www.acsaem.org Article Long-Term Autonomic Thermoregulating Fabrics Based on Microencapsulated Phase Change Materials Paula F. De Castro, Sergiy Minko, Vladimir Vinokurov, Kirill Cherednichenko, and Dmitry G. Shchukin* Cite This: ACS Appl. Energy Mater. 2021, 4, 12789−12797 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Microcapsules loaded with n-docosane as phase change material (mPCMs) for thermal energy storage with a phase change transition temperature in the range of 36−45 °C have been employed to impregnate cotton fabrics. Fabrics impregnated with 8 wt % of mPCMs provided 11 °C of temperature buffering effect during heating. On the cooling step, impregnated fabrics demonstrated 6 °C temperature increase for over 100 cycles of switching on/off of the heating source. Similar thermoregulating performance was observed for impregnated fabrics stored for 4 years (1500 days) at room temperature. Temperature buffering effect increased to 14 °C during heating cycle and temperature increase effect reached 9 °C during cooling cycle in the aged fabric composites. Both effects remained stable in aged fabrics for more than 100 heating/cooling cycles. Our study demonstrates high potential use of the microencapsulated n-docosane for thermal management applications, including high-technical textiles, foot- wear materials, and building thermoregulating covers and paints with high potential for commercial applications. KEYWORDS: phase change materials, encapsulation, thermal energy storage, textile materials, capsules 1. INTRODUCTION of both PCMs and the matrices in which PCMs are Phase change materials, PCMs, are materials that absorb and incorporated.5,9−12 Mechanical properties of the micro- release thermal energy when undergoing and/or overpassing encapsulated PCMs (mPCMs) play an important role since their phase change transition temperature. They attracted the shell needs to be flexible enough to overcome the volume attention because of their potential use as thermal energy changes during the solid−liquid transition but also be resistant storage materials and have found niche in applications such as against cracking to cope with the processes during their heat recovery, solar energy storage,1 intelligent and energy integration into the heat storage macroscale systems. Most of efficient building materials, air conditioning, and photovoltaics mPCMs had melamine−formaldehyde (MF) and urea− devices2−4 with a common goal of contributing to more formaldehyde (UF), 13−17 poly(methyl methacrylate) efficient and environmentally friendly energy use. PCMs with (PMMA),18,19 polystyrene (PS),20,21 silica-based-22,23 and solid−liquid phase change are the most practical in terms of silica shells.24,25 The selection of the core PCM depends mainly on the temperature range and application area. Organic latent heat capacity and design of the heat storage device.5 PCMs have been studied and used for many applications They work in such a way that when the temperature reaches because of their high latent heat capacities, noncorrosiveness, the melting temperature (Tm), the PCM absorbs the equivalent nontoxicity, and large melting temperature range. Among heat for melting. Reversely, when the temperature drops the them, n-octadecane, n-hexadecane, and n-eicosane have been PCM starts crystallizing at crystallization temperature (Tc) and in the focus of many studies because of their phase change releases back the heat that was previously stored.6−8 Successful PCMs use and implementation require them to be encapsulated in order to (1) confine the liquid phase during Received: August 16, 2021 the solid−liquid phase transition, (2) prevent their degradation Accepted: October 18, 2021 when exposed to the outside environment, (3) enhance heat Published: October 29, 2021 transfer, (4) avoid supercooling problems, and (5) improve their handling and flexibility of incorporation into different matrices without losing the functional and original properties © 2021 American Chemical Society https://doi.org/10.1021/acsaem.1c02170 12789 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article Figure 1. SEM images of the (a) cotton fabric and (b−d) 2 wt % mPCMs incorporated into fabrics at different magnifications, and (e) 8 wt % mPCMs and (f) 34 wt % of mPCMs. temperature of about 28 °C, 36 °C, and 18 °C, which are 2. RESULTS AND DISCUSSION convenient for targeting human comfort temperature.26−29 Figure 1 shows SEM images of the untreated fabrics (Figure However, these temperature ranges are lower when targeting 1a) and the fabrics impregnated with mPCMs (Figures 1b−f). thermoregulatory response in footwear materials, high- Fabrics were first impregnated with 2 wt % of mPCMs to study technical textiles, and building materials where temperature compatibility between cotton and mPCMs and then the may go above 37 °C. In this study, we have prepared mPCMs content was increased to 8 wt % and further to 34 wt polyurethane (PU) microcapsules of ∼2 μm size loaded with % to explore heat self-regulation efficiency. Figure 1c−f shows n-docosane by the improved methodology previously mPCMs attached to the cotton surface and located between developed by us30,31 with a phase change transition in the the fibers and on the fiber surface. Figure 1e demonstrates range of 40−45 °C and incorporated them into fabrics in order SEM images of the fabrics loaded with 8 wt % of mPCM. Well- to show long-term thermal buffering effect of the micro- dispersed capsules are seen along the surface of the cotton capsules which has not been reported yet. Incorporation of filaments. The textile fibers with 34 wt % of mPCMs in Figure phase change materials into textile has been previously 1f have a thick microcapsule coating with a high content of demonstrated for melamine formaldehyde capsules,32 poly- microcapsules in the fabrics. Figure S1 (see Supporting styrene capsules33 and capsules with SiO2 shell.34−36 However, Information) shows a comparative images of the fabrics at all papers previously mentioned do not demonstrate long-term different impregnation rate. Impregnation of the fabrics with heat uptake/release stability of the capsules and composites mPCMs leads to an increase in the rigidity of the material that and present DSC data for first heat uptake/release cycle. The is visually observed at the corners of the specimens which are main novelty of our research is the long-term cycling stability slightly bent inward. Optical microscope images of untreated of individual capsules during at least 100 heat uptake/release fabric (Figure S2a) and treated fabric with 8 and 34 wt % of cycles as well thermal properties of composite textile materials mPCMs (see Figure S2b,c, respectively) also show a higher by TGA, DSC, and FLIR measurements even after 4 years impregnation level of the textile at higher mPCMs content. (1500 days) of the aging of thermotextiles based on The adhesion stability of PCM capsules is demonstrated in . microencapsulated PCMs. In addition, we show the long- Capsules remain stable on the fabric surface after 24 h of term thermal buffering effect that mPCMs provide to the exposure in a water bath at 22 °C. No weight loss has been hosting matrices by carrying out time-dependent temperature found for composite thermoregulating textile after water measurements. treatment. 12790 https://doi.org/10.1021/acsaem.1c02170 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article Characteristic mPCMs peaks30 appear in the FTIR spectra the fabrics decrease: bending vibration of the −OH from the of the impregnated fabrics (Figure 2) and their intensity carboxylic acid radicals at 3400−2400 cm−1; stretching vibration of the −CH of the aldehydes that appear in the range of 2850−2750 cm−1; −CH bending at 1369 cm−1; −OH in-plane bending at 1335 cm−1; −CH wagging at 1314 cm−1; and the stretching vibrations from −CO at 1300−1101 cm−1. The difference between peaks in impregnated structure comparing to initial fabric and mPCM within the 1250−750 cm−1 range is associated with the interaction between OH and CO groups of fabric and CN and NH group of mPCM shell stabilizing microcapsules inside fabric network. Melting tests were visually done to determine the stability of the impregnated fabrics. Selected fabrics impregnated with 34 wt % of mPCMs and a reference prepared by impregnating 34 wt % of pure n-docosane were heated above 50 °C for 30 min and then cooled down to room temperature. The impregnated structures with