**4. Technologies for microencapsulation of PCMs**

Microencapsulation techniques are of several types which are broadly classified into three major categories on the basis of fabrication mechanism: (1) physical methods (2) chemical methods and (3) physico-chemical methods. All these techniques involve the formation of a solid shell/coat around small liquid or solid particles of 1–100 μm diameter to accomplish the desired properties such as, protection competency, time-dependent release of material, provision of the substance to the particular target, minimize interaction with the environment, corrosion prevention, steadiness of function and to facilitate the use of toxic materials. The microencapsulation of PCMs is a special packaging methodology in which solid–liquid PCMs can be enclosed in some wall materials by using physical or chemical process to make small particles termed 'microcapsules' [44]. The PCM in a microcapsule is named as the core material while the outer shell which encloses the PCM from the surrounding environment is called the wall material. Microencapsulation as an emerging technology, commonly applied in many fields like thermal energy storage, medicine, food preservation, catalysis, dyes, textile, cosmetics, self-healing, coatings, engineering and defense [45]. A detailed classification of the microencapsulation methods is listed in **Figure 1**.

#### **4.1 Physical methods**

Physical methods involve involves physical processes, like drying, dehydration and adhesion in the formation of microcapsule shells. The most frequently used physical methods for PCMs encapsulation are spray-drying and solvent evaporation. The spray-drying process can be accomplished in the following steps: (1) preparation of oil–water emulsion comprising PCMs and shell materials, (2) spraying of the oil– water emulsion in a drying chamber via an atomizer, (3) drying of the sprayed droplets by using a stream of drying gas at a particular temperature, and (4) separating the solid particles by cyclone and filter [46]. Borreguero et al. employed a spray drying method for microencapsulation of paraffin Rubitherm®RT27 core using polyethylene EVA shell with and without carbon nanofibers (CNFs) [47]. The CNFs addition improved the thermal conductivity and mechanical strength of microcapsules, and the heat storage capability was retained. Also, the DSC analysis shown that even after the 3000- thermal charge/discharge cycles the microcapsules still had good thermal stability. Hawlader et al. synthesized spherical shape and uniform size microcapsule with paraffin core and gelatin and Arabic gum using spray-drying method [48]. The microcapsules prepared at the core-to-shell ratio of 2:1 have heat storage and release capacity reached 216.44 J/g and 221.217 J/g, respectively.

The solvent evaporation method includes: (1) preparation of polymer solution by dissolving shell materials in a volatile solvent; (2) addition of PCMs to the polymer solution to form O/W emulsion; (3) developing shells on the droplets by evaporating the solvent; (4) filtration and drying to obtaining the microcapsules. Lin et al. encapsulated myristic acid (MA) with ethyl cellulose (EC) using the solvent evaporation

*Design and Fabrication of Microencapsulated Phase Change Materials for Energy/Thermal… DOI: http://dx.doi.org/10.5772/intechopen.102806*

**Figure 1.**

*Major physical and chemical microencapsulation methods for solid–liquid PCMs.*

method [49]. The melting and solidifying temperatures were observed to be 53.32°C and 44.44°C, while the melting and solidifying enthalpies were found 122.61 J/g and 104.24 J/g, respectively. Wang et al. applied the solvent evaporation method to synthesize high-performance microcapsules by using sodium phosphate dodecahydrate (DSP) as the core and poly(methyl methacrylate) (PMMA) as the shell [50]. The optimal preparation temperature for the microencapsulation process was 80–90°C, reaction time 240 min, and stirring rate 900 rpm. The microcapsules obtained had an energy storage capacity of 142.9 J/g at the endothermic peak temperature of 51.5°C.

