SMA concentration and emulsifying stirring speed. Moreover, decreasing the core/shell ratio results in an excessive amount of shell material adsorbed onto the surface of the microcapsules. In addition, the degree of aggregation and absorption of self-polymerized was reduced by decreasing MF/MMF ratio. In other studies, MF was etherified by methanol or/and _n_-butanol to reduce the number of amino groups, allowing its reactive activity and polymerization control. [PERSON] et al. employed methylated melamine-formaldehyde (MMF) and butylated melamine-formaldehyde (BMF) pre-polymers as a hybrid shell to encapsulate _n_-dodecanol [115]. They mentioned that the morphology of mPCMs with MMF/BMF was smoother than microcapsules with MF and MMF ones. They related this observation to the high solubility of the MF and MMF pre-polymers in the aqueous phase, leading to an increase in the self-polymerized nanoparticles deposited on the surface of microcapsules. The 1:1 mass ratio of MMF/BMF was considered as the optimum condition. The heat storage capacity and the encapsulation efficiency increased with the increase in the MMF/BMF ratio. The amino resin was also modified with high thermal resistance materials, such as graphene oxide [116], silver [117], and metal oxide, to improve the heat transfer and thermal stability. Thus, [PERSON] et al. used metal oxide (Fe\({}_{2}\)O\({}_{3}\), ZnO, and TiO\({}_{2}\)) to develop a modified inorganic-organic hybrid UF shell to encapsulate paraffin. They observed that the thermal performances were improved, and the decomposition of UF polymer was delayed to a higher temperature level [118]. [PERSON] et al. indicated that MF modified by cellulose nanofibers (CNFs) prevents breakage during application [119]. They mentioned that only 3.4 wt% of CNFs is required to improve the mechanical strength of the MF coating. Moreover, the encapsulation efficiency increased with the increase in the amount of CNFs. Therefore, the emulsion stage governs the average diameter and size distribution of the particles [120]. The use of an SMA improves the stability of the emulsion rather than a nonionic surfactant. Shell formation and particle surface morphology are related to pH adjustment, preferably around 4, with a low prepolymer drop rate. Controlling these parameters can suppress particle flocculation to achieve a smooth surface [121].