Energy Conversion and Management 50 (2009) 723–729

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Energy Conversion and Management

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Microencapsulation of butyl stearate as a phase change material by interfacialpolycondensation in a polyurea system

Chen Liang *, Xu Lingling, Shang Hongbo, Zhang ZhibinCollege of Material Science and Engineering, Nanjing University of Technology, Nanjing 210009, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 March 2008Accepted 30 September 2008Available online 17 November 2008

Keywords:Phase change materialButyl stearatePolyureaInterfacial polycondensationMicrocapsule

0196-8904/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.enconman.2008.09.044

* Corresponding author. Tel./fax: +86 025 8358723E-mail addresses: doseng_1982@hotmail.com, inte

For the last 20 years, microencapsulated phase change materials (MicroPCMs), which combine microen-capsulation technology and phase change material, have been attracted more and more interest. By over-coming some limitations of the PCMs, the MicroPCMs improve the efficiency of PCMs and make itpossible to apply PCMs in many areas. In this experiment, polyurea microcapsules containing phasechange materials were prepared using interfacial polycondensation method. Toluene-2,4-diisocyanate(TDI) and ethylenediamine (EDA) were chosen as monomers. Butyl stearate was employed as a corematerial. The MicroPCMs’ properties have been characterized by dry weight analysis, differential scan-ning calorimetry, Fourier transform IR spectra analysis and optical microscopy. The results show thatthe MicroPCMs were synthesized successfully and that, the phase change temperature was about29 �C, the latent heat of fusion was about 80 J g�1, the particle diameter was 20–35 lm.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Today the world consumes energy at a rate of approximately4.1 � 1020 J/yr. With the increase in population and the growth ofeconomy, the demand for energy will be more than double of whatis now in 2050, and more than triple by the end of the century [1].But with the current rates of consumption, the reserves of fossilfuels in the form of oil and natural gas will fall short of this demandin nearly 50 years.

With the aggravation of energy crisis, more and more attentionis being paid to energy saving. Because of the optimum use ofrenewable energies [2], thermal energy storage (TES), the tempo-rary storage system of high or low temperature energy for lateruse [3], has been attracting more and more interest. It is expectedto be used in many areas, such as solar energy storage, cool storage,spacecraft thermal systems, and building temperature fluctuationscontrol [2,3]. Phase change materials (PCMs) are one of the mostpromising areas of thermal energy storage. These can store/releaseenergy from/to the surroundings during phase change. The quan-tity of energy per weight of the material stored/released is so largethat less volume is required by the system to store the energy. Inaddition, during the phase change, the temperature remains nearlyconstant, which is beneficial for the control of the temperature ofthe surroundings [4]. As a phase change material, the main charac-teristics required are suitable phase change temperature, high

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9.r_1982@tom.com (C. Liang).

change enthalpy, high thermal conductivity in both liquid and so-lid phases, good thermal stability, high phase change speed, smallvolume change, compatibility with container materials, and shouldbe non-toxic, non-flammable, non-polluting, cheap and abundant[2,5].

However, this kind of heat storage system has also caused someproblems. Many PCMs can corrode the container. This means thatthey have short service lives, and the packing and maintenancecosts are high [3]. Also the PCMs may freeze on the heat exchangersurface. This will cause a poor heat transfer rate because of the lowthermal conductivities of some PCMs, such as paraffin wax [6].Many attempts have been made to solve these problems. Recently,a new technique of using microcapsulated PCMs in energy storagehas been developed. Microencapsulated PCMs (MicroPCMs) arecolloidal particles composed of a protective shell and one or morePCMs (core substance) [7,8]. Because these can overcome severallimitations of the PCMs, the MicroPCMs improve the efficiency ofPCMs and make it possible to apply PCMs more widely. Microcap-sulated PCMs have many advantages such as reducing the reactiv-ity of the PCMs with the outside environment, increasing the heattransfer areas of PCMs and permitting the core material to with-stand frequent changes in the volume of the storage material dur-ing the phase change [6]. Nowadays many researchers are studyingon this area. Shulkin and Stover [9] prepared the microcapsules byinterfacial polyaddition between styrene-maleic anhydride (SMA)copolymers and polyamines. Cho [7] microencapsulated octade-cane as a phase change material by interfacial polymerization withtoluene-2,4-diisocyanate (TDI) and diethylenetriamine (DETA)used as monomers in an emulsion system. In China, Guanglong

Fig. 1. The sketch map of interfacial polycondensation. Monomer: X, Y. Product:(X � Y)n or (X)n.