mPCMs (encapsulated n-docosane) demon- strate a remarkable stability compared to those ones with directly immobilized n-docosane. As it is shown in Figure 3a,b, the initial appearance of the impregnated fabrics with 34 wt % mPCMs remains stable while, as depicted in Figure 3d, fabrics Figure 2. FTIR spectra of fabrics where (a) untreated fabric, (b) treated with pure n-docosane show the release of n-docosane fabric with 8 wt % mPCMs, (c) fabric with 34 wt %, and (d) mPCMs. from textile matrix. Thermal stability of the thermocontrolled fabrics was also increases when comparing fabrics with 8 and 34 wt % loading. examined by thermal gravimetric analysis (TGA) (Figure 4). These peaks are asymmetric and symmetric stretching The untreated fabric shows weight increase of 5.4 wt % at vibrations of the −CH from methylene group at 2952 and about 86.7−326 °C due to moisture adsorption from air. The 2915 cm−1; stretching vibration of the −NCO appears at 2295 first main decomposition step takes place at about 326−345.2 cm−1; stretching vibration of free and bonded urethane groups °C with a weight loss of 74.5 wt % followed by the second step (−CO) at 1732 and 1709 cm−1, respectively; stretching that occurs in the temperature range of 345.2−486.3 °C with vibration of the aromatic ring of the shell (−CC) at 1594 26 wt % weight loss, which is the result of thermal degradation cm−1, peak corresponding to the −CO amide region at 1541 and oxidation of untreated fabric below 320 °C in air. cm−1, stretching vibration of −CN and deformation of Microcapsules, mPCMs, have two main decomposition steps. −NHassoc), 1509 cm−1, 1471 cm−1; asymmetric deformation The first one occurs in the range of 239.7−270 °C which of the − CH3 at 1411 cm−1; deformation of the NHassoc and involves a weight loss of 48.6 wt % related to the stretching vibration of the −CN at 1305 cm−1; and the peak decomposition of the capsule shell followed by the second that corresponds to the rocking vibration of the methylene one that takes place at about 270−561.5 °C with a total weight group at 717 cm−1. At the same time, peaks corresponding to loss of 49.7 wt % related to the decomposition of encapsulated Figure 3. Images of fabric with 34 wt % mPCMs (a) before and (c) after being heated above 50 °C, and of the fabrics with 34 wt % of pure n- docosane (b) before and (d) after being heated to 50 °C. 12791 https://doi.org/10.1021/acsaem.1c02170 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article appears much smaller for the fabrics with 8 wt % of mPCMs. The same tendency is observed in the DTA signal (Figure 4c); an exothermic process is observed at the same temperature interval for mPCMs and for the fabrics impregnated with 34 wt % mPCMs. The starting temperatures for the second and third main decomposition steps take place in the same temperature range as the first decomposition of the untreated fabric but also at the same temperature of the second decomposition of the mPCMs overlapping of processes. We performed DSC measurements to determine the heat capacity of the impregnated structures. Figure 5 presents DSC Figure 4. Thermogravimetric analysis measurements of impregnated and pure fabrics: (a) TG signal, (b) DTG signal, and (c) DTA signal. docosane.30 The fabrics impregnated with 8 wt % of mPCMs show a small decomposition step at about 24−242.3 °C total weight loss of 7 wt % due to water desorption. Then, the first main decomposition takes places at about 323.3−340.4 °C Figure 5. DSC thermograms for cotton fabric systems where (1) pure with a total weight loss of 65 wt % and the second one takes mPCMs, (2) untreated fabrics and loaded fabrics with (3) 8 wt % place in the range of 340.4−508.7 °C with a total weight loss of mPCMs and (4) 34 wt % mPCMs. 