Research PaperThermal performance of electrospun core-shell phase change fibrous layers at simulated body conditions
Introduction
In recent years, the development of environmentally-friendly systems and energy storage devices has led to growth in the global markets of Phase Change Materials (PCMs). PCMs can absorb or release large amounts of energy in the form of latent heat during the phase change processes [1], and therefore they can be applied in thermal energy storage systems to be used in different applications, such as buildings [2], solar systems [3], packaging and transportation [4], [5], and textiles [6], [7]. Textiles, including PCMs, can be applied in diverse products, such as outdoor wears, sportswear, bedding and accessories, and shoes [6]. Thermoregulating textiles containing PCMs can also be used in special applications, including protective vests, automotive and medical applications [7].
One of the main factors influencing thermo-physiological comfort is the creation of a stable microclimate surrounding the skin to support thermal regularity of the body during changes in the ambient temperature or the physical activities of the body. Therefore, clothing plays a major role in the heat balance between the body and the ambient conditions [8]. Thermal insulation of clothing can be achieved by active or passive insulating fabrics. Passive thermal insulation includes common fabrics produced by conventional fibers, such as cotton, wool, polyester and nylon, all of which depend on the amount of air trapped inside the fabric producing a certain amount of thermal insulation. However, active insulation materials, such as PCMs, have a behavior different than that of the conventional insulators. They create a thermoregulating effect as a result of sudden temperature changes. Clothing containing PCMs can absorb heat from the body or the environment during high physical activities or warm environmental conditions, and conversely can release heat during low physical activities or cold environmental conditions [7].
However, loading high levels of microcapsules into fibers or fabric structures faces a number of limitations, resulting in a decrease in the thermal performance of PCMs [9]. Therefore, direct encapsulation of PCMs into fiber structures can be a useful method to increase the insulating capacity of PCMs in final products.
The electrospinning process was initially introduced as a simple and inexpensive method to produce ultrafine fibers ranging from a few nanometers to several micrometers. Electrospun fibrous layers can directly be used as nonwoven fabrics or as coating layers on fabrics [10]. By means of the electrospinning method, PCMs can be embedded into nanofibers and ultrafine phase change fibers (PCFs) in submicron scale can be formed [11]. Over the past decades, ultrafine PCFs with various morphologies and structures were developed by several electrospinning techniques including uniaxial [12], coaxial [13] and multicomponent-jet electrospinning [14]. Compared with uniaxial electrospinning, PCM is completely encapsulated inside the polymeric shell via coaxial electrospinning. Moreover, compared with conventional method to produce PCM-containing textiles, using coaxial electrospinning, only in one step, nanofibrous PCM-containing layers can be produced without any additional processes, such as microcapsulation or fabric finishing. Furthermore, the final properties of the product, such as permeability, porosity, and mechanical properties, such as flexibility, remain unchanged, while the same properties fairly decline once the fabric is coated with the microencapsulated PCM [15], [16], [17]. This technique was first introduced by McCann et al. [13] in 2006. They integrated the melt-electrospinning process with the coaxial setup, and devised melt-coaxial electrospinning to encapsulate alkanes into the PVP shell. Thereafter, several PCM/polymers combinations including PEG/PVDF [18], [19], eicosane/PVDF [20], PEG/CA [21], [22], soy wax/PU [23], octadecane/PVB [24], paraffin wax/PCL [25], PEG/PA6 [26], hexadecane/PU [27], and PEG/PAN [28] was developed as the ultrafine PCFs with core/shell structure.
In addition to the DSC analysis, there are several test methods to evaluate the thermal performance of the clothing containing PCMs. For the first time, in 1995, Pause proposed a method for assessing the thermal performance of the PCM-containing textiles. In this method, the effect of phase changes was evaluated by measuring the dynamic thermal resistance [29]. The total thermal resistance was equal to the sum of the dynamic and the static thermal resistance. Pause method could not perfectly evaluate the thermoregulating fabrics as a constant value was reported for the dynamic thermal resistance. However, the dynamic thermal resistance of the thermoregulating fabrics was not constant and changed over the time during the measurement process. In 2002, another test method was introduced by Hittle and Andre [30]. In this method, the fabric layer was sandwiched between a hot plate and two cold plates. The temperature was kept constant for the cold plates, while the heat flux was kept constant for the hot plate. The temperature regulating factor (TRF) was identified based on the designed system.
