Reversible thermochromic microencapsulated phase change materials for thermal energy storage application in thermal protective clothing

Applied Energy 217 (2018) 281–294

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Reversible thermochromic microencapsulated phase change materials for thermal energy storage application in thermal protective clothing

T

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Xiaoye Genga, Wei Lia,b, , Yu Wanga, Jiangwei Lua, Jianping Wanga, Ning Wanga, Jianjie Lib, Xingxiang Zhanga a

StateKey Laboratory of Separation Membranes and Membrane Processes, Tianjin Key Laboratory of Advanced Fibers and Energy Storage, School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b TianjinColouroad Coatings & Chemicals Co Ltd, Tianjin 300457, China

H I G H L I G H T S thermochromic micro• Reversible encapsulated phase change materials

• • •

G RA P H I C A L AB S T R A C T Reversible thermochromic microencapsulated phase change materials for thermal energy storage.

(TC-MPCMs) were designed and fabricated successfully. The thermochromic function provided a visual evidence of energy storage or release performance in real time. TC-MPCMs expressed higher than 99% thermal storage capability and excellent cyclic durability performance. TC-MPCMs showed great potential applications in thermal protective clothing and other thermal regulation fields.

A R T I C L E I N F O

A B S T R A C T

Keywords: Reversible thermochromic Thermal energy storage Phase change materials Microcapsule In-situ polymerization

In this study, a series of reversible thermochromic microencapsulated phase change materials (TC-MPCMs), exhibiting excellent latent heat storage-release performance, were designed and fabricated successfully. The characterization and microstructure regulation of TC-MPCMs were conducted systematically as well. The core of TC-MPCMs was comprised of crystal violet lactone employed as thermochromic colorant, bisphenol A employed as developer and 1-tetradecanol employed as co-solvent, respectively. These influencing factors of encapsulation process such as the amount of emulsifier, stirring rate, feeding weight of core/shell ratio, acid resistance and thermal cyclic durability were carried out to clarify the effect of various experimental conditions. The surface morphology, shell thickness and core–shell structure of TC-MPCMs were characterized via optical microscope (OM), thermal field emission scanning electronic microscope (TFE-SEM), transmission electron microscope (TEM), respectively. From different scanning calorimetry (DSC) analysis, the performance of temperature of fusion and crystallization and enthalpy of TC-MPCMs under various conditions were measured as well. The results of thermogravimetric (TG) analysis illustrated the influence on thermal stability of TC-MPCMs. In addition, Lab color space obtained by colorimeter is certainly intuitive to observe the colorimetric characteristics of TC-MPCMs as well. More importantly, the reversible thermochromic property associated with phase state of the 1-tetradecanol could also provide a visual evidence of energy storage or release performance of the TC-MPCMs. Furthermore, The TC-MPCMs exhibited excellent stability even after 100th thermal cycling test without any obvious performance degradation, including the morphology, phase change properties and thermal stability. In the end, the fire fighter protective clothing containing TC-MPCMs was designed and fabricated, which could

⁎

Corresponding author at: School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China. E-mail address: [email protected] (W. Li).

https://doi.org/10.1016/j.apenergy.2018.02.150 Received 29 November 2017; Received in revised form 21 February 2018; Accepted 22 February 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

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provide adequate thermal protection in the various fire environments. Thus, TC-MPCMs developed in this work showed great potential applications in thermal protective clothing and other thermal regulation fields.

1. Introduction

monomers would inevitably remain in the resultant shell due to the insufficient reaction in free radical polymerization during encapsulation process (reaction temperature of emulsion system is usually lower than 100 °C under 1 atm). In addition, some solid nanoparticles selfpolymerized from vinylic monomers dissolved in aqueous phase, always make it difficult to obtain the microcapsule powder product. In this study, we discussed systematically the encapsulation, characterization and application of TC-MPCMs. The objectives of this work were to investigate the effects of different experimental conditions, i.e. dosage of surfactant, stirring rates and various core/shell ratios, on surface morphology, diameter and diameter distribution of TC-MPCMs. The influence on the melting and crystallization properties, thermal properties, as well as chromogenic properties of TC-MPCMs was discussed via optical microscope, field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), different scanning calorimetry (DSC), thermogravimetric (TG) and Colorimeter. Furthermore, the thermal cyclic durability of TC-MPCMs was carried out via heating and cooling microscope stage, and the fire fighter protective clothing with TC-MPCMs was also fabrication and investigated. The TC-MPCMs developed by this work showed excellent latent thermal energy storage-release performance, reversible thermochromic property and stability would offer tremendous potential applications in thermal energy storage especially thermal protective clothing.

Thermal energy storage through phase change materials (PCMs) [1–4] can absorb, store and release large amounts of latent thermal energy during the process of physical state change without changing its temperature, which is attracting increasing attention to enhance the energy utilization efficiency and thermal regulation [5–7]. A considerable research related on microencapsulated PCMs (MicroPCMs) [3,8–10] possessing excellent capacity of latent thermal energy storage [3,11] have been intensively investigated during recent decades. MicroPCMs have gained a considerable development in fundamental researches such as solar-thermal conversion systems [12], thermal energy storage [13,14] and so on [15–18]. In addition, great progress in the fields such as encapsulated autonomic healing materials [19,20], drug delivery [21], photochromic materials [22,23] and reversibly thermochromic materials [24] have made as well. The study related on MicroPCMs with a novel thermochromic function is considered as a promising research topic, which is attracting comprehensive and significant commercial interest [25,26]. The applications of reversible thermochromic microcapsules are being extensively applied such fields as inks [27], coatings of smart materials [28], cements, textiles [29], luminescent thermosensors [30] and color indicators [31]. Recently, luminescent dye or leuco dye based thermochromic (TC) systems [32] was selected as the core materials to fabricate thermochromic microcapsule which was sensitive to temperature [33]. Crystal violet lactone (CVL) consisted of the DMAP and MGL segments, is considered as a kind of common leuco dye used as the color former. Bisphenol A used as the color developer is a kind of weak acid, and 1-tetradecanol is used an organic solvent. Reversible thermochromic systems consists of electron donor (Crystal violet lactone, CVL), electron acceptor (Bisphenol A) and organic solvent (1-tetradecanol). The system shows color when electron donor and electron acceptor react, and acceptance and giving electron reversibly change with temperature. When the temperature is higher than the melting point of 1-tetradecanol, the color developer combined with protons, and the leuco dye favors the colorless, ring-closed state. When the temperature is lower the melting point of 1-tetradecanol, Bisphenol A release protons and gain electron, forming a complex with CVL that favors the colored ring-open, where molecular rearrange and conjugate double bond through, which show the color. Thus, the above threecomponent organic thermochromic materials [34,35], which could become blue colored under the crystallization temperature of 1-tetradecanol and turn colorless above melting point, were selected as the core materials of reversible thermochromic microcapsules. There are various encapsulation techniques available to synthesize microcapsules or nanocapsules, e.g. in-situ polymerization [36], spray drying [8], sol-gel [37], interfacial poly-condensation, complex coacervation, suspension polymerization and emulsion polymerization. And in-situ polymerization is adopted widely because of its simple, cheap, eco-friendly and technically feasible fabrication. Owing to low price, simple fabrication, good seal tightness and endurance, fire resistance, acid and alkaline resistance, the melamine–formaldehyde resin capsule has been successfully commercialized for decades [38,39]. The residual formaldehyde is actually considered as the main drawback that affects its application, however, the residual level of formaldehyde could be reduced to meet the requirements of formaldehyde in clothing and other textiles by heat treatment or addition of ammonium chloride and urea [40,41]. By contrast, there is no residual formaldehyde problem involved in capsules with vinylic shell such as acrylic-based polymer and styrene-based polymer [38,39], however, certain vinylic