28.3 wt %. The first main decomposition peak is related to the degradation of fabric matrix (very similar for TGA data of pure measurements for the pure fabrics and for fabrics with fabric) while the second demonstrates oxidation of the mPCMs. The mPCMs used for the impregnation of the encapsulated docosane. It is interesting that no separate fabrics have a melting temperature of Tm = 47.2 °C and degradation peaks were observed for the shell of the embedded melting enthalpy of ΔHm = 76 J·g−1 on heating, while the capsules, which indicates stabilization of the capsule shell of crystallization temperature is centered at Tc = 34.6 °C with a the fabric matrix, and the degradation of the capsule shell shoulder at 26.5 °C, associated with a crystallization enthalpy occurs in the same temperature range as for pure fabric. Fabrics of ΔHc = 76 J·g−1 on cooling. Both heat uptake and release impregnated with 34 wt % of mPCMs show a similar energy is the same, demonstrating stable heat uptake/release decomposition profile with a weight loss of 2.7 wt % and in cycling. the range of 88.8−234 °C. However, the first main The pure cotton fabrics exhibit no thermal events. DSC decomposition takes place in the range of 234−272.4 °C thermograms of the fabrics impregnated with 8 and 34 wt % of with a total weight loss of 12.2 wt % associated with the mPCMs show similar curves to those ones obtained for pure degradation of the capsule shell not strongly bounded to the mPCMs which means the microcapsules attain latent heat fabric fibers. The second one in the range of 272.4−342.3 °C capacity to the impregnated fabric network. The enthalpies of that involves a loss of 46.2 wt % is associated with the impregnated fabrics increase with the increase of mPCMs decomposition of the fabric and main part of the capsule wt % content. Impregnated fabrics with 8 wt % mPCMs show shell, and the third one takes place at about 342.3−517.9 °C melting temperature Tm = 44.3 °C and melting enthalpy ΔHm with a total weight loss of 33.9 wt %, demonstrating = 6.3 J·g−1 while the crystallization temperature and enthalpy decomposition of encapsulated PCM. Table S1 contains are Tc = 37.6 °C and ΔHc = 7.7 J·g−1. There are several TGA data with standard deviations. TGA weight losses can endothermic contributions during crystallization that take be also seen in the derivative signal (Figure 4b) that also shows place above and below Tc at 43 °C (small peak contribution) a clear peak contributing to mPCMs as well as to fabrics and at 28 °C related to the different crystalline phases of n- impregnated with 34 wt % of mPCMs. This peak contribution docosane.31,37,38 The fabrics impregnated with 34 wt % 12792 https://doi.org/10.1021/acsaem.1c02170 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article Figure 6. Dynamic heat storage measurements for (a) untreated cotton fabrics and treated cotton fabrics impregnated with 8 and 34 wt % of mPCMs, heating cycle; (b) untreated cotton fabrics and treated cotton fabrics impregnated with 8 and 34 wt % of mPCMs, cooling cycle. Results for pure mPCM are shown for comparison. mPCMs show melting temperature and melting enthalpy Tm = in temperature of just 1.1 °C compared to the one with 8 wt % 45.6 °C and ΔH m = 26.3 J·g−1, respectively, and a even though the content of microcapsules was 4 times higher. crystallization temperature and melting enthalpy Tc = 37 °C The cooling cycles show similar energy saving efficiency for and ΔHc = 27.4 J·g−1, respectively. The obtained average latent the composites impregnated with high quantity of the mPCM. heat values for the impregnated fabrics, ΔHm = 6.3 J·g−1, ΔHc Figure 6b demonstrates the cooling cycle for composites with = 7.7 J·g−1 (8 wt %) and ΔHm = 26.3 J·g−1, ΔHc = 27.4 J·g−1 the tendency for increasing thermal storage at a higher level of (34 wt %) were used to calculate the mPCMs content resulting capsule impregnation. In the very beginning of the cooling (up in 8.2 and 34.6 wt %, respectively, which is in accordance with to 1 min), the temperature drops abruptly with no clear the initial impregnation calculations for mPCM-fabric samples. difference between the samples because of the fast cooling rate. Table S2 contains DSC data with standard deviations. However, after 1 min of cooling, crystallization of PCM starts In the next step, we quantified the temperature reduction at about 36 °C and impregnated fabrics demonstrated higher caused by the mPCMs in the fabrics by dynamic heat storage temperatures than the ones without mPCM. After 7 min of measurements (see Figure S4) that consisted of heating the cooling samples with high concentration of mPCM (34 wt % samples up using an IR lamp (lamp on) for 30 min and then