Thermo-regulating textiles with phase-change materials
Incorporation of phase-change material (PCM) in textiles is an attractive way to make thermo-regulating textiles. This chapter provides a review of PCM for textile applications. The chapter discusses thermal comfort, working principles of PCM, different types of PCMs, incorporation of micro-encapsulated PCMs in textiles and various applications of PCM incorporated textiles. The chapter concludes with a discussion of some of the challenges for PCMs in textile applications.
Smart materials that can respond to external stimuli are now increasingly being applied in the manufacturing of innovative textile products. Garments made from a smart fabric whose functional properties adjust with changes in the external environment can provide superior protection to the wearer in extreme environmental conditions. Phase-change material (PCM) is one such smart material which has the ability to store and release energy in a certain temperature range . Whenever the supply of or demand for energy does not change dependently with time, energy storage is required . The thermal energy storage (TES) system bridges the time gap between energy requirements and energy use , and plays an important role in energy management of textile products to enhance thermal comfort . TES by PCM to improve thermal performance of clothing during environmental temperature fluctuation is becoming an attractive option . PCMs absorb energy in the heating cycle as phase change takes place from solid to liquid, and release energy to the environment in the phase-change range from liquid to solid during a reverse cooling process . Incorporation of PCM in textiles to make thermo-regulated smart textiles is of growing interest to researchers. There are many situations where this concept could find applications such as all those professions where a person is affected by extreme changes of external temperature .
Wide ranges of PCMs are researched by scientists. However, PCM with a phase-change temperature range of 18–35°C will be most useful for making thermo-regulating textiles . Selection of PCM for the textile substrate depends on the end application of textile materials. For underwear textiles, PCM with a phase-change temperature near skin temperature is appropriate. On the other hand, for lining material of a ski suit, a much lower phase-change temperature of PCM is required . This chapter provides a review of the PCMs for applications in textile fields. Concepts of thermal comfort, clothing for cold environments, PCMs and thermo-physiological comfort are discussed here and different types of PCMs are presented. This is followed by an account of the incorporation of PCM in the textile substrates. Some of the applications of PCM incorporated textiles are introduced. Finally, the chapter concludes with a discussion of some of the challenges for PCMs in textile applications.
Thermal comfort or discomfort is defined as the mental satisfaction or dissatisfaction respectively with the thermal environment. Thermal dissatisfaction may be due to the warm or cool discomfort of the body which is expressed by the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) indices. Thermal dissatisfaction may also be caused by local discomfort due to the unwanted cooling or heating of one particular part of the body . Thermal comfort/discomfort engendered by textile materials in direct contact with the skin is a complex phenomenon and depends on the integration of the following groups of sensations: thermal (warm or cold), wetness and tactile (contact). Weather conditions (humidity, temperature, wind, etc.), levels of physical activity, the physical and physiological status of individuals, and the properties of textile materials all influence the level of perceived sensations . The human body attempts to maintain a core body temperature around 37°C. The balance between the heat production by the body and loss of the same is known as the comfort factor. The body will be in a state of comfort when the body temperature is about 37°C and there is no perspiration on the skin surface . For predicting or evaluating the thermal environment, clothing has an important role to play because it determines how much of the heat generated by the human body can be exchanged with the environment [12,13]. There are many studies that have investigated the relationship between clothing properties and thermal environment [14–17]. In a cold environment, thermal insulation is required in order to ensure that the body is sufficiently warm while resting. During extensive physical activity, body temperature increases with enhanced heat production, and the body will perspire in order to withdraw energy from the body by evaporation of sweat. Overall thermal balance may be attained at numerous combinations of the thermal environmental parameters and clothing properties, even at high atmospheric humidity . The transport of dry heat through textiles is a complex process involving conduction, convection and radiation. The dry heat loss, Hdry, from the skin can be calculated from the heat balance equation  as follows:
where M is the metabolic rate, W is the external work, Cres is the convective respiratory heat loss, Eres is the evaporative respiratory heat loss, E is the evaporatory heat loss from the skin and S is the change in body heat content. All values in equation [7.1] are expressed in W.m− 2 . A heat balance between the heat produced by human body with heat exchange between the body and the environment maintains the human body temperature . Heat loss through respiration is only around 10% of the total heat loss from the body; therefore clothing plays an important role in the thermal sensations of the human body . In addition to its thermal protection role, clothing also influences the metabolic rate, and therefore plays a prominent role in overall heat balance. Lee and Choi stated that the body’s metabolic rate is lower with heavier clothing than with lighter clothing .
