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Synthesis and characterization of thermochromicenergy-storage microcapsule and application to fabricZhilei Wuab, Xiaoguang Maab, Xiling Zhengab, Wenfang Yangab, Qintao Mengab & ZhenrongZhengab

a School of Textile, Tianjin Polytechnic University, Tianjin, 300387, China.b Tianjin Key Laboratory of Fiber Modification and Functional Fiber, Tianjin PolytechnicUniversity, Tianjin, 300387, China.Published online: 03 Sep 2013.

To cite this article: Zhilei Wu, Xiaoguang Ma, Xiling Zheng, Wenfang Yang, Qintao Meng & Zhenrong Zheng (2014) Synthesisand characterization of thermochromic energy-storage microcapsule and application to fabric, The Journal of The TextileInstitute, 105:4, 398-405, DOI: 10.1080/00405000.2013.814753

To link to this article: http://dx.doi.org/10.1080/00405000.2013.814753

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Synthesis and characterization of thermochromic energy-storage microcapsule and applicationto fabric

Zhilei Wua,b, Xiaoguang Maa,b*, Xiling Zhenga,b, Wenfang Yanga,b, Qintao Menga,b and Zhenrong Zhenga,b

aSchool of Textile, Tianjin Polytechnic University, Tianjin 300387, China; bTianjin Key Laboratory of Fiber Modification andFunctional Fiber, Tianjin Polytechnic University, Tianjin 300387, China

(Received 23 July 2012; accepted 10 June 2013)

In this paper, 3,3-bis-(4-dimethylaminophenyl)-6-dimethylaminophthalide, 2,2-bis(4-hydroxyphenyl)propane, anddifferent mole ratios of binary eutectic mixture of aliphatic alcohol (myristyl alcohol and cetyl alcohol) were selected aschromophoric reagents, delomorphic reagents, and solvents of thermochromic energy-storage materials (TEMs),respectively. The thermochromic temperatures of TEMs were investigated according to Schroeder’s theory. The optimalTEMs with proper thermochromic temperature were chosen as the core material, and melamine–formaldehyde (M–F)resin served as the wall shell. The prepared microencapsulated thermochromic energy-storage materials (microTEMs)were characterized on the morphology, particle size, size distribution determination, and thermal properties. The resultsindicated that the spherical surfaces of microTEMs were smooth and compact. The diameters of microTEMs were foundin the range (0.9–4 μm) under the stirring speed of 10,000 rpm. Moreover, the microTEMs had good energy-storagecapacity and thermal stability. In the end, the prepared microTEMs were added to cotton/polyester fabric by coatingmethod to develop textile materials with thermochromic and energy-storage dual property.

Keywords: thermochomic; energy-storage; microTEMs; coating; functional fabric

Introduction

Recently, functional fibers have been studied to satisfythe esthetic needs of people in modern times. Diversefunctional fibers related to sensitivity of consumers havebeen developed (Hashemikia & Montazer, 2012; Li,Zhao, & Liu, 2013), and thus, needs for long-term dura-bility of functional fibers bring about the introduction ofmicroencapsulation technique.

In the past several years, the use of thermochromicmicrocapsule and energy-storage microcapsule havegained extensive attention with the aim of increasingthe stability of the thermochromic materials and energy-storage materials, which has been used in industrial,medical, security, clothing, daily decorations, etc. (Alkan,Sarı, Karaipekli, & Uzun, 2009; Onder, Sarier, & Cimen,2008; Sánchez, Sánchez-Fernandez, Romero, Rodríguez,& Sánchez-Silva, 2010; Sarier & Onder, 2007). There areseveral studies on the properties and performance of alarge number of thermochromic microcapsules andenergy-storage microcapsules. Ma and Zhu (2009) encap-sulated the thermochomic pigment into urea–formalde-hyde polymer. Sarı, Alkan, and Karaipekli (2010) usedpolymethylmethacrylate as shell materials to wrapenergy-storage materials (n-heptadecane). It was alsoreported that microcapsules containing energy-storage

material (n-octadecane) with melamine–formaldehyde(M–F) shell were fabricated by in situ polymerization(Cho, Kwon, & Cho, 2002; Shan, Wang, Zhang, &Wang, 2009). Apart from these, a series of polyurethanemicrocapsules containing n-octadecane were synthesizedas energy-storage microcapsules (Su, Wang, & Ren,2007).