In another test method, the fabric sample was placed in a skin simulator as a manikin or a laboratory device. Then, the skin simulator was transferred from the hot to the cold chambers in order to expose the sample to the sudden temperature changes of the environment. Using this method, Shim et al. [31] studied the thermo-manikin device for simulating the heat loss of the body in a cold environment and measured the amount of clothing insulation in fabrics containing micro-PCMs. Yoo et al. [32] applied the same method using a laboratory device to assess the effects of the number and position of the fabrics containing micro-PCMs on the thermoregulating properties of garments. After the specimen reached a steady state at warm conditions, the system was transferred to the cold condition; the results showed increased incorporation of PCM-containing fabrics led to increased heating and cooling effects.
The wear trial test, as a method evaluating the thermal performance of fabrics on human subjects, can be considered as the most accurate method to measure the thermoregulating effects of clothing containing PCMs [33]. However, it is costly and complex because people have different perceptions regarding the thermal comfort in the same condition. Thus, the efficient method is the use of skin simulator devices that can make sudden changes at the ambient temperature.
Wan and Fan [34] presented a new method for simulating the actual conditions of the body. In their method, a hot plate simulated the human body by producing a constant heat flux depending on the human activity. The hot plate was coated with the fabrics containing PCM and then was exposed to the ambient temperature variations. In fact, it simulated the state of a wearer moving from a warm indoor to a cold outdoor environment. The surface temperature and heat loss were recorded to describe the thermal properties of the PCM-containing fabrics. Recently, Safavi et al. [35] have simulated the actual condition considering the metabolism rate of body. Their setup was contained three important parts: (1) the skin simulator capable of producing heat flux proportional to metabolic heat rates, (2) the clothing layer placement frame and (3) the chamber simulating environmental conditions. The ambient temperature could suddenly be changed. Also, the number of different layers and the level of the body activity could be adjusted. The results showed that the system could properly assess the thermoregulating performance of micro-PCMs in the fabrics.
In most previous studies, the thermal performance of the electrospun ultrafine PCFs was investigated only by DSC analysis or simple test methods. McCann et al. [13] designed a simple test to determine the thermoregulating behavior of the nanofibrous samples, in which the glass vial containing 1 cc of 60 °C water was covered with a nanofibrous mat containing PCM and then was cooled in a 4 °C environment. The water temperature was recorded using a thermocouple and were compared with those of the control samples where no PCM was used. That is to say, a number of experiments, such as transferring the samples from the cold to the hot water [36], [37] or to the oil [38] bath, and using a hot stage with heating–cooling thermal cycles [39], were performed to measure the thermal properties of the nanofibers containing PCMs. But, no attempt has been made to evaluate the thermoregulating properties of the electrospun fibrous layers containing PCMs in the body conditions. Therefore, it is essential to study the thermal properties of electrospun ultrafine PCFs at simulated body conditions.
In our previous work, the processing parameters including the type and concentration of the polymer shell, voltage, and core/shell feed rate, were optimized to produce ultrafine PCFs with the core-shell structure [40]. In the present study, we aimed to assess thermal performance of fibrous layers containing ultrafine PCFs at the simulated body conditions. Therefore, firstly the fibrous layers containing ultrafine PCFs were successfully produced considering the earlier optimum conditions. Secondly, thermal performance was evaluated using DSC analysis and dynamic simulated body conditions. And finally, several indices including the mean and maximum temperature difference, the mean gradient temperature, and the cooling rate, were defined and derived from the experimental data to investigate the thermal performance of the produced layers.
Section snippets
Materials
Hexadecane and octadecane were purchased from the Merck as the PCMs with melting points about 18 and 28 °C, respectively. Polyvinylpirrolidon (PVP) with molecular weight Mw = 900,000 g mol−1 (Rahavard Tamin Pharmaceutical Co., Tehran, Iran) was employed as the polymer shell. Ethanol (99.9% purity, Merck) was used as the PVP solvent and n-hexane (purity 99%, Merck) was used as the solvent of hexadecane and octadecane to remove the core components before cross-sectional FE-SEM observations. PVP
Core-shell fiber morphology
Fig. 3 shows the longitudinal and cross-sectional FESEM images of the PVP fibers containing hexadecane and octadecane as the PCM core. As can be seen in Fig. 3a and b, the fibers containing octadecane had a uniform core-shell structure with no bead parts, while the fibers containing hexadecane had a varicose structure (Fig. 3c and d). This difference could be related to the difference in the melting point and the viscosity of two materials. Octadecane with the melting point of 28 °C was in the
Conclusion
A core–shell fibrous layer was produced by means of a coaxial electrospinning system using two types of alkanes, i.e. hexadecane and octadecane, as the PCM core, and the PVP solution as the shell materials. Morphological investigation showed that the types of alkanes had a considerable effect on the core–shell structure. The PVP fibers containing octadecane exhibited smooth cylindrical fibers, while the PVP fibers containing hexadecane exhibited a bead structure. It might be due to the
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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