#### **4.2 Chemical methods**

In chemical methods, microencapsulation is done by the polymerization or condensation of monomers, oligomers, or prepolymers as raw materials to form shells at an oil–water interface. The chemical methods mostly involve in-situ polymerization, suspension polymerization, interfacial polymerization, and emulsion polymerization. The schematic diagrams of these four polymerization methods are shown in **Figure 2**. In situ polymerization method (**Figure 2(a)**), involves the formation of a shell on the surface of the droplet by polymerization of the prepolymers which can be accomplished in the following steps [52]: (1) preparation of the O/W emulsion by adding PCMs to surfactant aqueous solution; (2) preparation of a prepolymer solution; (3) addition of the prepolymer solution to the O/W emulsion, followed by adjusting the appropriate reaction conditions; and (4) microcapsule synthesis. Konuklu et al. successfully utilized in situ polymerization method for microencapsulation of decanoic acid using poly(urea-formaldehyde) (PUF), poly(melamineformaldehyde) (PMF), and poly(melamine urea-formaldehyde) (PMUF) [53]. The microcapsules obtained by coating of PUF displayed higher heat storage capacity

**Figure 2.**

*Schematic diagrams of chemical methods for PCMs microencapsulation: (a) in situ polymerization, (b) interfacial polymerization, (c) suspension polymerization, and (d) emulsion polymerization [51].*

but weaker mechanical strength and lower heat resistance, while the microcapsules coated with PMF shells had higher thermal stability but lower thermal energy storage capacity. However, the PMUF-encapsulated microcapsules possessed seamless thermal stability and no leakage was found at 95°C. Zhang et al. utilized in situ polycondensation method for synthesizing dual-functional microcapsules containing *n*-eicosane cores and ZrO2 shells [54]. The microcapsules synthesized have a spherical shape with a size of 1.5–2 *μ*m have good thermal energy storage and possessed better thermal stability, and thermal properties almost unchanged after 100 thermal cycles. Su et al. used methanol-modified melamine-formaldehyde (MMF) prepolymer as shell material for microencapsulation of dodecanol and paraffin via in situ polymerization [55]. They observed that the average diameter of dodecanolbased microcapsules sharply decreased and encapsulation efficiency increased with increasing stirring rates. The maximum encapsulation efficiency was found to be 97.4%.

Interfacial polymerization is used in the preparation of organic shell materials such as polyurea and polyurethane. In this method, two reactive monomers are separately

#### *Design and Fabrication of Microencapsulated Phase Change Materials for Energy/Thermal… DOI: http://dx.doi.org/10.5772/intechopen.102806*

dissolved in the oil phase and the aqueous phase, then in the presence of an initiator polymerization occurs at the oil–water interface as shown in **Figure 2(b)**. This method includes the following steps: (1) preparation of an O/W emulsion having hydrophobic monomer and PCMs; (2) addition of the hydrophilic monomer under proper conditions to initiate polymerization; (3) filtration, washing, and drying to get microcapsules. Ma et al. successfully used the interfacial polymerization method for microencapsulation of binary core materials like butyl stearate (BS) and paraffin with polyurea/polyurethane as the shell material [56]. The microcapsules phase change temperature was adjusted by changing the ratio of the two core materials. The microcapsules obtained possessed high thermal stability. Lu et al. encapsulated the butyl stearate core with a polyurethanebased cross-linked network shell via interfacial polymerization [57].

In the suspension polymerization method, the dispersed droplets of PCMs, monomers and initiators are suspended in a continuous aqueous phase by using surfactants and mechanical stirring. The oil-soluble initiator free radicals are then released into the emulsion system to initiate polymerization of the monomers at a suitable temperature and stirring rate [46], as presented in **Figure 2(c)**. Wang et al. successfully employed the suspension polymerization method to encapsulate *n*-octadecane with thermochromic pigment/PMMA shells at five different pigment/MMA ratios varying as 0, 1.4, 4.3, 7.1, and 14.3 wt.% [58]. It was observed that the microcapsules without pigment achieved the highest melting and crystallization enthalpies of 149.16 J/g and 152.55 J/g, respectively. Tang et al. prepared spherical shape microcapsules with an average diameter of about 1.60 *μ*m using *n*-octadecane core material and *n-*octadecyl methacrylate (ODMA) methacrylic acid (MAA) copolymer as shell material via the suspension polymerization method [59]. The microcapsules attained the highest phase change enthalpy of 93 J/g at monomers to the *n*-octadecane ratio of 2:1. Sanchez-Silva et al. microencapsulated Rubitherm®RT31 with polystyrene via suspension polymerization by using different suspension stabilizers [60]. The DSC investigations have shown that when PVP and gum Arabic were used as suspension stabilizers the microcapsules obtained presented the lowest thermal storage capacity of 75.7 J/g and highest of 135.3 J/g.

