Composition and Preparation of Microencapsulated Phase Change Materials: History
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Danni Yang
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Sifan Tu
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Jiandong Chen
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Haichen Zhang
,
Wanjuan Chen
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Dechao Hu
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Jing Lin

Phase change materials (PCMs) have been extensively utilized in latent thermal energy storage (TES) and thermal management systems to bridge the gap between thermal energy supply and demand in time and space, which have received unprecedented attention in the past few years. To effectively address the undesirable inherent defects of pristine PCMs such as leakage, low thermal conductivity, supercooling, and corrosion, enormous efforts have been dedicated to developing various advanced microencapsulated PCMs (MEPCMs). 

  • phase change microcapsules
  • low-dimensional nanofillers
  • thermal conductivity

1. Core Materials of MEPCMs

Phase change materials (PCMs) is a kind of typical latent heat thermal energy storage (TES) material with the advantages of large heat storage density, operational simplicity, and high safety, which can control the temperature by reversibly absorbing or releasing thermal energy to the environment [1]. Thus, PCMs, as an excellent storage media, have a wide range of applications, including solar energy systems, buildings, and thermal management of vehicles battery and electronic devices [2][3][4][5][6][7][8]. Generally, PCMs can be divided into solid-solid, solid-liquid, solid-gas, and liquid-gas PCMs according to their phase transition states [9]. Although the latent heat of solid-gas and liquid-gas PCMs is larger than that of solid-liquid and solid-solid PCMs, it is difficult to be applied in engineering fields due to their great volume change in phase transition process. Besides, the solid-solid PCMs which stored heat by changing crystal form, have a relatively smaller phase transition enthalpy than solid-liquid PCMs [10]. Correspondingly, the solid-liquid PCMs have unique advantages in phase equilibrium, volume change and vapor pressure. Thus, the solid-liquid PCMs become the most significant core materials employing in phase change microcapsules [11]. When the external temperature is higher than the solid-liquid PCMs, the solid-liquid PCMs will absorb heat in the form of sensible heat, and the temperature begins to rise. When the phase transformation temperature is reached, the temperature will remain constant. This heating process can effectively store energy in the form of latent heat until it is completely converted into liquid and then raising the temperature in the form of sensible heat (and vice versa). The cooling process also has a period of releasing energy in the form of latent heat.
Solid-liquid PCMs can be further divided into organic, inorganic, and eutectic PCMs according to their composition [12]. Among them, organic PCMs have the characteristics of wide applicable temperature range and stable chemical properties, which mainly include paraffins, fatty acids, esters, and alcohols [13]. Typically, paraffins, as the most common commercial organic PCMs, contain a variety of high-purity straight chain alkanes, and their melting temperature and melting enthalpy can increase with the number of carbon atoms increasing. Compared with high-purity straight chain alkanes, paraffins are usually produced as the by-products in crude oil refining. Thus, they are widely available and inexpensive, and have higher practical application value [14][15]. Fatty acid is another organic PCMs produced from common vegetable oils and animal oils, whose cost is higher than that of paraffin. The commonly used fatty acids include stearic acid (SA) (melting point 69 °C), palmitic acid (melting point 56 °C), myristic acid (melting point 58 °C) and lauric acid (melting point 49 °C), etc. Due to their suitable phase transition temperature and high phase transition enthalpy, fatty acids are the most promising non-paraffin PCMs. Besides, sugar alcohol can also be used as PCMs, whose melting temperature and melting enthalpy are the highest among organic PCMs, so it has good application prospects in solar heating and industrial waste heat recovery. Notably, though organic PCMs have various advantages, almost all of them belong to low-temperature PCMs with a melting point lower than 220 °C [16]. Moreover, their thermal conductivity is relatively low, and usually suffer from high flammability and low thermal stability. Different from organic PCMs, the inorganic PCMs exhibit a relatively higher thermal conductivity and greater phase change enthalpy. They also have a clear melting point, excellent flame resistance, and good recyclability. Inorganic PCMs can be roughly divided into hydrated salts and molten salts. When the PCMs are hydrated salts, the solid-liquid phase change process is actually the dehydration and hydration process of hydrated salts, such as MgSO4·7H2O (melting point 49 °C), CaCl2·6H2O (melting point 33 °C), Ba(OH)2·8H2O (melting point 78 °C), Al(OH3)2·7H2O (melting point 75 °C). Usually, two or more hydrated salts are mixed to adjust the phase transition temperature to meet the requirements of practical applications. It is mainly suitable for medium and low temperature TES since the melting point of most hydrated salts is below 220 °C, while molten salts such as sulfate and nitrate can run at a temperature of more than 420 °C and can be used in high temperature TES applications [17][18]. Eutectic PCMs can be prepared by inorganic-inorganic, organic-organic or two kinds of mixed PCMs, which can achieve high thermal conductivity, small supercooling, and a relatively low cost through adjusting their specific proportion. Eutectic PCMs have a wide range of melting points and can be used in low, medium, and high temperature applications. Most organic-inorganic eutectic PCMs belong to low temperature PCMs, such as CO(NH2)2-NH4Br (melting point 80 °C), while most inorganic-inorganic eutectic PCMs belong to medium temperature PCMs with melting point between 220 °C and 420 °C [19][20]. Their advantages, disadvantages, and the melting points of typical organic, inorganic, and eutectic PCMs are summarized in Table 1.
Table 1. Advantages, disadvantages and melting points of typical PCMs.
Type PCMs Advantages Disadvantages Melting Point (°C)
Organic Paraffin C16-C18 Wide applicable temperature range
Less supercooling phenomenon
High crystallization rate
High chemical stability
Strong recycling performance
Safety and non-toxic
Little corrosion performance		 Low thermal conductivity
Poor thermal storage capacity per unit volume
Volatilize easily
Flammability
Expensive		 22
Paraffin C20-C33 50
Stearic acid 69
Palmitic acid 56
Myristic acid 58
Lauric acid 49
Inorganic MgSO4·7H2O Lower volumetric expansion
Large heat storage capacity
Low cost
High thermal conductivity
Greater phase change enthalpy
Non-flammable
With a clear melting point
Recyclable		 High supercooling
Prone to precipitation
Poor dimensional stability
Low thermal stability
Corrosive
Phase segregation
Poor compatibility with some building materials		 49
CaCl2·6H2O 33
Ba(OH)2·8H2O 78
Al(OH3)2·7H2O 75
KNO3 340
Na2CO3 850
Eutectic CO(NH2)2-NH4Br High thermal conductivity
Hardly segregation
Hardly supercooling		 Expensive
Strong odor		 80
Na(CH3COO)·3H2O-CO(NH2)2 30
NaCO3-LiCO3 490
NaF-MgF2 640