Fig. 2. FT-IR spectra of butyl stearate (a), microcapsulated PCMs (b) and emptymicrocapsules (c).

724 C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729

et al. [10] microencapsulated n-hexadecane as a phase changematerial in polyurea, and the microencapsulated PCMs showed agood potential as thermal energy storage materials. Li-xin et al.[11] prepared a kind of heat energy storage microcapsule usingmelamine formaldehyde resin as a shell material and a phasechange material, which melts at 24 �C as a core material. Also,Fan et al. [12] not only prepared microencapsulated phase changematerials with melamine and formaldehyde as shell-formingmonomers and N-octadecane as a core material, but also studiedthe effects of the nucleating agents (which can prevent thesuper-cooling), including sodium chloride, 1-octadecanol andpararffin, on the melting and crystallization behavior, morphologyand dispersibility of microcapsules.

Fig. 3. DSC curves of microcapsulated PCMs prepared with vario

Many methods are developed for microcapsulation includinginterfacial polymerization, in situ polycondensation, and complexcoacervation. The interfacial polycondensation is a kind of popularmethod. It has several advantages, such as high reaction speed,

us ratios by weight of core and shell, 2:1(a), 3:1(b), 4:1(c).

Table 1Thermal characteristics of microcapsulated PCMs prepared with various ratios byweight of core and shell.

No. TDI(g)

EDA(g)

Butyl stearate(g)

Core/shell

OP-10(g)

DHfus

(J g�1)Tonset (�C)

1 4.35 1.50 12.00 2:1 1.00 76.31 28.602 4.35 1.50 17.55 3:1 1.00 85.92 29.373 4.35 1.50 23.40 4:1 1.00 76.58 28.49

Table 2The capacity of microcapsulated PCM after different thermal cycles.

Thermal cycles DHfus (J g�1) Tonset (�C) Tpeak (�C)

1 85.92 29.37 33.8020 84.81 29.33 34.01

400 88.20 29.05 34.16

C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729 725

mild reaction course and also its products have low penetrability.So it seems to be more feasible than others. The sketch map ofinterfacial polycondensation is shown in Fig. 1 [13]. The schemeof interfacial polycondensation can be explained as follows. Thecore material is made into droplet. The capsule shell reactivemonomers polymerize on the surface of the droplets. When the ini-tially formed oligomers are insoluble at the interface of the drop-lets, they grow, and a thin monolayer membrane forms aroundthe droplets. The polycondensation leads the monolayer mem-brane to be a shell, and leads to the formation of a microscopicshell around the droplets at last [7]. Polyurea, polyurethane,polyester, polyamide and amine resin can be used as the shellmonomers in the interfacial polycondensation process. In this pa-per, butyl stearate as a phase change material was microencapsu-lated by interfacial polymerization with toluene-2,4-diisocyanate(TDI) and ethylenediamine (EDA) used as monomers in an emul-sion system.

Fig. 4. DSC curves of microcapsulated PCMs after different t

2. Materials and methods

2.1. Materials

TDI (toluene-2,4-diisocyanate, C.P.) and EDA (Ethylenediamine,A.R), used as shell-forming monomers, were obtained fromShanghai Lingfeng Chemical Reagent Co. Ltd. Butyl stearate (A.R.,Sinopharm Group Chemical Reagent Co., Ltd.) was employed as acore material. Nonionic surfactant, OP-10 (C.P., China NationalMedicines Group Shanghai Chemical Reagents Co.), was used asan emulsifier. Cyclohexane (A.R.), used as an assistant reagent(solvent media), was obtained from Shanghai Lingfeng ChemicalReagent Co., Ltd.

2.2. Preparation of microcapsules

The microcapsulation was carried out in a three-neckround-bottomed flask equipped with a mechanical stirrer. Beforeencapsulation, the core material (butyl stearate) was dissolved in

hermal cycles, 1 cycle (a), 20 cycles (b), 400 cycles (c).

Table 4Dry weight of MicroPCMs prepared with different emulsifier dosages.