As seen from the literature surveys above, a numberof different thermochromic microcapsules and energy-storage microcapsules have been prepared. Nevertheless,very little research has concentrated on the microencap-sulation of thermochromic energy-storage materials withM–F resin. In the meantime, M–F resin is a transparentpolymer with high durability for heat, which will notaffect the thermochromic properties of the core material.In additon, M–F resin is often used in a wide range offields because of its good properties, easy handlingprocessing, and low cost. It has been proved byYamagishi, Takeuchi, and Pyatenko (1999) that M–Fresin has good mechanical and good protection againstoutside environment. From this point of view, M–F resinis a promising polymer as shell material in the prepara-tion of microencapsulated thermochromic energy-storagematerials (microTEMs).

*Corresponding author. Email: xgmaemail@163.com

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In this study, novel microTEMs containing TEMs ascore material and M–F resin as shell materials weresynthesized by in situ polymerization. The microTEMswere characterized by using scanning electron microscopy(SEM), particle size distribution analysis, and FT-IRspectroscopy. Thermal properties and thermal stability ofthe microTEMs were determined by differential scanningcalorimetry (DSC) and TGA analysis. In addition,the microTEMs were added to cotton/polyester fabricby coating method, and the thermochomic and energy-storage properties of the fabric were also measured.

Experimental

Materials

Melamine and 37% formaldehyde as wall materials,styrene–maleic anhydride copolymer (SMA) as disper-sants, polyoxyethylene alkylphenol ether (OP-10) asemulsifier, and citric acid and sodium carbonate anhy-drous as pH controller were obtained from KermelChemical Reagent Factory, China, without any furtherpurification. 3,3-bis-(4-dimethylaminophenyl)-6-dimethyl-aminophthalide (BDPD) as chromophoric reagents inTEMs was obtained from Zhu zhou YaDi Industry,China. 2,2-bis(4-hydroxyphenyl)propane (BHPP) as delo-morphic reagents in TEMs was provided by ShentongChemical Co., China. Myristyl alcohol and cetyl alcoholwere procured from local market.

Preparation of the TEMs

BDPD (0.6 g) and BHPP (3 g) were added into 10 gbinary mixture of aliphatic alcohols, followed by uniformmixing at 95°C for 1 h.

Preparation of the microTEMs

About 2 g melamine and 3.86 g 37% formaldehyde in22.5ml of distilled water were adjusted to pH 9 with tri-ethanolamine. Even more preferably, the pH range wasfrom about 8.5 to 9.0. M–F precondensate was preparedby stirring at 70°C for 20min. Simultaneously, someamount of O/W emulsion of TEMs and 0.2 g NaOH in25ml of 8wt.% SMA copolymer aqueous solution wasprepared by 10,000 r/min stirring at 50°C for 10min. ThepH range for this aqueous solution should be adjusted to4.0–5.0 with 10% citric acid aqueous solution. O/W emul-sion was added into M–F prepolymer. Typically, a water-soluble M–F precondensate was dissolved in an aqueoussolution (known as the external phase). A discontinuousphase of a material to be encapsulated (known as the inter-nal phase or core material) was emulsified in the externalphase using emulsifier OP-10 as a protective colloid. Theresolution was then stirred at rotation speed of 400 r/min,

70°C for 120min. The step of causing the second M–Fprecondensate was also affected by decreasing the pH ofthe emulsion to below about 4.0 and adding heat thereto.After in situ polymerization on O/W emulsion surface,microTEMs slurry was decanted, washed with petroleumether to remove TEMs on the surface, and dried in theoven at 100°C for 24 h.

Characterization of the microTEMs

Infrared spectra of core material and microTEMs wereobtained with TENSOR37IR spectrometer (BRUKERCompany, Ettlingen, Germany). DSC measurements werecarried out utilizing Pekin–Elma DSC-7. Thermogravi-metric analysis measurement was performed with NET-ZSCH-STA409PC Thermogravimetric Analyzer (NETZSCH Corporation, Bayern, Germany) at room temperatureup to 573.15K at a 10K/min ramp rate under constantN2 flow. SEM was performed using a TM1000 (FEI,Akishima, Japan). Mean particle size and size distributionof microTEMs were determined using Image AnalyzerDelsa Nano C (Beckman Coulter, New York, USA.

Measurement of thermochromism

The fabric was tested before and after color change(under different temperatures) by using SF-600 color-measurement instrument (DATACOLOR Company,Luceme, Switzerland). The parameters of ΔE (total colordifference), ΔL (lightness difference), ΔC (chroma differ-ence), and ΔH (hue difference), indicating the thermo-chromic performance of the fabric based on the CIELABspace, were measured with the standard D65 light sourceand 10° visual field, respectively. Their relationship is4E= [(ΔL)2 + (ΔC)2 + (ΔH)2]1/2.