2. Shell Materials of MEPCMs

The shell of phase change microcapsules can effectively prevent the leakage of PCMs during the phase change process. Obviously, to maintain the structural stability of microcapsules, the shell materials need to have high encapsulation efficiency, excellent thermal and chemical stability, and superior compatibility. In the past few decades, extensive efforts have been dedicated to enhance the comprehensive properties of microencapsulated PCMs (MEPCMs) by the reasonable design of shell materials [21]. Usually, these shell materials can be divided into organic, inorganic, and organic-inorganic hybrid materials according to their chemical properties.

2.1. Organic Shells

Organic shell materials are generally composed of natural and synthetic polymer materials which have good elasticity, toughness, excellent compactness, and stable chemical properties. Typically, melamine-formaldehyde (MF) resin and urea-formaldehyde (UF) resin are the most reported and widely used organic shell materials of microcapsules [22][23][24][25][26][27][28][29]. For example, Yu et al. designed a type of MEPCMs by using MF as the shell material and dodecanol as the core material via in-situ polymerization, achieving the highest phase transition enthalpy of 187.5 J/g and a high encapsulation efficiency of 93.1% [28]. Compared with MF, UF has higher phase transition latent heat and relatively poor thermal resistance [30][31][32][33][34][35][36]. Thus, Tohmura et al. developed melamine-urea-formaldehyde resin (MUF) by the simultaneous reaction of cyanuric acid and urea with formaldehyde to effectively improve the intermolecular force, thereby improving its thermal stability [36]. Besides, polymethylmethacrylate (PMMA) is also a common shell material for MEPCMs in recent years due to its environmental stability, and low easy processing cost [37][38][39][40][41]. However, the residual small molecules (such as formaldehyde, acrylate, etc.) of the above resins pose a threat to the environment and human health. Therefore, more and more studies have been devoted to the development of polyurea or polyurethane (PU) encapsulated MEPCMs [42][43][44]. In particular, the initial reaction rate between diisocyanate and polyol is relatively lower, and the elasticity of MEPCMs shell can be improved by adjusting the soft segment structure of PU, which is very beneficial to the mechanical properties and compactness of MEPCMs [45]. Therefore, in the previous study, one team has realized the microencapsulation of methyl laurate with PU shell through interfacial polymerization [46][47][48]. In addition, some polymers including polystyrene (PS) [49][50][51][52][53] and starch [54], etc are also used as organic shell materials to prepare MEPCMs. However, it is noting that most organic materials face the challenges of high flammability and low thermal conductivity.