Heat loss through perspiration evaporation, and heat and water vapor flux through textiles are very important for the comfort of warm weather clothing. Perspiration is transformed into water vapor on the skin surface, and evaporation of moisture from the skin surface through clothing is extremely effective for the removal of body heat when the environmental temperature is greater than skin temperature. On the other hand, thermal insulation is one of the important factors for cold weather clothing. Besides insulation, cold weather clothing should ideally have three main features: it should be perspiration permeable, windproof and waterproof. Two types of fabrics are used in foul weather clothing, namely impermeable fabrics and breathable fabrics. An impermeable fabric is both wind- and waterproof but not perspiration permeable. In contrast, a breathable fabric meets all the features of foul weather clothing and it is perspiration permeable . In cold environments, both general body cooling and local cooling have a negative affect on human work performance due to thermal discomfort and, in more extreme cases, as a result of cold injury. Most probably, the insulation of different body parts is influenced by skin temperatures . Thus, to prevent cooling of the peripheral body parts, clothing insulation is very important for cold weather clothing.
Generally, material may exist in three states: solid, liquid and gas. In the phase-change process, a material converts from one state to another and vice versa. Three kinds of phase change of PCM may occur: (a) solid to liquid, (b) liquid to gas and (c) solid to solid. Relatively few solid–solid PCMs have been identified for the TES system. Due to large volume changes during phase transformation, liquid–gas PCMs are usually not considered for practical applications. Solid–liquid PCMs are useful as TES materials, as they store relatively large quantities of heat over a narrow temperature range, without corresponding significant volume change [23,24]. The modes of heat transfer are strongly dependent on the phase of the substances involved in the heat transfer processes . For substances that are solid, conduction is the predominant mode of heat transfer. For liquids, convection heat transfer predominates, and for vapors convection and radiation are the primary modes of heat transfer. Heat is absorbed or released during the solid–liquid and vice versa phase-change process of PCMs (Fig. 7.1). This absorbed or released heat content is called latent heat. The latent heat storage PCM can store large amounts of heat with only a small temperature swing as compared to a sensible heat storage system . Small temperature differences between storage and retrieval cycles, small unit sizes and low weight per unit storage capacity are advantages of PCMs for the TES system .
PCMs exist in various forms in nature. The most common example of a PCM is the water which crystallizes as it changes from liquid to solid (ice) at 0°C. A phase change also occurs when water is heated to a temperature of 100°C when it converts to steam [6,28]. The PCM, which can convert from solid to liquid and vice versa is used as latent heat storage material for the manufacturing of thermo-regulating textiles. Every material absorbs heat during a heating process while its temperature rises constantly. The heat stored in the material is released into the environment through a reverse cooling process. During the complete melting and crystallization process, the temperature of the PCM as well as its surrounding area remains almost constant . During the solid–liquid phase-change process, the PCM absorbs large quantities of latent heat from the surrounding area, and during the reverse phase-change (liquid to solid) process the PCM releases heat to the surrounding area. Therefore, PCMs should be able to repeatedly convert between solid–liquid and vice versa phases to utilize their latent heat of fusion/crystallization to absorb, store and release heat during such phase conversion cycles. The high heat transfer during melting and crystallization processes without significant temperature change makes PCM an interesting candidate for making thermo-regulating textiles. The thermal performance of PCMs can be evaluated by differential scanning calorimetry (DSC) (Fig. 7.2). Simplicity, speed and economy in its sample requirements are the advantages of DSC. The main disadvantage of DSC is that the result obtained is not absolute and it provides relative results .