2. Experimental section 2.1. Materials Three-component thermochromic mixtures cores are generally consist of a leuco dye used as an electron donor, a phenolic color developer used as an electron donor, and a high-melting organic solvent. Crystal violet lactone (6-dimethylamino-3,3-bis[p-(dimethylamino) phenyl] pahtalide, CVL, 96%, Guangzhou Qiao Xuan Chemical Co., LTD) was employed as color former, bisphenol A (2,2-bis(4-hydroxyphenyl) propane, BPA, Aladdin Industrial Corporation) was used as developer and 1-tetradecanol (AR, 99%, TianJin Guangfu Fine Chemical Research Institute) was selected as the co-solvent. The methylated melamine-formaldehyde (MMF) prepolymer solid contents of 73.0% were purchased from Tianjin Aonisite Chemical Trade Co., Ltd, and were used to synthesize the capsule shells. SMA (sodium styrenemaleic anhydride copolymer, 19 wt% aqueous solution, Shanghai Leather Chemical Workers) was employed as emulsifier. Both citric acid and sodium hydroxide were supplied by Tianjin Chemical Regents, Inc. and employed as pH regulators. All chemicals were obtained commercially and used without further purification. 2.2. Fabrication of TC-MPCMs Emulsion preparation: 0.31 g crystal violet lactone, 0.94 g bisphenol A and 18.75 g 1-tetradecanol were dissolved at 200 °C thermostatic oil bath and kept 1–2 min, then the obtained transparent and uniform oil phase was employed as core materials of capsules. The homogeneous oil solution, different dosages of SMA and 170 mL of distilled water were emulsified mechanically under 50 °C with a stirring rate of 7000 rpm for 15 min. During the process of emulsification, droplets of homogeneous oil solution were wrapped by colloid particles evenly with carboxyl groups because of negative charge, which played an important role to stabilize O/W emulsion and size of capsules. Then the pH value of the emulsion was adjusted 5.5 approximately with 20.0 wt 282

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specular reflectance and UV. The values of L∗, a∗ and b∗ of TC-MPCMs could be obtained directly at constant temperature. The cyclic durability of TC-MPCMs was carried out via heating and cooling microscope stage (Linkam THMS 600) in the range of 20–80 °C at the rate of 20 °C/min for 0, 5, 50 and 100 times, respectively.

% citric acid aqueous solution. Fabrication of TC-MPCMs: TC-MPCMs were prepared according to the process shown in Fig. 1. When the prepared emulsion was stirred at a rate of 500 rpm under the heating process from 50 to 75 °C, different doses of the MMF prepolymer solution were dropped into the prepared emulsion at rate of 0.43 mL/min. After the prepolymer was added, it was continuously stirred for 60 min under 75 °C. The system temperature was raised to 85 °C under stirring, and then 20.0 wt% citric acid was slowly incorporated into the flask at 0.43 mL/min to adjust the pH value to 4 approximately. Meanwhile, the system was maintained at 85 °C for 60 min to microencapsulate the ternary compound core. The system pH was adjusted to around 7 with 20 wt% sodium hydrate solution to terminate the reaction.

3. Results and discussion 3.1. Reversible thermochromic mechanism and encapsulation mechanism As presented in Fig. 2, the thermochromatic process of reversible thermochromic mixture emulsion core could be real-time observed from 50 to 25 °C approximately at various periods. The uniform oil phase containing of 1-tetradecanol, CVL and bisphenol A was stirred vigorously in the aqueous solution in presence of SMA at 50 °C, which was higher than the crystallizing point of 1-tetradecanol. As described in Fig. 2, the obtained emulsion could be observed using the OM under around 25 °C. The temperature of the surface of emulsion droplets decreased firstly, and the outer 1-tetradecanol of oil droplets started to crystallize firstly accordingly shown in Fig. 2(b), thus both the CVL and bisphenol A dissolved in the outer region of oil droplets were compelled to precipitate gradually and transfer into the inner core where the solubility remained relatively high. During the migration process, the thermochromic colorant and developer interact to be colored as presented in Fig. 2(b) and (c). When the temperature of oil droplets completely decreased to room temperature which was lower than crystallization point of 1-tetradecanol, the obvious core-shell structure with blue color former and developer as core and 1-tetradecanol acted as shell formed in emulsion droplets as shown in Fig. 2(d). The Fig. 1 illustrated the encapsulation mechanism of TC-MPCMs: the homogenous oil solution consisting of bisphenol A, 1-tetradecanol and CVL used as thermochromatic pigments, were prepared firstly and then added into the aqueous solution containing SMA surfactant. The stable O/W emulsion [33] could be obtained through vigorous emulsification in the end. During this process, the surfactant molecules trimly covered the surfaces of CVL/bisphenol A/1-tetradecanol oil droplets with hydrophobic chains orienting into the oil droplets and hydrophilic groups out of the oil droplets. In-situ encapsulation usually contains three steps, i.e. emulsification

