## 4 Microencapsulation Techniques
### Chemical Process
#### 4.1.2 Microencapsulation by In Situ Polymerization  
Encapsulation by in situ polymerization consists of the presence of the monomer or prepolymer in a single phase (continuous or dispersed phase) rather than in two phases, as in interfacial polymerization. The formation of coating materials occurs when the monomers or the pre-condensates diffuse at the interface of the two immiscible phases. An amino resin such as urea-formaldehyde (UF) or melamine-formaldehyde (MF) is the most common shell material used in this method. The polycondensation of the amino resin occurs in the continuous phase; the separation phase is controlled by adjusting the pH and the modification of melamine/formaldehyde or urea/formaldehyde ratios [87]. The encapsulation of PCMs carried out by in situ polymerization is based on three steps, i.e., (i) formation of tiny PCM droplets into the water phase with the adding of UF or MF under acidic conditions; (ii) primary shell formation by the self-deposition of amino resins onto the PCMs droplets; and (iii) crosslinking of the amino shell due to the increase in the amino resin concentration at the interface. The shell formation mechanism is based on the activation of the pre-polymers, yielding etherification reactions. Hence, the water solubility of pre-polymers decreases leading to the separation of materials from the continuous phase. The obtained materials are collected after filtration and drying [103]. In situ polymerization microencapsulation process using melamine-formaldehyde is described in Figure 6.  
Several studies investigated the optimization of process parameters to reduce free formaldehyde content. For example, [PERSON] and [PERSON] encapsulated _n_-octadecane by MF and analyzed the influence of the pH value, temperature, and MF dropping rate on the particle morphology and encapsulation yield [104]. Thus, they observed that well-defined microcapsules with the highest encapsulation efficiency were obtained at a moderate pH (between 4 and 8), at 75 \({}^{\circ}\)C, and with a medium dropping rate of MF (0.75 mL\(\cdot\)min\({}^{-1}\)). [PERSON] et al. encapsulated _n_-tetracosane by MF by switching the pH of the emulsion from acidic (pH = 4) to alkaline (pH = 9) to complete the polycondensation reactions to control the encapsulation efficiency [105]. The influence of the type and amount of surfactant in the formation of mPCMs was investigated by [PERSON] and al. [106]. They selected _n_-hexadecanol, MF, and anionic styrene-maleic copolymer (SMA) as core, shell, and surfactant materials. NaOH was used to enhance the solubility of SMA in water. They reported that 8% SMA to the core  
Figure 6: Schematic representation of the microencapsulation process by in situ polymerization [39]. material and a weight ratio of 3.3:10 between shell and core material are the optimal polymerization conditions to obtain spherical mPCMs with smooth surface and high encapsulation efficiency. [PERSON] et al. encapsulated 1-dodecanol by MF shell [107]. They noticed that the encapsulation efficiency was lower than other studies, probably due to nonionic emulsifiers (Tween(r) 20 and Span(r) 60) and the pH value in the range of 5-6. MF resin has several advantages, such as high prepolymer reactivity and short-time reaction, and a high weight ratio between the core and the shell compared to UF resins [108]. However, the strong tendency of MF to self-polymerization due to its high solubility leads to the formation of mPCMs with poor morphology. The washing and filtration steps are more difficult and toxic due to the presence of free formaldehyde. Modified MF resin by hydroxyl-terminated polydimethylsiloxane (HTPDMS) [109], carboxymethyl cellulose (CMC) [110], resorcinol [111] were used as an alternative to traditional MF resin. In addition, melamine-urea-formaldehyde(MUF) copolymers were used to enhance mPCMs stability. [PERSON] et al. encapsulated decanoic acid with Poly(UF) (PUF), Poly(MF) (PMF), and Poly(MUF) (PMUF) as shells [112].