The thermal comfort provided by clothing primarily depends on the physical activity engaged in, and on the surrounding conditions, such as temperature, relative humidity and wind. The quantity of heat produced by human body depends heavily on the nature of the physical activity involved and can vary from 100 W while resting to over 1000 W during maximum physical activity . The quality of insulation for a garment against cold is largely governed by the thickness and density of its component fabrics. Greater thickness and lower garment density improve insulation due to the air gaps between the garment layers. However, a garment made from a thick fabric will have greater weight, and the freedom of movement of the wearer will be restricted. People wearing heavy clothing in a cold climate need to adjust it frequently to cope with changes in activity and weather, which often causes thermal discomfort . Constant discomfort while wearing such protective garments can lead to reduced work efficiency and the likelihood of accidents. To improve the thermal performance of textiles, clothing with thermo-regulating properties are widely used and based on the application of PCM in textile structure. PCM is a highly effective thermal storage system, which can be used to absorb heat released by the human body under excessive physical activities and stored energy by PCM can be released to keep the body warm when the body/surrounding temperature decreases . The ability of the PCMs to change their physical state (solid to liquid or vice versa) within a certain temperature range makes them ideal for thermo-regulating textile applications. Suitable thermo-regulating properties of textiles depend on the PCM quantity applied to the active-wear garment with the level of physical activity and the duration of the garment use. PCM incorporated textile will be a dynamic and active thermo-responsive substance for some end applications .
A classical example of PCM is paraffin wax. Another common PCM is water. Higher hydrocarbons like hexadecane, octadecane, and so forth, have been extensively used as PCMs. Control of the melting point temperature and high latent heat are desirable for a good PCM, and the target temperature will depend on the end application . PCMs can be classified into three major categories: (a) inorganic compounds, (b) organic compounds and (c) eutectic compounds of inorganic–inorganic, organic–organic or i norganic–organic. Salts, salt hydrates, metals and their alloys are some examples of inorganic PCM compounds, whereas paraffin waxes, polyethylene glycols (PEGs) and fatty acids are examples of organic PCMs [23,34]. In most cases inorganic PCMs are cheaper than the organic PCMs. However, reliable data and information of life span, stability, toxicity and corrosion of inorganic PCMs are often unavailable . A brief account of the advantages and disadvantages of organic and inorganic PCMs are tabulated in Table 7.1. A wide spectrum of PCMs are available with different heat storage capacity and phase-change temperature as reported in Table 7.2.
Advantages and disadvantages of organic and inorganic PCMs 
|Type of PCMs||Advantages||Disadvantages|
|Organic||Most organic PCMs are non-corrosive, chemically stable, high latent heat per unit weight, low vapor pressure, exhibit little or no subcooling||Low thermal conductivity, high changes in volume as compared to inorganic PCM on phase-change cycles, flammability|
|Inorganic||High latent heat per unit volume, high thermal conductivity, non-flammable, low in cost as compared to organic PCMs||Corrosive to most metal, suffer from decomposition and subcooling, higher packing cost|
Hydrated salts are attractive PCMs due to their high volumetric storage density (~ 350 MJ/m3), relatively high thermal conductivity (~ 0.5 W/m.°C) and relatively low cost as compared to the organic PCMs [35,43,44]. Hydrated inorganic salts with ‘n’ molecules of water usually has a heat-absorbing and heat-releasing temperature interval of about 20–40°C, which can be used for the manufacturing of thermo-regulated textiles. Hydrated sodium sulfate produces or absorbs heat by the following chemical reaction between the decahydrate crystal and the water solution :
Melting and super cooling of most inorganic salt hydrates applied to fabrics withstands several heating/cooling cycles. Salt hydrate PCMs have low material costs, but high packing costs .
