**1. Introduction**

Paraffin waxes are defined as the materials consisting of saturated carbonhydrogen chains integrated with branched, straight and ring-like (aromatics) structures [1] that are relatively complex by nature. This chemical configuration endows the amorphous characteristics and inertness to paraffin waxes, resulting in inactive functional groups where the external chemical reactions become impossible. Based on this stance, paraffin waxes are supposed to be the green thermal reservoirs, lying within the sustainable targets of the current era. Therefore, the applications of paraffin waxes ranging from biomedical [2] to thermal storage/release [3–5] are declared relatively safe and environmental-friendly.

In thermal storage/release applications, phase change of the paraffin waxes is the backbone that governs the under-lying mechanism depending on thermal excitation or de-excitation driven by the heat source. Thermal excitation is the phase change process of melting during which thermal energy is absorbed, while thermal de-excitation is the phase change process of solidification during which thermal energy is released. These both processes build up the reversible functionality involving the sensible heat and latent heat storage/release. In case of paraffin waxes (or generally for PCMs), the sensible heat is counted before the phase change process, while latent heat is considered during the phase change process. By definition, phase change [6] process refers to either structural change or state change, as shown in **Figure 1**. In structural change, a single phase of paraffin wax undergoes thermal excitation that brings about the conversion of the internal structures and it is called the solid–solid phase change. Whilst, in the state change, two phases of the paraffin wax undergoes thermal excitation, resulting in conversion of solid phase into liquid phase at the melting temperature, which is known as solid–liquid phase change. The occurrence of either kind of phase change is dependent on the melting temperatures of the paraffin waxes. The paraffin waxes with low-melting temperature such as <40°C (or also called soft paraffin waxes) do not demonstrate structural change, so they only undergo state change. While, the paraffin waxes with high-melting temperature (or also called hard paraffin waxes) provide both structural change and state change.

#### **Figure 1.**

*Classification of phase change and paraffin wax-based phase change materials. A few contents of this figure are partially restructured from references [6, 7].*

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

In between structural change and state change, another phase consisting of solid flakes and liquid sites ensues which is called mush phase. Normally, the temperature range of the mush phase lies closer to the onset temperature of the liquid phase. In brief, all these phases are responsible for the total latent heat, while before and after these phases, total sensible heat is considered, depending on the melting temperatures of the involved paraffin waxes. Therefore, it is emphasized that the total thermal energy of the paraffin waxes should carefully include thorough observation of the onset and endset temperatures which can be determined either by DSC curves (**Figure 1**) or by the transient heat diffusion process (temperature versus time analysis). With help of transient heat diffusion process, the thermal storage/ release performance of paraffin waxes can precisely be assessed by looking into the time consumed during the melting (thermal excitation) and solidification (thermal de-excitation) processes, which is called charging time and discharging time, respectively. It is generally deducted that, the charging/discharging time becomes short, if the heat diffusion occurs fast, and vice versa. The heat diffusion is further related to the intrinsic property of materials known as thermal conductivity, which is defined as the ability of material allowing the fast or slow transfer of heat. In short, with high thermal conductivity, the charging/discharging time are reduced on behalf of faster heat transfer. However, heat transfer in paraffin waxes is sluggish, and their charging/discharging time is sufficiently long based on the low thermal conductivity. The possible reason for the low thermal conductivity of paraffin waxes is their amorphous nature wherein tightly-packed and inter-connected thermal paths are unavailable. However, a great care is essential because every application may not need fast heat transfer rates, for example, thermal management of buildings where the objective is to keep the heat either outside the buildings in summer or inside the buildings in winter, which is possible only if paraffin wax serves as a thermal insulator necessitating slow heat transfer rates, but the challenge of liquid drainage needs to be simultaneously addressed. Therefore, depending on the target applications, enhancing the thermal conductivity of paraffin waxes is of great practical interest so that the charging/discharging can be reduced. For example, the long charging/discharging time of paraffin waxes is the major bottleneck that can potentially hampers their functionality for thermal management of batteries where the heat generation rate is prone to high and the objective is to dissipate the heat so that hot-spots can be avoided. Keeping different practical scenarios ahead, thermal reinforcements are essential to improve the thermo-physical bottlenecks of the base materials (paraffin waxes), helping create paraffin wax-based thermal composites (PWTCs) which are discussed henceforth.