In emulsion polymerization (**Figure 2(d)**), first, the PCMs and monomers dispersed phase is suspended in a continuous phase in the presence of surfactants at constant stirring, followed by the addition of water-solution initiators to start the polymerization process [61]. This method is used to prepare microcapsule shells by polymerizing organic materials like PMMA and polystyrene. Şahan et al. encapsulated stearic acid (SA) with poly(methyl methacrylate) (PMMA) and four other PMMA-hybrid shell materials via emulsion polymerization technique [62]. The average diameter of microcapsules so obtained was found to be 110–360 *μ*m, the thickness of 17–60 *μ*m, heat storage capacity below 80 J/g and degradation temperature above 290°C. Sarı et al. successfully utilized the emulsion polymerization technique to microencapsulate paraffin eutectic mixtures (PEM) containing four different contents with PMMA shells [63]. The microcapsules obtained were spherical with a particle size of 1.16–6.42 *μ*m, heat storage capacity of 169 J/g and melting temperature in the range of 20–36°C.

#### **4.3 Physico-chemical methods**

In the physical–chemical method, microencapsulation is accomplished by combining the physical processes like phase separation, heating and cooling, with chemical processes, like hydrolysis, cross-linking and condensation. Normally, the coacervation and sol–gel methods are the most frequently employed methods. The coacervation method is of two types, one is single coacervation which requires only one type of shell material and the other is complex coacervation which requires two kinds of opposite-charged shell materials for microcapsules preparation. The microcapsules synthesized by the complex coacervation method usually have a more uniform size, better morphology and stability.

The complex coacervation processes involve the following key steps: (1) formation of emulsion by dispersing PCMs in polymer aqueous solution; (2) addition of a second aqueous polymer solution with opposite charges and deposition of shell material on droplet surface by electrostatic attraction and (3) Getting of microcapsules by crosslinking, desolation or thermal treatment. Hawlader et al. encapsulated paraffin cores with gelatin and acacia by using a complex coacervation process [48]. The melting and solidifying enthalpies of microcapsules obtained reached 239.78 J/g and 234.05 J/g, respectively, when the amount of cross-linking agent was 6–8 ml, homogenizing time was 10 min, and the ratio of core to the shell was 2:1. Onder et al. employed complex coacervation to microencapsulate *n*-hexadecane, *n*-octadecane and *n-*nonadecane core materials with natural and biodegradable polymers, like gum Arabic-gelatin mixture [64]. The microcapsules having *n*-hexadecane and *n*-octadecane cores showed good enthalpies of 144.7 J/g and 165.8 J/g, respectively, were obtained at the dispersed content of 80% in the emulsion and the microcapsules containing *n*-nonadecane prepared at the dispersed content of 60% in the emulsion presented enthalpy value of only 57.5 J/g.