Step-cool curve of the fabric

In the experiment, step-cool curve was measured by thetesting device of thermal property (see Figure 1) to studythe adjusting temperature function of the treated fabric.

The fabric was put into the testing device of thermalproperty, and made sure that the thermal probe contactsthe fabric tightly before testing. The fabric was naturallycooled down at room temperature after being heated by

Figure 1. Testing device of thermal property.

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infrared light till 40°C. The temperature of the fabricwas recorded at intervals of 10 s.

Application of microTEMs to fabric

The dried microTEMs powders were mixed with polyac-rylate adhesive, forming a uniform mixture, which wasevenly coated to the cotton/polyester blends fabric. Then,the fabric was precured and cured using LTE-S87609coating machine (Werner Mathis AG Company, Zurich,Switzerland). The conditions of curing and theconcentration of microTEMs are given in Table 1.

Results and discussion

Determination of the solvent in TEMs

TEMs were used as the core materials of microTEMs inthis experiment, which consisted of three components:the chromophoric reagent (BDPD), the delomorphicreagent (BHPP), and the solvent (binary eutectic mixtureof myristyl alcohol and cetyl alcohol). Reversible colorchange occurred via two competing reactions, namely,that between chromophoric reagents and delomorphicreagents and the other between solvent and delomorphicreagents. The first of these interactions prevailed at lowertemperatures where the solvent existed in its solid formand gave rise to colored chromophoric reagent–delomor-phic reagent complexes, the solvent melted at highertemperature, causing the solvent–delomorphic reagentinteraction to dominate. Thus, chromophoric reagent–delomorphic reagent complexes were destroyed whichconverted the system into its colorless state (seeFigure 2). The temperature at which decolorization/color-ization occurred was controlled by the melt temperatureof the solvent. However, the melt temperatures ofavailable solvents are too high, which cannot meet theneed of practical application in terms of the thermochro-mic temperature. It is a bottleneck in the development ofour domestic TEMs.

As we all know, a binary eutectic mixture ofphase-change materials presents three phases equilibrium,and the minimum eutectic temperature is calculatedaccording to Schroeder’s theory:

T ¼ 1=½ð1=Tf Þ � ðR ln xA=D1sHAÞ�: ð1Þ

Every parameter in formula (1) is explained as follows:

xA is the molar fraction of main constituent A in

mixtures; D1sHA is the heat of fusion of pure compound

A, J/mol; Tf is the melt temperature of pure compoundA, K; T is the melt temperature of the mixturescontaining compound A, K; and R is the universal gasconstant, 8.315 J/kmol.

Based on the view mentioned above, different moleratios of binary phase-change materials (myristyl alcoholand cetyl alcohol) were blended as the solvent of TEMsaccording to Schroeder’s theory to prepare differentTEMs with different thermochomic temperature (shownin Figure 3). The theoretical eutectic curve of binaryeutectic mixture (myristyl alcohol and cetyl alcohol) wascalculated based on the Equation (1) (shown in Figure 4).

The thermochomic temperature curve was found tomove in line with the theoretical eutectic curve by com-paring Figures 3 and 4. The mole ratio of the myristylalcohol/cetyl alcohol binary eutectic, 7:3, was selected asthe solvent of TEMs in order to decrease the thermo-chromic temperature of TEMs.

The color change of the TEMs (core materials) aboveand below the melt temperature of solvent is shown inFigure 5, in which the color of specimen can be changedfrom blue below melt temperature to white above melttemperature. Meanwhile, the color change is accompa-nied by energy absorption and release due to the statechange of solvent in TEMs from solid to liquid. There-fore, it is necessary to be encapsulated by polymer shellto form microTEMs.

Morphologies of microTEMs

Particle shape, size, and size distribution of the micropar-ticles are very important for the properties and applicabil-ity of microTEMs, which were all evaluated from theimages taken from electron microscopy at several magni-cations. SEM photograph of surface morphologies of themicroTEMs at different magnications are shown inFigure 6. They have relatively uniform sizes, sphericalshape, and smooth surface. The TEMs are encapsulatedin the M–F resin, solving the problem of liquid release inthe process of phase change. As shown in Figure 8, themicroTEMs display good thermal stability under theroom temperature and the elevated temperature situations.There are few changes of the overall weight with theincrease of time due to the protection of M–F resin. Thesmall weight loss during the previous 10 days is result ofthe evaporation of water in the microTEMs, which isconsistent with the result of TGA before 100°C (shownin Figure 12). So, it may be also advantageous of highheat transfer ratio and applicability.