for textile fabrics) exhibited effective energy capacity keeping cooling them down (lamp off) for another 30 min with the 10 °C temperature difference comparing to pure host measurements of temperature decrease FLIR T640 thermoca- materials. All samples went back to room temperature after mera. Figure S4 illustrates the setup for the dynamic heat 20 min of the cooling period (50 min of the heating/cooling storage measurements during the heating step: a thermocouple cycle). The curve obtained for the pure mPCMs that was used was connected to a recording system and placed in between also as reference (gray line), showing the mPCMs with the two layers of the sample specimen and the surface temperature highest temperatures as well as the slowest temperature was recorded over time of the heating for 30 min. Cooling decrease due to the stored amount of heat that is being cycle (lamp was switched off for another 30 min) was released during the cooling process. monitored by FLIR T640. DSC and dynamic heat storage measurements clearly Figure 6a shows the time-dependent temperature profiles for demonstrate that when the mPCMs are heated they absorb over 30 min of heating and cooling cycles for the impregnated fabrics with 8 and 34 wt % mPCMs and the untreated fabric the thermal energy equivalent to their heat capacity and used as a reference. The obtained curves for the heating cycles undergo phase change transition from solid to liquid state show a temperature gap for capsules’ impregnated fabrics as which corresponds with the exothermic event in DSC (heating compared with the pure one. In the beginning of the heating (1 cycle). This phase change transition produces a reduction of min), the heating rate is fast. After several minutes at a the temperature increase when the mPCMs are present in the temperature above the starting melting temperature of the n- fabrics as a consequence of the PCM up-take of thermal docosane, Tm,onset = 37 °C and within its phase change energy. Reversibly, when the mPCMs are cooled down they temperature range (37−45 °C) the heating rate is drastically release the previously stored heat while transitioning from lowered and temperature difference can be clearly seen liquid to solid state which corresponds to the endothermic between the capsules’ impregnated fabrics and the untreated peak in DSC (cooling cycle). This is in accordance to what has cotton fabrics. When PCM melting temperature is reached, the been observed in the dynamic heat storage measurements, that PCM is absorbing the heat during the phase change from solid is, when the impregnated structures show higher temperatures to liquid state leading to a delay and reduction of the peak than the pure structures due to the releasing of the heat. temperature. This temperature gap is still observed at the end The FTIR spectra were also obtained to determine whether of the heating cycle (until 30 min, Figure 6a). Fabrics with 8 wt the chemical nature of the impregnated fabrics was modified or % of mPCMs show considerable reduction of peak temper- not by their exposure to the IR source. As depicted in Figure 7, ature of ∼5.2 °C compared to the untreated fabrics while the the impregnated fabrics show neither a peak modification nor impregnated fabrics with 34 wt % of mPCMs showed 6.3 °C of decreased intensity. The process of up-taking and releasing of temperature reduction. It is noticeable that impregnated fabrics thermal energy by the mPCMs-impregnated structures is with the highest mPCMs content of 34 wt % show a reduction reversible and repeatable over a 100 cycles as we previously 12793 https://doi.org/10.1021/acsaem.1c02170 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article Moreover, we tested the energy saving efficiency of 4 years (1500 days) old fabrics with and without mPCM. Samples were stored at room temperature in the dark for more than 4 years and then their thermal uptake and release efficiency was tested (Figure 8b). At the heating step, pure aged fabric absorbed more heat leading to the increase of temperature by 3−4 °C comparing to the new fabric. The temperature of the aged fabrics with embedded capsules remained within the same range as for new samples (Figure 8a). As a result, the