2.3. Characterization of TC-MPCMs The emulsification and dispersion process of the ternary component thermochromic oil phase was real-time monitored with optical microscope (OM, XSP-06): a drop of the uniform emulsion was taken out at 50 °C to observe under circumstance temperature of 25 °C approximately, and the colouration process was then recorded with the OM equipped with computer. Both the morphology and microstructure of TC-MPCMs were observed by thermal field emission scanning electron microscope (TFESEM, ZEISS G500). One drop of TC-MPCMs dispersion washed with ethanol was dropped onto holey carbon support film and allowed to dry, and the core-shell structure of capsules could be obtained by transmission electron microscope (TEM, HITACHI H-7650). The thermal properties of TC-MPCMs were measured using differential scanning calorimetry (DSC, NETZSCH 200F3). Under nitrogen atmosphere, the phase changes were characterized from −20 to 80 °C at a heating or cooling rate of 10 °C min−1 after elimination of the thermal history. The thermal stabilities of dried TC-MPCMs were characterized via a thermogravimetric analyzer (TG, Netzsch409pc) with the temperature range of 25–600 °C. The measurement process was carried out in a nitrogen atmosphere with a heating rate of 10 °C min−1. The chromogenic properties of TC-MPCMs were carried out via the Colorimeter (SC-10). The colorimeter consists of the small aperture,

Fig. 1. Formation process of TC-MPCMs prepared by in-situ polymerization.

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Fig. 2. Real-time observation of reversible thermochromic mixture emulsion core at various periods: (a) 25 s (b) 45 s (c) 80 s (d) 92 s.

further curing to form solid microcapsules with a define shell strength.

of core materials, capsules formation and curing process. Fig. 1 depicted the simple schematic diagram of the preparation of TC-MPCMs, the prepolymer was added into the uniform and stable emulsion system consisting of the oil phase droplets, water phase and emulsifier. When the pH value of the emulsion system was regulated below 7, the SMA macromolecules were charged negatively, and the oil phase materials containing ternary reversible thermochromic mixture were dispersed in certain amount of emulsifier aqueous solution of SMA, forming O / W emulsion under the mechanical emulsification. SMA macromolecules were subsequently attached to the interface of the inter phases, and the lipophilic end extended into the interior of the droplets and the hydrophilic end of carbonyl groups extended to the aqueous phase, thus the surfaces of the oil droplets were negatively charged. The amine, imine groups and > N-CH2OH groups in the structure of performed polymer dissolved in a weak acid aqueous solution turned negatively charged due to combining with hydrogen ion. When the prepolymer was added to the emulsion, part of the prepolymer was enriched to form the self-assembled structure of the prepolymer molecules through the interaction between charges of SMA molecules in the droplet surface. Under certain conditions of pH and temperature, the prepolymer molecules in the aqueous phase diffused continuously to the interface of the oil phase and aqueous phase until the reaction is complete and

3.2. Thermal properties of TC-MPCMs The melting and crystallization properties of TC-MPCMs were obtained from DSC analysis and summarized in Table 1. The melting enthalpy (△ Hm ) and crystallization enthalpy (△ Hc ) are important parameters to evaluate the thermal storage capability of TC-MPCMs during the phase change process. As shown in Figs. 3 1-tetradecanol was measured to have considerably high phase change enthalpies, and the △ Hm of MicroPCMs and TC-MPCMs was 191.3 J/g and 165.9 J/g and △ Hc was 191.5 J/g and 165.3 J/g from Table 1, respectively, indicating that the MicroPCMs and TC-MPCMs could store high latent heat. From Fig. 3 and Table 1, there was a decrease on the phase change enthalpies of TC-MPCMs in comparison with those of 1-tetradecanol and MicroPCMs, this is mainly attributed the fact that the additional “impurity” effects of organic thermochromatic pigments decreased crystal perfection of 1-tetradecanol. The Tc,γ of TC-MPCMs decreased to lower temperature (from 18.4 °C to 5.4 °C), meaning that the crystallization became more difficult which mainly caused by the addition of organic thermochromic pigments. And super-cooling phenomenon should be taken into consideration in the application of TC-MPCMs. In

Table 1 Thermal properties of 1-tatradecanol, MicroPCMs and TC-MPCMs. Samples

1-tetradecanol MicroPCMs TC-MPCMs

Melting

Crystallization

Tm,0 (°C)

Tm (°C)

△Hm (J/g)

Tc,α (°C)

Tc,β (°C)

Tc,γ (°C)

△Hc (J/g)

Tc,0 (°C)

36.6 29.9 33.1

42.7 40.1 39.0

228.4 191.3 165.9

– – 21.3

– 28.0 15.2

27.6 18.4 5.4

229.7 191.5 165.3

24.3 10.2 -1.5

Theoretical core content (Wt%)

Actual core content (Wa%)

Energy storage efficiency (E%)

Thermal storage capability (C%)

– 73.3 73.3

– 83.8 72.6

– 83.6 72.3

– 99.7 99.6

Note: Tm,0 , onset temperature on DSC melting curve; Tm , Peak temperature on DSC melting curve; ΔHm , Enthalpy on DSC heating curve; Tc,α , Alpha peak temperature on DSC cooling curve; TC ,β , Beta peak temperature on DSC cooling curve; TC ,γ , Gamma peak temperature on DSC cooling curve; Tc,0 , onset temperature on DSC cooling curve; ΔHc , Enthalpy on DSC cooling curve.