2.2. Inorganic Shells

Compared with the organic polymers, the inorganic shell materials have higher thermal conductivity and superior mechanical strength, which can not only improve the durability and reliability of the microcapsules, but also improve the heat transfer performance of MEPCMs. Therefore, the use of inorganic materials as microcapsules shells has become another significant trend [21]. Common inorganic shell materials include silica oxide (SiO2) [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71], calcium carbonate (CaCO3) [72][73][74][75][76], graphene [77][78][79], titanium dioxide (TiO2) [80][81][82][83], etc. Among them, SiO2 is a commonly used shell due to its wide source, low cost, and mature preparation method. For example, Liang et al. developed n-octadecane-based MEPCMs with silica shell by interfacial polymerization, and the phase change enthalpy and encapsulation rate of the microcapsules were as high as 109.5 J/g and 51.5%, respectively [56]. Besides, the graphene with superb intrinsic thermal conductivity can significantly improve the thermal conductivity of MEPCMs and provide a new material for thermal energy management [79]. In addition, metal oxides have also been widely used as an inorganic shell, including TiO2, Al2O3, Fe3O4, and ZnO, etc [84]. For example, Liu et al. synthesized n-eicosane MEPCMs with TiO2 as shell by interfacial polymerization, and found that the obtained microcapsules not only had durability and highly stable shape, but also exhibited superior antibacterial function, showing great potential in medical applications [82]. However, it is undeniable that the inorganic shell materials inevitably suffer from the disadvantage of poor stability and brittleness.

2.3. Organic-Inorganic Hybrid Shells

Organic-inorganic hybrid shells can combine the high inherent thermal conductivity of inorganic materials and the toughness of polymer shells, and achieve a high thermal conductivity, permeability resistance and thermal stability of MEPCMs. Therefore, the organic-inorganic hybrid shell exhibit promising potential in next-generation shell materials of MEPCMs [21][85]. It can be expected that the incorporation of inorganic nanofillers in organic shells can endow MEPCMs with high thermal conductivity and some unique properties [86][87][88][89][90][91]. Typically, Pickering emulsion polymerization is a classical method to obtained the polymer/inorganic hybrid shells. For example, Yin et al. prepared n-hexadecanol MEPCMs using MF-SiO2 as the hybrid shell by Pickering emulsion polymerization [86]. The SiO2 particles in the shell significantly improved the mechanical strength and thermal reliability of the MEPCMs, and received a high phase change enthalpy of 163.76 J/g. Furthermore, Zhang et al. synthesized a unique graphene oxide (GO)/polyaniline (PANI) hybrid shell through emulsion polymerization, which combined the excellent barrier properties of GO with the anti-corrosion function of PANI. The results show that the MEPCMs have strong solvent resistance to organic solvents, and the MEPCMs are dispersed in waterborne epoxy resin, which can prepare intelligent coatings with dual functions of anti-corrosion and self-healing [92]. In short, the MEPCMs with polymer-inorganic hybrid shells have become the current research focus of phase change microcapsules, which hold great potential in thermal management applications. Table 2 summarizes the phase change enthalpy and encapsulation rate of MEPCMs encapsulated by some representative shell materials.
Table 2. Typical shell materials in the preparation of MEPCMs.
Shell Type Shell Materials Core Materials Encapsulation Methods Phase Change Enthalpy (J/g) Encapsulation Rate (%) Refs.
Organic shells MF Lauryl alcohol In-situ polymerization 187.5 93.1 [28]
UF Paraffins In-situ polymerization 72.4 52.8 [32]
PU Methyl laurate Interfacial polymerization 136.2 - [46]
PMMA Na2HPO4·12H2O Solvent evaporation 142.9 80.4 [41]
Polyurea CaCl2·6H2O Interfacial polymerization 118.3 71 [43]
PS N-tetradecane In-situ polymerization 98.71 44.7 [53]
Starch N-heptadecane Interfacial polymerization 187.27 78.27 [54]
Inorganic shells SiO2 N-octadecane Interfacial polymerization 109.5 51.5 [56]
Mannitol Sol-gel 252.66 89.6 [69]
CaCO3 N-tetradecane Self-assembly 58.54 25.86 [74]
rGO SA Pickering emulsion polymerization 159 74.3 [79]
TiO2 N-eicosane Interfacial polymerization 188.27 - [82]
Organic-inorganic hybrid shells MF-SiO2 N-hexadecanol Pickering emulsion polymerization 163.76 74.6 [86]
PMMA-BN/TiO2 Paraffin Pickering emulsion polymerization 124.4 72.1 [87]
P(MMA-co-BA)-TiO2 Paraffin Pickering emulsion polymerization 90.12 36.09 [88]
PUA-TiO2 N-octadecane Interfacial polymerization 181.1 77.3 [89]
PS-GO N-hexadecane Pickering emulsion polymerization 186.8 78.5 [91]