Hydrophobic linear hydrocarbon is becoming more and more attractive as a PCM (by-product from oil refining). This material has a general chemical formula of CnH2n + 2. The melting and crystallization temperature of n-alkane depends on the number (odd or even) of carbon atoms in its molecular structure [46,47]. In solid state, pure n-alkanes form a single crystal or four crystal system, namely hexagonal (α-phase), rhombic (β-phase), monoclinic (γ-phase) and triclinic (δ-phase), and the system depends on the temperature and the number of carbon atoms in its molecular structure . The melting temperature of hydrocarbon increases with the number of carbon atoms (n) in its molecular structure (Table 7.3). A wide range of latent heats, melting points, densities and specific heat of n-alkanes offers a good choice for low-temperature TES applications .
The repeating unit of PEG is oxyethylene (–0CH2CH2–), with either end of the chain containing a hydroxyl group. These water-soluble semi-crystalline polymers are another important candidate of PCMs for making thermo-regulating textiles. The melting temperature of PEG is proportional to the molecular weight, when its molecular weight is lower than 20 000. Its melting point temperature is in the range of 35–63°C when its molecular weight varies from 1000 to 20 000 [7,53]. The heat of fusion also increases with increasing molecular weight of PEG (Fig. 7.3) . Analysis of the molecular weight of PEG on its melting point and heat of fusion revealed that there is an increased tendency for higher molecular weight PEGs to form in the crystalline phase due to their lower segmental mobility and more convenient geometrical alignment. In the cooling cycle, a higher molecular weight PEG causes an increase in the solidification temperature and heat of crystallization. The advantage of using PEG blends as compared to its pure form is related to the opportunity of changing the phase transition temperature range and heat content associated with melting/crystallization cycles .
7.3 Influence of molecular weight on heat of fusion of PEG .
Melting temperatures of fatty acids (capric, lauric, palmitic and stearic) are in the range of 30–65°C, and their heat of fusion is in the range of approximately 153–182 kJ/kg as reported by Feldman et al. . The mixture of 65 mol% capric acid and 35 mol% lauric acid (C–L acid) having a melting temperature of 18°C can be used as potential thermal storage material for textile applications . Polyethylene paraffin compound (PPC) which consists of paraffin as a dispersed PCM and a high density polyethylene (HDPE) as a supporting material is reported as a stable PCM. The melting temperatures and latent heat of fusion for composite PCMs are 37.8–55.7°C, and 147.6–162.2 kJ/kg respectively . A new PCM for low-temperature heat storage which consists of a stable mixture of water with water-soluble polymerized and cross-linked monomer such as acrylamide is described by Royon et al. . The melting and thawing temperature of this new PCM is 0°C and latent heat is 292 kJ/kg .
One of the major disadvantages of the use of latent heat storage materials is the useful life of a PCM’s container system and the number of cycles it can withstand before degradation of its functional properties . Before application of PCM on the textile substrate, PCMs need to be kept in a very small container to protect them while in a liquid state. The microcapsules should be resistant to mechanical action, heat and most types of chemicals. Effective microencapsulation of PCM is defined by particle size, thickness of shell wall, thermal capacity and conductivity, durability, and so forth . It is important to retain as high enthalpy as possible for the microcapsule, whereas the microcapsules should not coalesce during thermal cycles of melting and crystallization . Therefore, a higher core to shell ratio is required to improve the thermo-regulating efficiency of PCM on textiles substrate . In most cases, these microcapsules possess approximate diameters of around 40 μm.
Microencapsulated PCMs (MicroPCMs) are colloidal particles composed of a protective polymer shell and one or more PCMs in the core [8,59–61]. Encapsulation is one of the most important parameters for the effective utilization of PCM, and has a significant influence on the formation of capsules and thermo-physics performance . Microencapsulation is opening up new market opportunities for performance textiles. In microencapsula-tion, tiny particles (small particle with few micrometer) of solids or liquids are surrounded by a coating (Fig. 7.4). The microcapsules are produced by depositing a thin polymer coating on small solid particles or liquid droplets, or on dispersions of solids in liquids .