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tetradecanol are crystallization enthalpies of microcapsules and 1-tetradecanol, respectively. These encapsulation parameters derived from Eqs. (1) to (4) was summarized in Table 1. As the data listed in Table 1, the values of the actual core content of MicroPCMs show higher level than theoretical core content, indicating some small molecule by-products such as H2O, methanol or formaldehyde might generate during the shell forming process. The reason why the values of the actual core content of TCMPCMs existed lower than the theoretical might be that not all of 1tetradecanol in emulsion was microencapsulated by MMF shell. The actual core contents were deduced from Eq. (2) in terms of the phase change enthalpies, and the value reflects the effective encapsulation of the microcapsules. In addition, Wa and E of TC-MPCMs were found to be lower than MicroPCMs, which means addition of organic thermochromatic pigments might influence the latent heat of 1-tetradecanol in TCMPCMs storage and release when the feed ratio kept consistent. The energy storage efficiency plays an important role to describe the phase change performance for latent heat storage and release after phase change materials was encapsulated [42]. And the energy storage efficiency was much closed to their actual core content in samples, which indicated that microcapsules could release almost all of latent heat during the phase change process. Furthermore, thermal storage capability of MicroPCMs and TC-MPCMs could be evaluated via Eq. (4), and Wa was the actual core content via Eq. (2). From Table 1, the thermal storage capability of all of samples was higher than 99%, which might mean that the most of 1-tetradecanol had been well encapsulated and all of the encapsulated 1-tetradecanol could effectively store and release the latent heat through phase change, indicating that TCMPCMs have promising practical application in latent heat storage system.

Fig. 3. DSC curves of (a) 1-tetradecanol, (b) MicroPCMs and (c) TC-MPCMs.

addition, the theoretical core content (Wt ), the actual core content (Wa ), the energy storage efficiency (E) and the thermal storage capability (C) are four important parameters and introduced to evaluate the microencapsulation ratio of 1-tetradecanol, which can be calculated by DSC results via the following formula:

Wt =

mcore × 100% mcore + mshell

(1)

Wa =

|ΔHm,micro | × 100% |ΔHm,1 − tretradecanol |

(2)

E=

|ΔHm,micro | + |ΔHc,micro | × 100% |ΔHm,1 − tetradecanol | + |ΔHc,1 − tetradecanol |

|ΔHm,micro | + |ΔHc,micro | × 100% C= (|ΔHm,1 − tetradecanol | + |ΔHc,1 − tetradecanol |) × Wa

3.3. Effects of SMA on morphology and microstructure of TC-MPCMs (3)

Emulsification is generally considered as a key influencing factor to determine the size and distribution of resultant capsules, in which liquid is dispersed in other incompatible liquid medium in the way of tiny droplets to form a heterogeneous dispersed system with considerable stability. In the experiment, the amount of emulsifier SMA was regulated to investigate the influence on thermochromic microcapsules,

(4)

where Δ Hm,micro and Δ Hm,1-tetradecanol are melting enthalpies of microcapsules and 1-tetradecanol, respectively; Δ Hc,micro and Δ Hc,1-

Fig. 4. Morphology and microstructure of TC-MPCMs added with various concentration of SMA:(a) 2.16 wt%, (b) 2.48 wt%, (c) 2.79 wt%, (d) 3.10 wt% and (e) 3.41 wt%.

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product can be clearly found from Fig. 4(a2) to (e2), the shell thicknesses of TC-MPCMs added with various SMA were 61, 59, 58, 59 and 60 nm, respectively. During the experiments, there was no distinct influence of emulsifier SMA dosage on the shell thickness of TC-MPCMs. However, the concentration of emulsifier had a great effect on the stability of the emulsion. During emulsification process, certain amount SMA would form a protective film absorbing around the oil phase droplets to inhibit flocculation of the dispersed phase, and the residual SMA would form the frame structure in the solution to keep the emulsion droplets in stable state. Most of the dispersed droplets could be wrapped completely without adhesion between each other only

while the rest experimental parameters were kept constant. Fig. 4 showed the SEM micrographs and TEM images of TC-MPCMs added with various amounts emulsifier SMA. From Fig. 4(a1) to (e1), with the increase of emulsifier, most of the TC-MPCMs surface were smooth and compact, however, there was a slight morphology difference among these TC-MPCMs, i.e. the size of capsules exhibited a slightly tendency to decrease. In addition, the number of self-polymerized nanoparticle adhered onto the surface increased accordingly presented in Fig. 4 (c1), which was probably caused by the fact that excessive emulsifier existing among oil droplets were employed as “nucleating” agent for self-polymerization. In addition, the core-shell microstructure of TC-MPCMs

Fig. 5. Distribution of particle size of TC-MPCMs with various concentration of SMA emulsifier: (a) 2.16 wt% (b) 2.48 wt% (c) 2.79 wt% (d) 3.10 wt% and (e) 3.41 wt%; (f) the relationship curve between the amounts of emulsifier and the capsule size distribution.

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3.4. Effects of emulsification condition on the TC-MPCMs