Figure 7 shows the particle size distribution of themicroTEMs. The size of all the resulted particles isbelow 4 μm by stirring at the rate of 10,000 rpm, andtheir size distribution is narrow. Thus, the prepared

Table 1. MicroTEMs application conditions to the fabric.

MicroTEMsconcentration (%)

Coatingweight(g/m2)

Precuringcondition

Curingcondition

1.5–2 35 80°C, 3min 170°C, 2.5min

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microTEMs in this study seem to be adequate for thepreparation of thermochomic energy-storage fabric dueto their monodispersed size distribution.

Structure of microTEMs

Generally, FT-IR spectra are used to characterize themicrocapsules structurally because it is possible to provethe existence of materials in the microcapsules. In thiswork, the FT-IR spectra of the M–F resin, TEMs, andmicroTEMs are given in Figure 9 to prove the co-presenceof M–F resin and TEMs in the microTEMs.

It is clearly seen that the FT-IR spectra of TEMs andmicroTEMs both constitute of the peaks of TEMs bycomparing the FT-IR spectra of M–F resin, TEMs, andmicroTEMs. For example, the peaks at 2850–3000 and1731 cm�1 present C–H stretching vibration of aliphaticalcohol (myristyl alcohol and cetyl alcohol) in TEMsspectra and C=O stretching vibration of the BDPD inTEMs spectra, respectively. To the in-plane rockingvibration of the CH2 group, 721 cm�1 in the spectra ofTEMs and microTEMs are assigned. Similarly, nocharacteristic bands of CH2 groups can be observed fromthe FT-IR spectra of M–F resin. Therefore, it can beconcluded that microTEMs had been successfullyfabricated.

Thermal properties of microTEMs

TEMs and microTEMs were investigated by using DSC,and their curves are shown in Figures 10 and 11. It isseen from Figure 10 that TEMs melt at 33.9°C, andcrystallize at 29.4°C. The latent heats of melting andfreezing of TEMs are measured as 85.92 and �61.12 J/g,respectively. The thermal properties of the microTEMsare displayed in Figure 11. The microTEMs melt at

O

N

N N

H3C CH3

CH3

CH3

H3C

H3C

+O

CH3H3C

OH

OH

N

N N

H3C CH3

CH3

CH3

H3C

H3C

O

CH3

CH3

OHHOO-

delomorphic reagent

(BHPP)

chromophoric reagent

(BDPD)

chromophoric reagent-delomorphic reagent complexes

Figure 2. The reaction mechanism of TEMs.

Figure 3. Thermochomic temperature curve of TEMs.

Figure 4. Theoretical eutectic curve of binary eutectic mixture.

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30.4°C, and crystallize at 28.3°C. The latent heat ofmelting and freezing of microTEMs are 19.05 and�18.82 J/g, respectively. These results show that thethermal properties of microTEMs are quite satisfactoryfor energy-storage applications.

The percentage of TEMs in microTEMs is also a keyfactor, which determines directly enthalpy and thusenergy-storage efficiency of microTEMs. The microen-capsulation ratio of TEMs in M–F resin is calculatedapproximately with the following formula (2) based onenthalpy values as 22wt.% in the experiments (Alay,Alkan, & Göde, 2011).

PTEMs ¼ ðDHmicroTEMs=DHTEMsÞ � 100%; ð2Þ

where PTEMs is the microencapsulation ratio of TEMs,and ΔHmicroTEMs and ΔHTEMs are the measuredenthalpies of microTEMs and TEMs itself, respectively.

By comparing DSC results shown in Figures 10 and11, phase-change temperature range of microTEMs isslightly narrower than pure TEMs, and the phase-change

temperature of the microTEMs is slightly lower thanTEMs, since the molecules of TEMs connect each otherby hydrogen bonding due to the fact that there areabundant higher fatty alcohols in TEMs, which enduethe pure TEMs with higher phase-change temperature.However, the amount of hydrogen bonding between mol-ecules in microTEMs decreases greatly owing to theTEMs being encapsulated in the M–F resin. In summary,the microTEMs have higher phase-change enthalpy andideal property of energy storage, which lay the founda-tion for its application in intelligent textiles.