The sol–gel method is a cheap and mild process for synthesizing PCMs microcapsules by inorganic shells, such as SiO2 and TiO2 shells. The major steps involved in the preparation of microcapsule by the sol–gel method are as follows: (1) preparation of colloidal solution by uniformly dispersing the reactive materials like PCMs, precursor, solvent and emulsifier in a continuous phase via hydrolysis reaction; (2) formation of a three-dimensional network structured gel system through condensation polymerization of monomers and (3) drying, sintering and curing processes to obtain microcapsules [65]. Cao et al. used the sol–gel process to microencapsulate paraffin core with TiO2 shells. They found that the sample with a microencapsulation ratio of 85.5% had melting and solidifying latent heat of 161.1 kJ/kg (at the melting temperature of 58.8°C) and 144.6 kJ/kg (at the solidifying temperature of 56.5°C), respectively [66]. Latibari et al. successfully employed the sol–gel method to synthesize nanocapsules containing palmitic acid (PA) core with SiO2 shell by controlling solution pH [67]. The nanocapsule obtained presented an average particle size of 183.7, 466.4 and 722.5 nm, at pH 11, 11.5 and 12, respectively, and the corresponding melting latent heats values of 168.16, 172.16 and 180.91 kJ/kg, respectively.

### **5. Design of microencapsulated PCMs for versatile application**

#### **5.1 PCMs microencapsulation with function inorganic shells**

Microencapsulation with conventional polymeric, inorganic or composite shells can provide only protection for the PCM core, but at the same time, these inert wall materials cause a reduction in their latent heat-storage capacities which make them unsuitable for thermal energy storage and thermal management systems. In view of that various inorganic materials have a feature of functional diversity, it will be possible to synthesize bi-function PCMs-based microcapsules by encapsulating the PCM core with a functional inorganic shell. This idea was first used by Fei et al. [68] and successfully synthesized a novel multi-functional microcapsules based on an anatase

### *Design and Fabrication of Microencapsulated Phase Change Materials for Energy/Thermal… DOI: http://dx.doi.org/10.5772/intechopen.102806*

TiO2 shell and *n*-octadecane/titania aerosol core via the hydrothermal method. The microcapsules obtained presented multi-functional properties with photocatalytic activity and UV-blocking effectiveness as well as a thermal energy-storage function. Chai et al. [69] introduced a new synthetic strategy by fabricating a well-defined core-shell structured PCM microcapsule based on a functional TiO2 shell. The crystallization of amorphous TiO2 was initiated by adding fluorine ions when the *in-situ* polycondensation of titanic precursors was performed in a nonaqueous O/W emulsion-templating system. The microcapsules so prepared have excellent thermal energy-storage capacity and show photocatalytic and antibacterial functions. Liu et al. [70] introduced a new technology by modifying the brookite TiO2 shell of the *n*-eicosane core with graphene nanosheets. It was observed that graphene promotes the charge transfer and separation ability of microcapsule which leads to a significant increase in its photocatalytic activity. Liu et al. [39] also explored that modification of TiO2 shell with ZnO boosts the latent heat-storage capacity and photocatalytic activity of the resultant microcapsules. A study on the utilization of microcapsules doped with ZnO presented good thermal regulation and thermal management properties when incorporated into the gypsum-matrix composites. These explorers make the modified microcapsules good candidates for direct solar energy utilization. Additionally, Liu et al. [71] introduced a morphology-controlled synthetic technology to fabricate PCM-based microcapsules with crystalline TiO2 shells by using different structure-directing agents and effectively obtained the microcapsules in the tubular, octahedral and spherical shapes. They also studied the influence of structural morphology on the thermal energy-storage capacity of these microcapsules and observed the highest latent heat-storage efficiency with microcapsules of spherical morphology while the tubular ones displayed the fastest heat response rate. Li et al. [38] successfully encapsulated *n*-eicosane with ZnO shell via *in-situ* precipitation reaction of Zn(CH3COO)2·2H2O and NaOH in an emulsion templating system. The microcapsule prepared exhibited good thermal energy-storage capability and high working reliability as well as high photocatalytic activities and antimicrobial effectiveness against *Staphylococcus aureus*. These microcapsules, therefore, have gained potential applications in medical care and surgical treatment. Gao et al. [72] designed multifunctional microcapsules by a microencapsulating *n*-eicosane core with a Cu2O shell, through emulsion templated *in-situ* precipitation and reduction. The microcapsules obtained exhibited multifunctional properties of effective photothermal conversion, high latent-heat storage/release efficiency for solar photocatalysis and solar thermal energy storage, as well as demonstrated sensitivity to some toxic organics gases due to a *p*-type semiconductive feature of Cu2O shell.