when the concentration of the emulsifier reaches critical micelle concentration, thus resulting in droplets with smaller particle size and uniform distribution. In this experiment, the number of self-polymerized nanoparticle adhered onto the surface increased accordingly from 2.79 wt%, the reason was that excess SMA dispersed in water turned into the sites of self-polymerization of prepolymer to form selfpolymerized nanoparticles, adhering onto the surface of TC-MPCMs. From the microscopic structure of SMA macromolecular, there are both carboxyl hydrophilic groups and aromatic hydrophobic groups attached on the long-chain carbon backbone. SMA would be arranged in alignment onto the surface of hydrophobic core of microemulsion droplets, forming the structure of which the carboxyl groups extend outward and the aromatic groups extends inward to adsorb while the oil core is emulsified under the severe shearing force. After emulsification, the droplets of core materials could be obtained as colloid particles with negative charge in virtue of directional alignment of carboxyl groups to maintain the size of colloid particles of the microcapsules. More than 200 TC-MPCMs of each sample were randomly selected to carry out particle size statistics and distribution, and the particle size distribution was determined via Origin 8.5 plotting as described in Fig. 5. When the emulsifier concentration increased, the average particle size of capsules decreased from 720 to 440 nm gradually, however, the changes of particle size were no longer significant when the SMA content increased to a certain value. Besides, there was an increasing tendency of average particle size when the weight percent of SMA increased to 3.41 wt% as Fig. 5(f) shown. The reason might be that an increase of SMA contents probably resulted in the insufficient emulsification caused by high viscous solution system or the self-aggregation of capsule. The particle size of TC-MPCMs decreased mainly attributed to that the surface tension between the oil phase and water phase gradually reduced with SMA increasing, so the particles size of the droplets became smaller, and thus resulting in the smaller capsules size. Owing to its unique molecular structure containing both hydrophobic benzene ring and hydrophilic hydroxyl groups, 1-tetradecanol could be easily emulsified and dispersed into stable emulsion. However, the changes of particle size became no longer apparent due to the smaller variation of surface tension when the concentration of emulsifier reached a certain value. In addition, excessive SMA was probably contributed to high viscous solution system and self-polymerization of MMF prepolymer. The phase change properties of TC-MPCMs with various concentration of emulsifier were investigate via DSC, and the resultant curves of which were presented in Fig. 6 and thermal properties data were summarized in Table 2. The phase transition peak temperatures of samples were close to each other, and both the melting and crystallizing enthalpies of TC-MPCMs fabricated with higher dosages of emulsifiers SMA showed a slight increase. The onset temperature and peak temperature of heating curve were around 31.0 and 37.8 °C, respectively, and the enthalpies of melting and crystallizing increased from 90.3 to 119.7 J/g. Thus, the various dosages of emulsifiers had no obvious effect on melting temperature of TC-MPCMs, while the influence on the enthalpies of melting and crystallizing of TC-MPCMs was distinct. The reason might be that the emulsifier formed a protective film to stabilize the droplet on the surface of the oil phase and increased the encapsulation efficiency of the microcapsules. In addition, there appeared alpha-phase, beta-phase and gamma-phase of crystallization inside the TC-MPCMs with various dosages of emulsifiers SMA on the cooling curve as given in Fig. 6. This might be attributed to the fact as follows: the number of crystal nucleus located in each capsule decreased as the particle size of the microcapsules diameter minimized, meaning that the number of the inoculating crystal that induced to produce heterogeneous nucleation decreased. The crystallization peaks of phase change material inside TC-MPCMs were mainly composed of crystallization induced by the heterogeneous nucleation (α, β) and homogeneous nucleation (γ).

SEM micrographs of TC-MPCMs prepared under various emulsification rates, and the size distributions were presented in Fig. 7. The particle distribution curve of TC-MPCMs was plotted by Origin 8.5 through measuring more than 200 microcapsules with software Nano Measurer. As seen in Fig. 7, it could be clearly observed that most of capsules exhibited regular spherical shape, and particle size of TCMPCMs turned smaller from (a) to (d). TC-MPCMs were not so uniform in size range of 7.02–1.12 μm under emulsification rate of 3000 rpm, and the mean size was measured to be 3.11 μm. While the diameter size of TC-MPCMs reduced to 2.37–0.49 μm approximately with emulsification rate of 5000 rpm, and the mean size was 1.09 μm. When the shear emulsification rate enhanced to 7000 rpm, the particle size reduced to about 1.50–0.38 μm, the mean size became 770 nm and its distribution of TC-MPCMs turn narrower as well. Under 9000 rpm of shear emulsification rate, TC-MPCMs were uniform in size about at 720–170 nm and the distribution of particle size became more concentrated, the mean size reached 420 nm. Thus, larger shear emulsification rate contributed to smaller particle size as showed in Fig. 7(a1) to (d1) and narrower distribution of particle size of TC-MPCMs as showed in Fig. 7(a2) to (d2). Meanwhile, the shrinkage of the volume of compound core, mainly attributed to 1-tetradecanol, would lead to irregularity and roughness of the surface of TC-MPCMs. The 1-tetradecanol has unique molecular structure containing both hydrophilic hydroxyl group and hydrophobic C14 alkane, so it is easy to be emulsified and dispersed. The emulsified dispersants were stably dispersed in emulsion system uniformly, and thus enhanced the charge attraction force between shell-forming pre-polymer and the surface of core materials covered by SMA macromolecules. Lab color space developed by the international Commission on Illumination (CIE) in 1976 employs three mutually perpendicular coordinate values L∗, a∗ and b∗ to represent a color space. L∗ axis indicates brightness that black is at the bottom, while white is at the top. The positive value of a∗ expresses fuchsin, and the negative value of a∗ expresses green. The positive value of b∗ expresses yellow, while the negative value of b∗ for blue. As illustrated in Fig. 8, the curves presented relationships of emulsification rates with particle size distributions of capsules and color rendering of TC-MPCMs. From Fig. 8, the particle size of TC-MPCMs decreased with an increase of emulsification rates. While the value of b∗ tended to be positive with the increasing of emulsification rates, in other words, as particle size of TC-MPCMs decreased. When particle size of TC-MPCMs less than 1 μm, the phenomenon of super-cooled crystallization intensified, which likely affected coloration of TC-MPCMs. In the condition of volume constancy, the number of TC-MPCMs increased when core particle size decreased,

Fig. 6. DSC curves of TC-MPCMs with various concentration of emulsifier SMA: (a) 2.16 wt%, (b) 2.48 wt%, (c) 2.79 wt%, (d) 3.10 wt% and (e) 3.41 wt%.

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Table 2 Thermal properties of the TC-MPCMs with various dosages of SMA. Samples

a b c d e

Melting

Crystallization

Tm,0 (°C)

Tm (°C)

△Hm (J/g)

Tc,α (°C)

Tc,β (°C)

Tc,γ (°C)

Tc,0 (°C)

△Hc (J/g)

30.8 30.8 31.2 31.2 30.8

37.6 37.1 37.4 37.2 37.8

90.3 96.1 106.1 109.8 119.7

21.3 20.6 20.8 21.3 20.6

14.1 13.9 14.1 14.3 13.3

4.7 3.7 4.2 4.2 4.3

−1.0 −3.2 −1.1 −0.3 −1.9

90.7 96.2 105.8 108.6 118.5

Note: Tm,0 , onset temperature on DSC melting curve; Tm , Peak temperature on DSC melting curve; ΔHm , Enthalpy on DSC heating curve; Tc,α , Alpha peak temperature on DSC cooling curve; TC ,β , Beta peak temperature on DSC cooling curve; TC ,γ , Gamma peak temperature on DSC cooling curve; Tc,0 , onset temperature on DSC cooling curve; ΔHc , Enthalpy on DSC cooling curve; a 2.16 wt%, b 2.48 wt%, c 2.79 wt%, d 3.10 wt% and e 3.41 wt%.