average temperature difference increased to about 14 °C but still remains stable for over 100 heating/cooling cycles. Similar observations were found for fabrics during the cooling step. An aged pure textile cooled down faster than new one with 2−3 °C temperature difference while the temperature of the fabrics with capsules did not change. The total temperature difference increased to 9 °C stable for over 100 cycles. Four years of storing fabrics with impregnated PCM capsules did not Figure 7. FTIR spectra (a,c,e) before and (b,d,f) after dynamic heat decrease heat control performance of the composite textile storage measurements for (a,b) pure fabrics, (c,d) fabrics impregnated materials. with 8 wt % of mPCM, and (e,f) fabrics impregnated with 34 wt % of mPCM. 3. CONCLUSIONS In summary, we impregnated cotton fabrics with n-docosane loaded polyurethane microcapsules (mPCMs) for thermal reported for highly stable PCM microcapsules for over a 100 energy storage by simple dipping of the textile fabrics into the heating and cooling cycles.31 microcapsule suspension. We demonstrated the temperature Fabrics impregnated with 8 wt % of the mPCM showed control during heating and cooling of the modified textile almost similar heat capacity as fabrics impregnated with 34 wt fabrics stable for more than 100 heating/cooling cycles. We % of the capsules; the same as for heating cycle. We continued characterized the morphology, latent heat capacity, and our cycling and aging stability research with impregnated thermoregulating properties of the impregnated structures. cotton fabrics samples with 8 wt % loading of mPCM to avoid SEM images showed that mPCMs were successfully located overloading of the host material with capsules. Figure 8a inside textile network before and after 24 h washing in water. demonstrates stability of the temperature control for fabrics Impregnated structures showed thermal stability up to 190 impregnated with 8 wt % of the capsules. During the heating °C and high heat capacity. The observed heat capacity for step, thermoregulating fabrics continuously keep about 11 °C fabrics impregnated with 8 wt % of mPCMs was 6.3 J·g−1 of temperature reduction for over 100 heating/cooling cycles. On the latent heat which provided 11 °C of temperature buffering the cooling step, fabrics with 8 wt % capsules retain effect during heating as compared to nonimpregnated fabrics approximately 6 °C temperature increase for over 100 cycles for over 100 heating/cooling cycles, while the one with 34 wt after switching off the heating source. This confirms % of mPCMs had 26.3 J·g−1 of latent heat and exhibited 12 °C sustainable effect of the incorporated capsules on the temperature buffering. On the cooling step, impregnated autonomic thermal response of the fabrics. fabrics demonstrated 6 °C temperature increase for over 100 Figure 8. Cycling heat storage measurements for (a) freshly prepared, untreated cotton fabrics, and treated cotton fabrics impregnated with 8 wt % mPCMs at the 30th minute of the heating cycle and 10th minute of the cooling cycle; (b) 4 year old untreated cotton fabrics and treated cotton fabrics impregnated with 8 wt % mPCMs at the 30th minute of the heating cycle and the 10th minute of the cooling cycle. 12794 https://doi.org/10.1021/acsaem.1c02170 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article cycles after switching off the heating source. Similar thermo- Thermogravimetric analysis (TGA) experiments evaluated thermal regulating performance was observed for impregnated fabrics stability and decomposition behavior of the fabrics. Thermogravi- stored for 4 years at room temperature and ambient metric analysis was undertaken using a TGA STA PT1000 instrument conditions. Temperature buffering effect increased to 14 °C from Linseis (Germany). Measurements were taken from room during heating cycle and temperature increase effect reached 9 temperature up to 950 °C with a ramp of 10 °C·min−1 under air atmosphere and repeated three times for each sample. °C during cooling cycle. Both effects remained stable in aged Differential scanning calorimetry (DSC) was used to determine fabrics. FTIR spectra showed no change in the chemical latent heat storage properties. DSC