**2.2 Organic encapsulants**

The organic encapsulants may be developed in two structural scales, such as nano or micro. They include polymers and surfactants that are being introduced to develop PWTCs. It is worth-noting that the surfactants are normally polymers, but precisely, they are thermo-plastic elastomers consisting of co-polymers blocks. The potential polymeric encapsulants include low-density polyethylene (LDPE), highdensity polyethylene (HDPE), melamine-formaldehyde (MF), polypropylene and polyacrylate, etc., while surfactant-based encapsulants are styrene-b-(ethyleneco-butylene)-b-styrene (SEBS) tri-block copolymer, styrene–butadiene–styrene (SBS), olefin block copolymer (OBC) and polystyrene, among others. However, the main challenge with organic encapsulants is that their thermal conductivity is approximately similar to that of paraffin waxes [25]. Therefore, organic encapsulants are only SSs. The need of TCEs with encapsulated PWTCs definitely results in a triplex thermal composite, implying that the latent heat has to be further sacrificed. Such a design challenge can only be resolved by recommending the encapsulated PWTCs for those applications where thermal energy storage is moderately acquired but fast charging/discharging is not a primary objective, for example, in flame retardancy and thermal management of buildings, fabrics and food packages. The fabrication of encapsulated PWTCs is based on dry or wet-physical methods;

however, the fabrication of the encapsulants is usually enabled by the chemical reactions which are complex due to the inclusion of several preparation steps and process conditions [26]. The little variance in process conditions can result in broken capsules (shells), posing the liquid drainage threats for PWTCs. Until recently, encapsulated PWTCs have been successfully fabricated. For example, duplex encapsulated PWTCs have been presented comprising of paraffin wax (melting temperature of 52–54 C, latent heat of 182.6 Jg−1) as a core and MF as shell, as shown in **Figure 9** [27]. With 10 g of paraffin wax and 61.6% encapsulation efficiency, the latent of encapsulated PWTC is 107.4 Jg−1.

It is thus obvious that the encapsulation process covers the several steps at various process constraints to create the final PWTCs. Normally, the morphology is spherical and it is filled with paraffin wax, as shown in Figure, therefore encapsulation efficiency (how much core material can be surrounded and uptaken by the shell of encapsulant) is the fundamental parameter to be emphasized for encapsulated PWTCs. In another example of triplex PWTC is presented where HDPE has been employed as an encapsulant for paraffin wax (melting temperature of 54–56°C and latent heat of 199 jg−1), and copper metal foam has been introduced enhancing the thermal conductivity of HDPE/paraffin wax from 0.72 Wm−1 K−1 to 2.14 Wm−1 K−1 and reducing the latent heat as minimum as 151.6 jg−1. The PWTCs also

#### **Figure 10.**

*Thermal stability of paraffin wax and PWTCs via TGA [30].*

provided favorable shape-stability, allowing 2.3% mass loss of paraffin wax after 50 thermal cycles [28]. In a word, the stronger the encapsulant shell is, the more effective is the shape-stability. Generally, several other examples are also found where carbonic thermal reinforcements have been introduced to accelerate thermal response of encapsulated PWTCs, such as EG [25, 26].

Apart from the salient merits of providing shape-stabilization, organic encapsulants are thermally stable too [29], imparting the adequate thermal stability to PWTCs. For example, SEBS has been employed to encapsulate paraffin wax (melting temperature of 52–54°C and latent heat of 176.6 jg−1) in different percent contents (5–20%), and 20% of SEBS enhances the thermal decomposition temperature of PWTC to around 221.4°C, as demonstrated in **Figure 10** [30].

## **3. Bottlenecks, recommendations and design standards for PWTCs**

The fabrication of PWTCs is simple until investigated recently; nonetheless thermo-physical bottlenecks of the paraffin waxes still exist. For example, their inert features do not allow chemical reactions, so the utilization of chemical methods for fabrication of PWTCs can be overlooked. Thus, physical methods are only viable options for the fabrication of PWTCs. However, physical methods can also be wet or dry, for example, mixing the melted paraffin waxes with thermal reinforcement is the dry-physical method, while dissolving the paraffin wax into a solvent and then mix with thermal reinforcement is the wet-physical method or encapsulating the paraffin wax in the capsules is also the wet-physical method. Meanwhile, the shape-stabilization can be achieved via both methods, but thermal conductivity improvements have abundantly been reported based on dry-physical methods. Besides, thermal conductivity of PWTCs prepared via dry-physical methods is not sufficiently high, leaving a wide research roam and urging to put rigorous efforts into this serious matter. However, the PWTCs with increased thermal conductivity suffer from another challenge which consists of a short and uprising isothermal zone, indicating the reduced duration of temperature-control capacity. The uprising isothermal zone dictates that the temperature-control capacity of PWTC is not perfectly constant, but nearly constant and fast. A case-study describing this standpoint can be seen above in **Figure 7**.

Thus, either maximum or minimum, threshold limit of the thermal reinforcements is very important in PWTCs, which is however not standardized yet. Thermal reinforcements always demand the equal replacement of base material in PWTCs, signifying that the equivalent reduction in latent heat storage capability [12]. In such a situation, the design parameters of PWTCs should be adjusted according to the target applications. For example, thermal management of photovoltaic panel is required to be done through PWTC. In this case, the liquid drainage of the PWTC may not be considered a design parameter of the primary importance because the mechanical enclosure can assist in controlling the liquid drainage, meaning that the form-stability of PWTC can be neglected. However, the heat accumulation at the interface of PV and the hot-spots in the PWTC body are altogether supposed to be the primary design parameters. Therefore, thermal reinforcements for such applications should solely be the TCEs that can help fabricate the duplex PWTCs. Counter-institutively, Another case can be discussed regarding the thermal management of buildings where PWTCs should capable of ensuring prolonged temperature-control capacity as well as adequate shape-stability. So, thermal reinforcements for such applications should solely be the SSs, but the SSs needs to conform the property prerequisites of the building ingredients like cement, sand, and clay, etc., which altogether leads to the triplex PWTCs. In between these two scenarios, special applications such as thermal management of satellites, robots, and astronauts can be put forth, where the total quantity of PWTC emerges is another design parameter together with consideration of temperature-control capacity and shape-stabilization.

PWTCs can also suffer from the challenge of saturation energy storage limit which is dependent on the total thermal energy storage (sensible heat + latent heat)

**Figure 11.** *Proposal of design standards for PWTCs.*

#### *Paraffin Wax-Based Thermal Composites DOI: http://dx.doi.org/10.5772/intechopen.97195*

of paraffin wax. This challenge is expected to emerge in all kinds of applications. Therefore, it is strongly recommended to evaluate the total saturation duration that can anticipate saturation energy storage limit. Accordingly, the periodic heat regeneration of PWTCs, i.e., extracting the stored thermal energy, becomes essential to keep the whole thermal management system fresh for the coming cycles. The possible methods for periodic heat regeneration encompass the natural convection or forced convection achieved either via air or water for which deep design considerations of the whole thermal management systems are required.

Among thermal reinforcements, EG can act both as TCE and SS [29, 31], pointing out that duplex PTWCs can be fabricated for thermal management with high charging/discharging rates. Therefore, EG is declared to be the most effective carbonic thermal reinforcement.

Although the design standardization of the optimal PWTCs is complicated, the parameters as-proposed in **Figure 11** may serve as the preliminary design principles.