3. Preparation Strategies of MEPCMs

The microencapsulation of PCMs with various shell materials can not only effectively solve the leakage problem during the phase change process, but also improve the thermal conductivity and other properties of PCMs, which shows a good development prospect in many applications. Most of MEPCMs are spherical with a particle size from 1 to 1000 μm, and their mass of PCMs cores vary from 20% to 95% in the total mass of MEPCMs [93]. Typical preparation strategies of the MEPCMs were described as follows. It can be mainly divided into physical method, chemical method, and physical-chemical method according to the synthesis mechanism.

3.1. Physical Method

In general, the physical methods for preparing MEPCMs mainly include spray drying and solvent evaporation [94][95]. The spray drying method is that the core material and the shell material are co-dissolved in a solvent, and the mixture is sent into a heating chamber in the form of small droplets for heating. During the heating process, the solvent will evaporate, and finally the microcapsules are separated. For example, Borregro et al. microencapsulated the paraffin by spray drying, and the encapsulation efficiency reached 63%. Besides, they found that the properties of these MEPCMs were closely related to where they were collected in the spray dryer, and the MEPCMs was highly stable and reversible even after 3000 cycles [95]. Overall, the spray drying method is relatively simple to operate and has a high production efficiency, but it is not suitable for inorganic PCMs [96]. Another physical synthesis method is the solvent evaporation method [97]. Specifically, the polymer shell materials were dissolved in volatile solvent, and then adding PCMs into above solution to form emulsion, finally the shell is formed on the droplet surface by evaporating the solvent. 

3.2. Chemical Method

Chemical method is the most widely used method to prepare MEPCMs. It usually uses free radical polymerization to form oil-in-water or water-in-oil emulsions, and reacts on the oil/water (O/W) interface to form shell materials. The representative and commonly used chemical methods include in-situ polymerization, interfacial polymerization, suspension polymerization and emulsion polymerization. Typically, the in-situ polymerization method is to emulsify the core material to form oil-in-water emulsion droplets under the action of emulsifier, and then the obtained O/W emulsion droplets were catalytically polymerized with monomers or prepolymers under certain conditions to form a polymer shell, and finally cured to form MEPCMs. Generally, most of MEPCMs with MF and UF shells are synthesized by in-situ polymerization. Notably, this synthesis method can achieve the synergetic integration of high encapsulation efficiency and uniform coating, but the operation process is relatively complex and may cause some environmental pollution [98]. Another chemical method for preparing MEPCMs is interfacial polymerization. In the interfacial polymerization, the core material and hydrophobic monomer are used as the oil phase, while the emulsifier and deionized water serve as the water phase, which are emulsified to form an oil-in-water emulsion, and finally polymerize at the O/W interface to form a shell under appropriate conditions. Zhang et al. synthesized a n-octadecane-based MEPCMs with a polyurea shell by interfacial polymerization. It was found that when the core-shell ratio was 7/3, the MEPCMs exhibited better phase transition performance, better permeation resistance and higher encapsulation efficiency [99]. In general, interfacial polymerization is easy to operate and low cost, but the reaction speed is often too fast, resulting in the properties of final product is more difficult to control [100]. Besides, suspension polymerization is to mix the PCMs, reaction monomers and initiator as the oil phase, which is then suspended in the water phase to form emulsion droplets under the action of surfactants, finally polymerize to form MEPCMs [101]. Typical MEPCMs with organic shell materials such as PMMA and PS are usually prepared by suspension polymerization. Compared with above polymerization methods, the suspension polymerization usually has a higher encapsulation rate and strong thermal regulation ability, but the production cost is also relatively higher [102]. In addition, the emulsion polymerization method is to mix the PCMs with reaction monomers, and add emulsifiers to form the O/W emulsions, finally initiate polymerization to form microcapsules under the action of initiator [103][104]. Particularly, Pickering emulsion polymerization has been widely applied to prepare MEPCMs in recent years due to the superior mechanical properties and thermal stability of as-obtained MEPCMs. Moreover, Pickering emulsion polymerization is more environmentally friendly and easy to obtain polymer/inorganic hybrid shells [102][105]. Wang et al. prepared a MEPCMs with Pickering emulsion polymerization by introducing modified SiO2 and TiC nanoparticles as emulsifiers [106]. The obtained MEPCMs exhibited well-defined core-shell structure, and have good heat storage and release properties, superior thermal stability, and enhanced thermal conductivity. So far, Pickering emulsion polymerization has become an important part of the methods for preparing of advanced MEPCMs due to its simplicity and economy.