With PCM microencapsulation, the ultimate aim is not only to make PCMs easier and safer to handle – but also to reduce reactivity, to improve thermal properties by increasing the heat transfer area and permit the core material to withstand frequent changes in the volume of the storage during the phase-change cycles [35,64,65]. The solid–liquid and vice versa phase-change temperature and the latent heat associated with phase changes are strongly dependent on the encapsulation ratio, and influence the microcapsules’ practical applications . Physical or chemical techniques may be employed to prepare microencapsulation production of PCM. Physical methods include mainly spray drying or centrifugal and fluidized bed processes which are not able to prepare microcapsules smaller than 100 μm . Many chemical methods are developed for microencapsulation of PCM, including interfacial polymerization , suspension polymerization , emulsion method , in situ polycondensation [66,68] and complex coacervation . In a recent study, nanoencapsulation of PCMs using ultrasonic technique and mini-emulsion in situ polymerization have been reported by Fang et al. . The core material is made into droplets and the capsule shell reactive monomers polymerize on the surface of the droplets. When the initially formed oligomers are insoluble at the interface of the droplets, they grow, and a thin monolayer membrane forms around the droplets. The polymerization leads the monolayer membrane to become a microscopic shell around the PCM droplets . Polyurea, polyurethane (PU), polyester, polyamide, polyethylene and amine resin can be used as the shell materials for PCMs.
PCMs can be incorporated into the textile structure by coating application. A typical coating composition includes wetted microsphares containing PCMs dispersed throughout the polymer binder, surfactant, dispersant, anti-foaming agent and thickener. In order to prepare the coating composition, microsphare containing PCMs can be wetted and dispersed in a dispersion of water solution containing surfactant, dispersant, antifoaming agent and polymer binder such as PU or acrylic mixture . The coating paste can then be applied to the textile substrate by a suitable coating method. There are various available coating methods, including knife-over-roll, knife-over-air, pad-dry-cure, gravure, dip coating, transfer coating, and so forth .
In order to improve the thermo-physiological wear comfort of non-woven protective garments, PCM could be incorporated into the thin polymer film and applied to the inner side of the fabric system by lamination . The quality of lamination primarily depends on the strength of adhesion of films to the textile substrate, which may be lost due to mechanical and/ or extreme environmental conditions . PCM laminated textile can be applied in chemical protective suits. Beside chemical protective suits the PCM laminates can also improve the thermo-physiological wearing comfort of other protective garments made of non-wovens such as surgical gowns, uniforms or garments worn in clean rooms . Prior to the lamination, PCM microcapsules can be mixed into a water-blown PU foam mixture. An excellent honeycomb structure is formed during water evaporation. Still air entrapped in the honeycomb structure increases the passive insulation which is an added advantage of the PCM–PU foam laminated system .
MicroPCM can be incorporated into the textile fibers. Micro PCM can be added to the polymer solution before the spinning process starts. The PCM incorporated fibers are able to absorb heat, and store this heat for long periods of time . The composition and properties of sheath/core composite polypropylene fiber non-wovens with different PCM contents in the core have been investigated by Zhang et al. The PCM content in the fiber and sheath/core ratio affect the temperature-regulating capability of the fibers . A novel method of sheath–core photo-thermal converted thermo-regulated fibers by using the fiber-forming polymer containing photo-thermal conversion ceramic as sheath and the fiber-forming polymer containing micro PCMs as core is reported by Shi et al. Photo-thermal converted and thermo-regulated fibers have better temperature-regulating capabilities when compared with the control one .
Micro PCMs can be applied on the textile substrate by impregnation or by the exhaustion method with acrylic resin. Treated fabric can then be heat-treated in order to fix the microcapsules on the textile substrate. Significant difference in the number of microcapsules remaining on the fabric was observed by different methods. The number of microcapsules existing on the fabric is higher for the impregnation process than for bath exhaustion (Fig. 7.5) . Onder et al. encapsulated PCM by complex coacervation, and 2.5–4.5 times enhancement of energy absorption capacities of the coacervate added fabrics were observed .