measurements were carried out in morphology after dynamic heat storage measurements the range of −40 to 100 °C under nitrogen atmosphere with a 10 °C· indicating thermal and structural stability of the samples. min−1 ramp, using a DSC 204 F1 model from NETZSCH (Germany) Energy microcapsules are promising materials for various and repeated three times for each sample. thermal-regulating applications including high-technical tex- Dynamic heat storage performance of the impregnated fabrics was tiles, apparel, building materials, and paints. evaluated by heating and cooling cycles from 20 to 55 °C using an IR lamp (150 W) to imitate solar radiation on the heating step. The 4. EXPERIMENTAL SECTION heating/cooling cycles were carried out by switching on the IR lamp for 30 min and then switching off for another 30 min for the cooling 4.1. Microcapsule Preparation. The microencapsulated PCMs cycles. The environmental temperature was 19 °C, and the (mPCMs) were prepared by miniemulsion interfacial polymerization dimensions of the sample specimens were 1.5 cm × 1.5 cm. For following the procedure previously developed by us.30 PU shell was that purpose, a thermocouple sensor, with a resolution of 0.03 °C selected for their outstanding thermal stability and proven elasticity to from 0 to 40 °C and of 0.1 °C from 40 to 100 °C, was placed in overcome temperature and volume changes of the encapsulated PCM between the layers of the fabrics which was connected/plugged to a during heat uptake and release as confirmed in our previous LABQUEST system from Vernier (U.S.A.) to record the time- studies.39−41 Docosane was chosen as n-alkane because of its high dependent temperature profile and to measure temperature profile heat capacity, noncorrosiveness and its transition temperature (Tm) of during heating. Temperature profiles during the cooling step were 37−40 °C suitable to be used in thermoregulating applications as functional textiles. The microcapsules have 2 μm size with melting measured by FLIR T640 w/25° and 15° thermocamera (FLIR and crystallization temperature (Tm and Tc) 47 and 35 °C, Systems Inc., U.K.) because of its high sensitivity during sample respectively, and melting and crystallization enthalpies (ΔHm and cooling. It is not possible to use FLIR instrument on the heating step ΔHc) of 76 J·g−1. Encapsulation yield was 77% of docosane (per because of the external heating exposure from IR lamp. Heat storage capsule), which is close to theoretical capsulations considering shell performance was measured in the cycling regime for fresh samples thickness (79%). High elasticity of PU shell maintains integrity of the and 4 year old samples to demonstrate chemical and environmental capsule load, as shown by DSC measurements.31 The microcapsules stability of the mPCM incorporated into fabrics. ■ were kept in water for their subsequent use for impregnation of fabrics. Resulting microcapsules have ∼3 ± 1 μm size. ASSOCIATED CONTENT 4.2. Impregnation of the Fabrics. A plain weave white cotton fabric with unit weight of 100 g·m−2 (purchased from ZS Fabrics, * sı Supporting Information U.S.A.) was selected as hosts to show the thermoregulating effect of The Supporting Information is available free of charge at the microencapsulated n-docosane in polyurethane shell. Impregnated https://pubs.acs.org/doi/10.1021/acsaem.1c02170. fabrics with mPCMs were prepared by simple dipping of the cotton fabrics (1 cm2) into 10 mL of the 8 or 32 wt % microcapsule solutions Optical images of cotton fabrics impregnated with (prepared by dilution of initial 55 wt % capsule suspension in water) microencapsulated PCMs; SEM images of microPCM for 12 h under mild stirring followed by a drying step at 100 °C for 20 impregnated textiles after explosion to water for 24 h; min in an oven and then cooling down to ambient conditions. This step was repeated with new microcapsule solutions until the desired heat transfer characteristics; setup for dynamic heat impregnation concentration of microcapsules was achieved in the storage measurements (PDF) ■ fabrics. Thermoregulatory properties are influenced by the mPCMs content in the fabrics; therefore, two different concentrations of the PCMs were studied: (1) the first one