Fig. 7. Morphology and size distribution of TC-MPCMs with various emulsification rates: (a) 3000 rpm, (b) 5000 rpm, (c) 7000 rpm and (d) 9000 rpm.

emulsification rates were presented in Fig. 9. The melting and crystallization properties of these samples were obtained from DSC analysis and summarized in Table 3. The changes of peak temperature of melting curve of all the samples had no distinct differences, the peaks temperature of the cooling curve decreased as illustrated in Table 3. While the onset melting temperature values slightly were increased with increasing of emulsification rates. The β peak temperature

thus resulting in specific surface area increasing of TC-MPCMs. The decreasing of particle size leaded to increasing of specific surface area of TC-MPCMs, which resulted in confinement of crystallization of thermochromic pigments within tiny TC-MPCMs, thereby, the coloration of TC-MPCMs probably became weaker with the decreasing of TCMPCMs size. Both DSC heating and cooling curves of the TC-MPCMs with various 288

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3.5. Effects of core/shell ratio of TC-MPCMs The morphology and core-shell structure of TC-MPCMs with various core/shell ratios were characterized via TFE-SEM and TEM as shown in Fig. 10. The mean particle size of TC-MPCMs was uniform, and most TC-MPCMs exhibited spherical profile and smooth surfaces. From Fig. 10(a1) to (d1), TC-MPCMs still kept a hollow spherical shape. However, the shell thickness turned thinner with an increasing of core/ shell ratios, thus resulting in the fact that deformation shape of TCMPCMs became more obvious. And Fig. 10(a2) to (d2) showed the shell thickness and core-shell structure of TC-MPCMs. The outcomes clearly revealed that the resulted capsules had typical core-shell structure and distinct shell thickness. As measured in Fig. 10, the shell thicknesses of TC-MPCMs with increasing of core/shell ratios in proper order were measured as 67, 52, 50 and 47 nm, respectively. TC-MPCMs of each sample were randomly selected to measure the average of the shell thickness using the software Nano Measurer, and the curve of relationship between the average shell thickness of the TC-MPCMs and various core/shell ratios was shown in Fig. 11. The shell thickness of TC-MPCMs tended to be thinner with an increase of core/shell ratios, which was in accordance with the TEM micrograph results within a certain range. Furthermore, the curves described relationships between various core/shell ratios and b∗, which could be considered to be approximate linear relation, and the variation tendency was in accordance with shell thickness. The value of b∗ also changed from −15 to −40 with core/shell ratios increase, meaning that the color of TC-MPCMs turned bluer as the shell thickness became thinner. Besides, the thinner shell of TC-MPCMs probably facilitated the crystallization of 1-tetradecanol and formed more perfect crystal structure, and resulted in the enhancement of color performance of TC-MPCMs. In addition, TC-MPCMs powder sample with core/shell ratio of 1.4 were measured reversible thermochromic property as shown in Fig. 12. TC-MPCMs presented blue color below the melting point of 1-tetradecanol, while the color turned white caused by capsule shell materials when TC-MPCMs were heated to higher temperature than phase transition temperature of 1-tetradecanol. As a result, the reversible thermochromic property associated with the phase state of 1-tetradecanol could provide visual evidence of the latent heat absorption or release performance of TC-MPCMs. Fig. 13 showed typical DSC curves of the TC-MPCMs with various core/shell rations, and the melting and crystallization properties of all the samples were obtained and summarized in Table 4. Tm of TCMPCMs was increased closely to the phase transition temperature of 1tetradecanol with the increase of core/shell ratios, indicating that the changes of core/shell ratio had effect on phase transition temperature of TC-MPCMs. From the data listed in Table 4, the enthalpies of melting curve increased from 99.8 to 145.6 J/g with increasing of the core/shell ratios and the enthalpies of crystallizing curve enhanced from 95.8 to 146.3 J/g. That mainly attributed to that the decrease of shell materials amount leaded to the relative increase of the purity core materials content inside the capsules. TGA was employed to determine thermal degradation behaviors of

Fig. 8. Relationship curves of (a) emulsification rates and particle size distributions of capsules, (b) emulsification rates and b*of TC-MPCMs.

Fig. 9. DSC curves of TC-MPCMs with various emulsification rates: (a) 3000 rpm, (b) 5000 rpm, (c) 7000 rpm and (d) 9000 rpm.

possibly corresponded with unstable mesophase transition on cooling curve, and this peak became weaker when the particle size of TCMPCMs became smaller (i.e. sample c and sample d), which was resulted from the confined space caused by the reduced particle size. In addition, enthalpy of absorption curve enhanced from 135.4 to 165.9 J/ g and enthalpy of exothermic curve changed from 136.8 to 165.3 J/g with the emulsification rates increasing. Furthermore, the phenomenon of super-cooling crystallization became obvious gradually, which could be mainly attributed to the confined crystallization caused by sequentially decreasing of TC-MPCMs particle size. The influence on the latent heat absorption field would be less than that of latent heat release field.

Table 3 Thermal properties of the TC-MPCMs with various emulsification rates. Samples

a b c d

Melting

Crystallization

Tm,0 (°C)

Tm (°C)

△Hm (J/g)

Tc,α (°C)

Tc,β (°C)

Tc,γ (°C)

Tc,0 (°C)

△Hc (J/g)

31.9 32.5 32.5 33.1

38.5 38.6 38.7 39.0

135.4 148.0 158.0 165.9

20.6 21.2 20.9 21.3

14.8 15.3 15.2 15.2

7.5 6.5 5.6 5.4

1.0 −1.1 −0.1 −1.5

136.8 148.6 157.6 165.3

Note: Tm,0 , onset temperature on DSC melting curve; Tm , Peak temperature on DSC melting curve; ΔHm , Enthalpy on DSC heating curve; Tc,α , Alpha peak temperature on DSC cooling curve; TC ,β , Beta peak temperature on DSC cooling curve; TC ,γ , Gamma peak temperature on DSC cooling curve; Tc,0 , onset temperature on DSC cooling curve; ΔHc , Enthalpy on DSC cooling curve; a 3000 rpm, b 5000 rpm, c 7000 rpm and d 9000 rpm.