7.5 Scanning electron micrographs of cotton fabrics with microcapsules: (a) applied by bath exhaustion and (b) applied by impregnation method (Reproduced with permission from P Monllor, M. A. Bonet and F. Cases, Characterization of the behaviour of flavour microcapsules in cotton fabrics. European Polymer Journal 43 (2007): 2481–2490 © 2007 Elsevier Ltd ).
PCM incorporated textiles create clothing that keeps the wearer at a comfortable temperature, even in extreme weather conditions, by solid–liquid and vice versa phase-change cycles. PCM incorporated textiles have been used in space, medical, insulation, protective clothing, automotive textile and many more applications . A brief account of the application of PCM incorporated textiles is given in the following paragraphs.
PCMs were first developed in textiles for use in space suits and gloves to protect astronauts from temperature fluctuations while working in space . Phase-change material incorporated textiles keep astronauts comfortable in space due to their thermo-regulating properties .
From the original applications in space, PCMs are now increasingly used in consumer apparel products. The development of PCM technology has made it possible to create fabrics to maintain comfortable temperatures for the wearer without adding significant mass to the bulk of the garment . Active wear needs to provide a thermal balance between the heat generated by the body and the heat released into the environment while engaged in a sports activity to reduce thermal stress. PCM incorporated active wear, lifestyle apparel and outdoor sports apparel can provide an active thermal insulation effect acting in addition to the passive thermal insulation effect of the garment system . Excessive body heat from the wearer can be absorbed by the PCM incorporated apparel during physical activity and released during periods of rest between activities as less heat is generated to keep the human body in a comfortable state. The popularity of cold winter sports such as alpine climbing, ice climbing and recreational skiing has increased the demand for textiles which keep the wearer dry, warm and comfortable .
Micro PCMs can be applicable in automobile textiles such as seat covers. Paraffin PCM is used in automobile textile applications due to the high capacity of its heat storage capability, low toxicity, lack of hygroscopic properties and low cost, and the target temperature range can be obtained by blending. Application of PCM incorporated textiles in car interiors and seat covers can provide superior thermal control to the passengers .
PCMs interact with the microclimate around the human body, and respond to the fluctuations of temperature which may caused by changes in physical activity levels and external environment changes . Therefore, the textiles treated with PCM microcapsules have potential applications in surgical apparel, bedding materials, bandages and other medical textiles products to regulate patient temperatures in intensive care units .
PCM incorporated fabrics can also be applicable in bedding accessories such as quilts, pillows, bed sheets and mattress covers. PCM incorporated textiles could be used to actively control temperature in bed by absorption of body heat (to cool down) and the release of stored energy (to warm up) when required. The PCMs are also used in footwear, especially in ski boots, mountaineering boots, racing drivers’ boots, and so forth . Helmets , fishing waders and firefighters’ suits are some other examples of potential application of PCMs in the textile field.
PCMs found in today’s consumer textile products were originally developed to protect astronauts from extreme temperature fluctuations while working in space [7,76]. There are many challenges facing the use of these smart materials in consumer apparel applications. The use of these interesting materials and their integration into garments requires the development of new types of testing methods and standards. One problem for this kind of smart material is probably its low thermal conductivity [82–86], for example paraffin PCM has thermal conductivity of 0.22 W/m.K as compared to 2–90 W/m.K for graphite powder . Low thermal conductivity decreases the rate of heat storage and release of PCM during the melting and crystallization processes respectively . Hence, thermal conductivity enhancement of core PCM microcapsules and their shell wall may be needed in order to obtain fast response properties of these smart materials with environmental temperature changes. Furthermore, effective microencapsulation of PCM (such as control of size, stability and shell wall thickness of microcapsules), mechanical properties, durability or functionality of developed PCM incorporated textiles in repeated cycles under various conditions, and flammability of organic PCM should also be evaluated. More research in these areas will offer opportunities for material scientists to make PCM ideal for thermo-regulating textiles.
The author would like to acknowledge Elsevier Ltd for providing the required permission to reproduce Fig. 7.5.
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