of about 8 wt % and (2) the AUTHOR INFORMATION second composition with higher content of mPCM of about 34 wt % to obtain textiles with higher energy storage capacity. The amount of Corresponding Author mPCM in fabrics was experimentally measured by the ratio between Dmitry G. Shchukin − Gubkin University, 19991 Moscow, melting enthalpy of fabrics with mPCM to the melting enthalpy of Russia; Stephenson Institute for Renewable Energy, University pure mPCM from DSC data. of Liverpool, Liverpool L69 7ZF, United Kingdom; 4.3. Characterization. The morphology of the impregnated orcid.org/0000-0002-2936-804X; Email: d.shchukin@ fabrics was determined using a JEOL JSM-7001F (Japan) scanning liverpool.ac.uk electron microscope (SEM). Samples were prepared by placing composite samples onto carbon tape on an aluminum SEM stub and Authors further coating with chromium. Compatibility and adherence Paula F. De Castro − Leitat Technological Center, 08225 evaluation of the mPCMs to the fabrics were determined by immersing the impregnated structures into a water bath at 22 °C Barcelona, Spain for 24 h and then examining by SEM. Sergiy Minko − Department of Chemistry, University of Chemical characterization of the untreated and impregnated fabrics Georgia, Athens, Georgia 30602, United States; was done using Fourier transform infrared spectroscopy (FTIR) on a orcid.org/0000-0002-7747-9668 Bruker TENSOR II (Germany) instrument equipped with all Vladimir Vinokurov − Gubkin University, 19991 Moscow, reflective diamond ATR. Measurements were taken on transmittance Russia mode with 64 scans from 400 to 4000 cm−1. Kirill Cherednichenko − Gubkin University, 19991 Moscow, Visual stability tests were carried out by heating the samples up to Russia 50 °C, above the melting point of the PCM n-docosane for 30 min and leaving them to cool down. Photos were taken to follow the Complete contact information is available at: release of the PCM through the fabric structure. https://pubs.acs.org/10.1021/acsaem.1c02170 12795 https://doi.org/10.1021/acsaem.1c02170 ACS Appl. Energy Mater. 2021, 4, 12789−12797 ACS Applied Energy Materials www.acsaem.org Article Author Contributions using Different Types of Styrene-Maleic Anhydride as Emulsifier. The manuscript was written through contributions of all Colloid Polym. Sci. 2009, 287, 549−560. authors. All authors have given approval to the final version of (16) Khakzad, F.; Alinejad, Z.; Shirin-Abadi, A. R.; Ghasemi, M.; the manuscript. Mahdavian, A. R. Optimization of Parameters in Preparation of PCM Microcapsules Based on Melamine Formaldehyde through Dispersion Funding Polymerization. Colloid Polym. Sci. 2014, 292, 355−368. ERC Enercapsule project (project 647969), NATO Science for (17) Rochmadi, A. P.; Hasokowati, W. Mechanism of Micro- Peace Program (project G5330), and RSF project (project 19- encapsulation with Urea-Formaldehyde Polymer. Am. J. Appl. Sci. 79-30091). 2010, 7, 739−745. (18) Sari, A.; Alkan, C.; Bicer, A.; Altuntas, A.; Bilgin, C. Micro/ Notes Nanoencapsulated n-nonadecane with Poly (Methyl Methacrylate) The authors declare no competing financial interest. ■ Shell for Thermal Energy Storage. Energy Convers. Manage. 2014, 86, 614−621. ACKNOWLEDGMENTS (19) Wang, H.; Wang, J. P.; Wang, X.; Li, W.; Zhang, X. Preparation We thank ERC Enercapsule project (project 647969), NATO and Properties of Microencapsulated Phase Change Materials Science for Peace Program (project G5330), and RSF project Containing Two-Phase Core Materials. Ind. Eng. Chem. Res. 2013, 52, 14706−14712. (project 19-79-30091) for funding this research. ■ (20) Sari, A.; Alkan, C.; Doguscu, D. K.; Bicer, A. Micro/Nano- Encapsulated n-heptadecane with Polystyrene Shell for Latent Heat REFERENCES Thermal Energy Storage. Sol. Energy Mater. Sol. Cells 2014, 126, 42− (1) Weinstein, L. A.; Loomis, J.; Bhatia, B.; Bierman, D. M.; Wang, 50. E. N.; Chen, G. Concentrating Solar Power. Chem. Rev. 2015, 115, (21) Fang, Y.; Kuang, S.; Gao, X.; Zhang, Z. Preparation and 12797−12838. Characterization of Novel Nanoencapsulated Phase Change Materials. (2) Ma, T.; Yang, H.; Zhang, Y.; Lu, L.; Wang, X. Using Phase Energy Convers. Manage. 2008, 49, 3704−3707. 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