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Fig. 10. SEM micrographs and TEM images of TC-MPCMs with various core/shell ratios: (a) 1, (b) 1.11, (c) 1.25 and (d) 1.4.

Fig. 11. Relationship curves of (a) shell thickness and core/shell ratio and of TC-MPCMs, (b) b* and core/shell ratio of TC-MPCMs. Fig. 13. DSC curves of TC-MPCMs with various core/shell rations: (a) 1.0 (b) 1.11 (c) 1.25 and (d) 1.4.

TC-MPCMs with various core/shell ratios, and the resultant TGA thermograms were described in Fig. 14. The weight loss of oil phase cores containing ternary thermochromic mixture started at about 170 °C and almost no residual materials remained at 280 °C, suggesting that the oil phase experienced volatilization caused by gasification or decomposition. The weight loss of MMF shell started at 180 °C and remained other impurities till complete reaction in virtue of keeping losing weight. The weight loss curves of TC-MPCMs presented similar process, i.e., a twostep weight loss process during 130–270 °C and 300–650 °C, respectively. The first step was mainly attributed to the decomposition of the

oil phase core, while the second step was resulted from the decomposition of shell materials that was in accordance with pure shell materials. From Fig. 14, the onset decomposition temperature of TCMPCMs had no obvious difference even though the shell thickness increased, however, the weight loss end temperature of oil phase core increased slightly from 260 to 280 °C with shell materials increase, suggesting that the compactness of MMF shell was excellent. However, it could be seen that the weight losses of TC-MPCMs at first step were

Fig. 12. Images of TC-MPCMs at (a) 25 °C and (b) 45 °C.

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Table 4 Thermal properties of TC-MPCMs with various core/shell rations. Samples

a b c d

Melting

Crystallization

Tm,0 (°C)

Tm (°C)

△Hm (J/g)

Tc,α (°C)

Tc,β (°C)

Tc,γ (°C)

Tc,0 (°C)

△Hc (J/g)

31.1 32.0 29.1 29.9

37.0 37.3 37.5 37.7

99.8 107.2 125.8 145.6

21.0 21.3 21.3 21.2

13.7 14.0 – 14.1

4.1 4.9 5.5 5.7

−1.6 −1.0 −1.9 −0.1

95.8 108.7 125.8 146.3

Note: Tm,0 , onset temperature on DSC melting curve; Tm , Peak temperature on DSC melting curve; ΔHm , Enthalpy on DSC heating curve; Tc,α , Alpha peak temperature on DSC cooling curve; TC ,β , Beta peak temperature on DSC cooling curve; TC ,γ , Gamma peak temperature on DSC cooling curve; Tc,0 , onset temperature on DSC cooling curve; ΔHc , Enthalpy on DSC cooling curve; a 1.0, b 1.11, c 1.25, d 1.4.

durability test. The comparison of surface micrographs changes of TCMPCMs after different cycling times were presented in Fig. 16. From SEM micrographs, most of TC-MPCMs still exhibited smooth and compact surface and spherical shape even after 100th cycling test. Some surface of microcapsules presented slightly sunken, however, cracked capsules were almost non-existent. It could be attributed to the excellent strength and flexibility of MMF resin as shell. In addition, Fig. 17 showed typical DSC and TG curves of TC-MPCMs with undergoing different times cycling test. The melting and crystallization properties and TGA data of all the samples were summarized in Table 5. From Fig. 17, there was no obvious difference among the DSC and TGA curves of TC-MPCMs undergoing 5, 50, 100 cyclic times respectively, compared with that of blank sample. The onset decomposition temperature and the weight loss of TC-MPCMs of two steps undergoing 0th, 5th, 50th, 100th cycling test were nearly accordant with each other. The trends of weight loss of samples were almost consistent, indicating which was not affected by thermal cycling times. It could be concluded that the thermal stabilities and durability of TC-MPCMs was excellent. As shown in Table 5, the Tm,o, Tm and △Hm of the four TC-MPCMs sample were measured as around 30 °C, 37 °C and 145 J/g, respectively. And the Tc,γ, Tc,o and △Hc were 5.3 °C, −0.5 °C and 145 J/g, respectively. Thus, the cyclic durability of TC-MPCMs was excellent. The TC-MPCMs exhibited outstanding latent thermal energy storage and thermal regulation properties, and the TC-MPCMs could be assembled into narrow-disperse composite macrocapsules simply using a piercing-solidifying incuber method according our previous reports [25]. The fire fighters mainly rely on fire fighter protective clothing (FFPC) to provide adequate thermal protection in the various fire environments. The flash fire may occur with a heat flux up to 80 kW/m2 or more [43] in some fire cases, so the FFPC must provide the wearers with adequate thermal protection for fire fighter’s safety. In our application experiments, these millimeter-scale macrocapsules were inserted into small knitted sacks and then incorporated to the inner surface of the vest, i.e. near the skin, as shown in Fig. 18. The latent heat absorbed during phase change could decrease the temperature of fire fighter’s skin surfaces significantly, contributing to the thickness reduction of the gear while maintaining similar thermal protection performances. The temperature of macrocapsules containing TC-MPCMs wouldn’t increase until the solid-liquid phase change (i.e. crystalline state to amorphous state) completed, and the color of which would turn from blue to white when the solid-liquid phase change was accomplished. Therefore, the reversible thermochromic property associated with the phase state of PCMs could provide simple visual evidence of the latent thermal energy absorption performance of macrocapsules containing TC-MPCMs. In addition, TC-MPCMs developed in this work also showed great potential in application for intelligent textiles or fabrics, food and medicine package, thermo-sensors and solar energy storage, etc.