Thermal stability of the microTEMs

The thermal stabilities of the microTEMs were evaluatedby means of TGA. The degradation data obtained fromthe TGA thermograms in Figure 12 are given in Table 2.The TGA curve has three steps for microTEMs. ThemicroTEMs start to lose weight at approximately 61.3°C,showing a small weight loss (6.20%), which can beassigned to the volatilization of residual water in micro-

Figure 5. Color change states of TEMs (core materials) at different temperatures: (a) 15°C and (b) 40°C.

Figure 6. The morphologies of microTEMs. Figure 7. Particle size distribution of microTEMs.

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TEMs during the first stage between 60 and 140°C with amaximum rate at 78.5°C, and the following high weightloss (18.98%) between 140 and 400°C indicates that partof the core materials (TEMs) leak from the microTEMs.The last big weight loss (41.93%) between 400 and 600°Cis the result of the decomposition of wall material M–Fresin and substantial volatilization of TEMs.

The DTG curve of the microTEMs also has threeweight loss peaks, and shows lower weight loss ratebelow 180°C with a maximum rate at 170.9°C. Itindicates that the microTEMs have excellent thermalproperties, which are stable chemically and feasible to betreated on the fabric.

Morphology of the microTEMs treated fabric

Cotton/polyester fabric treated with microTEMs by theoptimal coating condition was chosen to investigate thefabric morphology. SEM images of the cotton/polyester

Figure 9. FT-IR spectra of M–F resin, TEMs and microTEMs.

Figure 10. DSC analyses of TEMs.

Figure 8. The weight change of microTEMs under differenttemperatures.

Figure 11. DSC analyses of microTEMs.

Figure 12. TGA analysis of microTEMs.

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fabric before and after treated by coating, given inFigure 13, show that the fibers are coated by theadhesive, and microTEMs are adhered to the surface offibers intensively. The fabric lines cannot be seen clearly.It can be thought that microTEMs are linked to the fibersurface by creating cross-linking.

Thermochomic and temperature-adjustable propertiesof the treated fabric

The thermochomic property of the fabric treated withmicroTEMs is shown in Table 3. The high ΔE valueindicates that the treated fabric has an excellent thermo-chomic property. The fabric can change color from pur-plish-blue to light blue, which is correspond to thenegative value of ΔL. Apart from these, the ΔC and ΔHalso undergo a tremendous change. The larger the abso-lute value of the above parameters is, the more obviousthe thermochomic property of fabric is. In all, the micro-TEMs can impart excellent thermochomic property tothe fabric.

The temperature-adjustable property of the fabrictreated with microTEMs was studied by step-cool curves,as shown in Figure 14. It can be found that the treatedfabric can keep a higher temperature at each time duringthe cooling process compared with the fabric untreatedwith microTEMs. An obvious “buffer platform” oftemperature can be obtained in the step-cool curve. Themaximal temperature difference is about 4°C. Itdemonstrates that the treated fabric has obvioustemperature-adjustable property.

Table 2. TGA data of microTEMs.

Degradation interval (°C) Mass loss (%)

MicroTEMs 60–140 (step 1) 6.20140–400 (step 2) 18.98400–600 (step 3) 41.93

Figure 13. SEM photos of the cotton/polyester fabric before (a) and after (b) treated with microTEMs by coating method.

Table 3. The thermochomic properties of the fabric treatedwith microTEMs.

Sample ΔE ΔL ΔC ΔH

Treated fabric 17.281 �10.607 13.063 3.936

Figure 14. The effect of step-cool curves of the treated anduntreated fabric.

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Conclusions

Binary eutectic mixture of aliphatic alcohol (mole ratiomyristyl alcohol/cetyl alcohol = 7:3) was selected to servethe solvent of TEMs according to Schroeder’s theory,decreasing the thermochomic temperature greatly. ThemicroTEMs were prepared successfully by using M–Fresin as wall shell. The TEMs in the microTEMs wereproved by FT-IR spectroscopy analysis. SEM andparticle size distribution analyses showed that themicroTEMs had relatively uniform sizes, spherical shape,and smooth surface. By using DSC analysis method, thetemperatures of melting and freezing and latent heats ofmelting and freezing of the microTEMs were determinedas 30.4 and 28.3°C and 19.05 and �18.82 J/g, respec-tively. TGA investigation showed that the microTEMsdegraded in three steps and were resistant to hightemperatures. The microTEMs were added to cotton/polyester fabrics by coating method, which impartthermochromism and temperature-adjustable functions tothe fabric.

AcknowledgementsThe Key Project of Tianjin Science & Technology ResearchProgram (The Project Number: 11ZCKFSF01900) isacknowledged for supporting this work.

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