#### **5.2 Advanced design of microencapsulated PCMs for versatile applications**

In recent years due to the fast development in microencapsulation technology, a large number of innovative designs have been introduced for fabricating bi- or multifunctional PCMs-based microcapsules. Jiang et al. [73] designed magnetic PCM-based microcapsules as an applied energy microsystem for bio-applications as thermoregulatory enzyme carriers. They synthesized the magnetic microcapsules by encapsulating *n*-eicosane with a TiO2/Fe3O4 hybrid shell by Pickering emulsion-templated interfacial polycondensation and then *Candida rugosa lipase* (CRL) was immobilized onto the microcapsules obtained by covalent bonds through a series of complicated surface modification and immobilization reactions. The microcapsules obtained were observed to have higher thermal stability, longer storage stability, higher biocatalytic

activity and better reusability compared to traditional inert enzyme carriers. Likewise, Li et al. [74] also developed thermoregulatory enzyme carriers based on the magnetic microcapsules containing *n*-docosane core and SiO2/Fe3O4 hybrid shell with α-amylase immobilized onto the microcapsule and examined the effect of ambient temperature on their biocatalytic activity. They found that the biocatalytic activity was increased considerably for the immobilized α-amylase on the developed enzyme carriers due to the thermoregulation microenvironment around the microcapsules. These innovative designs provide a novel approach for the preparation and applications of microencapsulated PCMs in areas of bioengineering and biotechnological.

Choi et al. [75] designed a novel, temperature-sensitive drug release system based on PCMs. They first prepared the gelatin nanoparticles containing fluorescein isothiocyanate-dextran as a drug via emulsification technique, and then 1-tetradecanol was used to synthesize the PCM-matrix microbeads containing these gelatin nanoparticles by using a simple fluidic device based on an O/W emulsion. Moreover, Wang et al. [58] designed and synthesized thermochromic microencapsulated PCM by encapsulating *n*-octadecane with PMMA shell with simultaneous dispersion of thermochromic pigments in core and shell by suspension-like polymerization. The microcapsules obtained showed a visible color change with change in temperature, confirmed the occurrence of thermal energy storage or release at the specific temperature.

Geng et al. [76] designed a three-component core consisting of 1-tetradecanol as a PCM, leuco dye and phenolic color developer as an electron donor and fabricated reversible thermochromic microcapsules for application in thermal protective clothing. They encapsulated the three-component core with a poly (methylated melamine-formaldehyde) (PMMF) shell via emulsion-templated copolymerization. The as synthesized microcapsules exhibited thermochromic reversibility with good energy storage/release capability and have a great potential for applications in thermal protective clothing of firefighters as well as intelligent textiles or fabrics, food and medicine package and so on. Wu et al. [77] synthesized reversible thermochromic microcapsules by encapsulating 1-hexadecanol with modified gelatin and gum Arabic via a complex coacervation process. The wall materials of this microcapsule system were fused with 2-phenylamino-3-methyl-6-di-*n*-butylamino-fluoran as a color former and 2,2-bis(4-hydroxyphenyl) propane as a color developer. The microcapsule prepared acts as an indicator for the states of energy saturation and consumption through color changes. In addition, Zhang et al. [78] introduced polysaccharideassisted microencapsulation as an innovative methodology for encapsulation of volatile PCMs with a fluorescent retention indicator to determine the retention of microencapsulated volatile PCM in diverse working environments. They microencapsulated heptane core with polymeric shell by one-step *in-situ* polymerization path using Nile red as a fluorescent indicator which was incorporated into the heptane core during the synthetic process, and therefore the fluorophores in Nile red could give a clear indication for the core and shell structures of microcapsules.