Fig. 14. TGA curves of (a) MMF shell, core/shell ratio (b) 1.0, (c) 1.11, (d) 1.25 and (e) 1.4, (f) oil phase.

similar, but the weight loss of TC-MPCMs in second step showed obvious increase with core/shell ratios increasing. It is obvious to gain the conclusion that the thermal stability might decrease with less shell materials, and enhanced compactness and thermal stability of TCMPCMs could be obtained by increasing shell thickness of TC-MPCMs. 3.6. Acid resistance of capsule shell materials To investigate the acid resistance of MMF resin shell, the MMF nanoparticle resin were treated in acid solution with pH value from 4.5 to 2, and the treated MMF nanoparticles were compared with TFE-SEM as shown in Fig. 15. It can be observed from Fig. 15(b) to (e) that the MMF nanoparticles still kept spherical profile and smooth surfaces. However, there were some impurities appeared among the surface of MMF nanoparticles and the quantity density also decreased after suffering from strong corrosion with pH value 2.5 and 2, suggesting that MMF particles were nonresistant to corrosion and tended to be dissolved partially. Further polymerization might lead to higher degree of crosslinking when the MMF resin was treated with acid solution firstly above the degradation threshold, and the insoluble MMF particles could be obtained. However, the amino and ether bond of hydrolyzed degraded or replaced in the acid solution with lower pH value and longer reaction period. The results were in consistent with the fact that for an optimal MMF core decomposition the pH value in the bulk should always below 2. In conclusion, the MMF shell material presented good acid resistant when the pH value above 4 approximately. 3.7. Thermal cyclic durability of TC-MPCMs and its application

4. Conclusions

Thermal cyclic durability is considered as an important factor influencing the applications of TC-MPCMs, so that the TC-MPCMs with core/shell ration of 1.4 was selected to conduct thermal cyclic

In 291

this

study,

a

series

of

TC-MPCMs

containing

ternary

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Fig. 15. SEM micrographs of MMF nanoparticles treated with acid solution of various pH values: (a) blank MMF nanoparticles, (b) pH = 4.5, (c) pH = 4, (d) pH = 3.5, (e) pH = 3, (f) pH = 2.5 and (g) pH = 2.

volatilization of oil phase had no obvious changes with the increasing of wall thickness. The weight loss end temperature of oil phase core increased slightly from 260 to 280 °C with shell materials increase, suggesting that the compactness of MMF shell was excellent. The MMF shell material also presented good acid resistant when the pH value above 4 approximately. Furthermore, the cyclic durability of TCMPCMs was still excellent after 100th cycling test. In the end, the fire fighter protective clothing macrocapsules composed of TC-MPCMs was designed and fabricated to provide adequate thermal protection in the various fire environments, showing great potential in thermal protective clothing. Owing to the excellent latent heat storage-release performance, reversible thermochromic property and stability, TC-MPCMs developed in this study would also offer tremendous potential applications for the intelligent textiles or fibers, printing inks, reversible thermochromic coating materials, food and medicine package, thermosensors and solar energy storage, etc.

thermochromic mixture exhibiting both excellent thermal energy storage-release performance and reversible thermochromic property, were successfully prepared via in-situ polymerization. The average particle size of TC-MPCMs could be regulated from 720 to 440 nm by changing the amount of emulsifier SMA. With emulsification rates gradually increasing, the particle size of TC-MPCMs reduced from 3.11 μm to 0.42 μm, the distribution of particle size got narrower, and enthalpy increased from 135.4 to 165.9 J/g gradually. Besides, TC-MPCMs were susceptible to be influenced by confinement of crystallization and super-cooling phenomenon, which affected chromogenic properties of TC-MPCMs. The measurement results showed that the increasing of core/shell ratios from 1.0 to 1.4 leaded to the lower intensity of TCMPCMs and the thinner shell thickness ranging from 67.30 to 47.75 nm. The melting enthalpies varied from 99.8 to 145.6 J/g and the enthalpies of crystallizing from 95.8 to 146.3 J/g with an increase of core/shell ratio of TC-MPCMs. In addition, the value of b∗ obviously changed from -15 to -40 and the color of TC-MPCMs enhanced as core/shell ratios increased. The onset decomposition temperatures related to the 292

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Fig. 16. SEM micrographs of TC-MPCMs with various cyclic times: (a) 0th, (b) 5th, (c) 50th and (d) 100th.

Fig. 17. Curves of (I) DSC and (II) TGA of TC-MPCMs with various cyclic times: (a) 0th, (b) 5th, (c) 50th and (d) 100th.

Acknowledgements

Application Foundation and Advanced Technology (No. 16JCYBJC17100), China Postdoctoral Science Foundation (2017M611167) and National Key Research and Development Project of China (2016YFC0400509).

The authors gratefully acknowledge the financial supports for this research from the National Natural Science Foundation of China (No. 51573135 and No. 51203113), Tianjin Research Program of Table 5 Thermal properties of TC-MPCMs with various cyclic times. Samples

a b c d

Melting

Crystallization

TGA data

Tm,0 (°C)

Tm (°C)

△Hm (J/ g)

Tc,α (°C)

Tc,β (°C)

Tc,γ (°C)

Tc,0 (°C)

△Hc (J/ g)

T1,onset (°C)

T1,max (°C)

Mass (wt %)

T2,onset (°C)

T2,max (°C)

Mass2 (wt %)

29.9 30.2 30.3 30.4

37.5 37.2 37.0 37.1

145.8 144.2 144.6 145.0

21.0 – – –

– – – –

5.3 5.1 5.5 5.2

−0.1 −1.1 −0.1 −1.4

146.3 144.8 144.7 144.8

204.64 205.66 205.26 203.88

245.90 246.76 247.27 245.38

59.04 58.98 58.73 58.95

358.47 357.97 358.24 356.07

366.49 364.87 365.21 363.11

27.29 28.72 28.21 28.45

Note: Tm,0 , onset temperature on DSC melting curve; Tm , Peak temperature on DSC melting curve; ΔHm , Enthalpy on DSC heating curve; Tc,α , Alpha peak temperature on DSC cooling curve; TC ,β , Beta peak temperature on DSC cooling curve; TC ,γ , Gamma peak temperature on DSC cooling curve; Tc,0 , onset temperature on DSC cooling curve; ΔHc , Enthalpy on DSC cooling curve; a 0th b 5th c 50th d 100th; T1,onset , the onset decomposition temperature of first step on TGA curve; T1,max , the peak temperature of first step on TGA curve; Mass1: weight loss of first step on TGA curve; T2,onset , the onset decomposition temperature of second step on TGA curve; T2,max , the peak temperature of second step on TGA curve; Mass2: weight loss of second step on TGA curve.

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Fig. 18. Morphology and structure of (a) fire fighter protective clothing, (b) macrocapsules composed of TC-MPCMs and (c) cross section micrograph of macrocapsule.

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