3.3. Physical-Chemical Method

Physical-chemical methods are usually used to prepare microcapsules under external force and chemical reaction by combining physical processes such as heating and cooling with chemical processes such as crosslinking and condensation. Coacervation is a typical and commonly used physical-chemical method, which can be further divided into single coacervation and complex coacervation method [107][108]. For example, complex coacervation method is to form emulsion by mixing PCMs with a polymer, then emulsifying with another polymer to form stable emulsion, and finally condensing to form MEPCMs. Sol-gel method is to hydrolyze the reaction monomer to form sol solution, and then mix it with the PCMs to form emulsion [109][110]. Under certain reaction conditions, the gel shell is generated around the PCMs droplet through polycondensation reaction [111].
In summary, there are many encapsulation methods that can be used to prepare MEPCMs. However, as shown in Table 3, different preparation methods all have some advantages and disadvantages. For example, although physical methods are simpler and more economical, the prepared MEPCMs are usually agglomerated together. Moreover, Pickering emulsion polymerization is particularly suitable for the preparation of MEPCMs with organic-inorganic hybrid shells, which is of great significance for achieving the enhanced thermal conductivity and functionalization of MEPCMs, but still faces some challenges for large-scale implementation. Therefore, in practical applications, the selection of an encapsulation method not only depends on the performance and parameters of MEPCMs (e.g., size, size distribution, chemical stability, and thermal stability), but also is inextricably linked to the polymerization operability, cost, etc.
Table 3. Comparison and summary of the different preparation methods of MEPCMs.
Types Methods Advantages Disadvantages Scope of Applications
Physical method Spray drying Simple operation
High production efficiency
Wide range of sizes		 Low packaging rate
Not used for inorganic PCMs		 Organic PCMs
Heat sensitive material
Solvent evaporation Low cost Low encapsulation efficiency Inorganic PCMs
Chemical method In-situ polymerization High encapsulation efficiencyStable shape
Uniform coating		 Complex operation
Harmful for the environment		 Organic PCMs
Inorganic PCMs
Organic shell material such as MF and UF
Interfacial polymerization High reaction speed
Simple operation
Low permeability		 The monomer is required to have a high reactivity
Harmful for the environment		 Organic PCMs
Inorganic PCMs
Organic shell material such as UF
Suspension polymerization Environmentally friendly
Facile reaction condition
High packaging rate		 High energy consumption Expensive
Not used for inorganic PCMs		 Organic PCMs
Large grained
Organic shell material such as PMMA
Emulsion polymerization Stable
High packaging rate
High preparation efficiency Environmentally friendly		 Complicated
Expensive		 Organic PCMs
Inorganic PCMs
Organic shell material such as PMMA
Polymer/inorganic hybrid shell
Physical-chemical method Coacervation Simple equipment
Good control of the particle size and thickness		 Difficult to scale-up
Not used for inorganic PCMs
Agglomeration		 Organic PCMs
Organic shell material
Sol-gel method High thermal stability
Strong controllability		 Non-insulated and limited in building applications Organic PCMs
Inorganic PCMs
Inorganic shell material

This entry is adapted from the peer-reviewed paper 10.3390/polym15061562

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