No. TDI(g)

EDA(g)

Butylstearate(g)

OP-10 (g)

Dry weight ofmicrocapsules (g)

Calculated dryweight (g)

1 4.35 1.50 12.00 0.00 16.22 17.852 4.35 1.50 12.00 0.50 16.16 17.853 4.35 1.50 12.00 1.00 16.37 17.854 4.35 1.50 12.00 1.50 16.98 17.855 4.35 1.50 12.00 2.00 16.56 17.856 4.35 1.50 12.00 2.50 16.50 17.85

Table 5Dry weight of microcapsules prepared with different cyclohexane dosages (OP-10,1.0 g).

726 C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729

an organic solution containing cyclohexane and TDI (oil solublemonomers), and an aqueous solution of OP-10 was prepared.Then the organic solution was added to the aqueous surfactantsolution and the mixture was emulsified mechanically at a stir-ring rate of 500 r/m to form an O/W emulsion. After stirring forfew minutes, the aqueous soluble monomers (EDA), which werediluted in distilled water before, were added into the emulsionsystem slowly and the mixture was heated to 65 �C. Then theinterfacial polycondensation reaction took place between TDIand EDA at the oil–water interface. The reaction lasted for about2–3 h. The resultant microcapsules were filtered, washed anddried.

2.3. Analysis of the microcapsules

The Fourier transform IR spectra (FT-IR) of the microcapsuleswere obtained to identify the structure of the shell polymer. Thethermal properties of the microcapsules containing phase changematerials (such as heat of fusion and melting point) were evalu-ated by Differential Scanning Calorimetry. The dry weight of sam-ples was measured by BL-220H electronic balance. The shapes ofthe microcapsules were observed by optical microscope. The parti-cle size and the distribution of microcapsules were measured by la-ser particle analyzer. The thermal stability of microcapsules wasevaluated by testing through cycles of alternative heating andcooling.

No. TDI(g)

EDA(g)

Cyclohexane(g)

Dry weight ofmicrocapsules (g)

Calculated dryweight (g)

1 4.35 1.50 9.00 4.30 5.852 4.35 1.50 12.00 4.40 5.853 4.35 1.50 15.00 4.52 5.854 4.35 1.50 18.00 4.80 5.85

Table 6Dry weight of microcapsules prepared with different core–shell mass I (OP-10, 1.0 g).

No. TDI(g)

EDA(g)

Butylstearate(g)

Core/shell

Dry weight ofmicrocapsules (g)

Increased dryweight (g)

1 4.35 1.50 0.00 Empty 4.40 0.002 4.35 1.50 12.00 2:1 16.37 11.873 4.35 1.50 17.55 3:1 21.89 17.49

Table 7Dry weight of microcapsules prepared with different core–shell mass II (OP-10, 1.5 g).

No. TDI(g)

EDA(g)

Butylstearate(g)

Core/shell

Dry weight ofmicrocapsules (g)

Increased dryweight (g)

1 4.35 1.50 0.00 Empty 4.70 0.002 4.35 1.50 12.00 2:1 16.58 11.883 4.35 1.50 17.55 3:1 21.17 16.474 4.35 1.50 23.40 4:1 24.95 20.25

3. Results and discussion

3.1. FT-IR spectra

FT-IR spectra of butyl stearate (a), empty microcapsules (b) andmicrocapsulated PCMs (c) are shown in Fig. 2.

In spectra (b) and (c), an absorption band near 3,320 cm�1 is as-signed to NH, and a band near 1640 cm�1 is assigned to C@O. Theseprove that the amine group (CONH) is formed. Also, in the twospectra there is no absorption band at 2200–2280 cm�1, which isassigned to an isocyanate group that indicates there is almost nounreacted isocyanate group in the microcapsule core. In spectra(a) and (b), we can find the absorption band near 1168 cm�1 ofC–O–C of butyl stearate. This proves that chemical structure of bu-tyl stearate, used as a kind of core material, has almost no change.From this discussion, it is possible to know that butyl stearate canbe microencapsulated well by using polyurethane as a phasechange material.

3.2. Thermal characteristics of the microcapsules

The thermal properties of the microcapsules containing phasechange materials were tested using Differential Scanning Calorim-eter. Fig. 3 and Table 1 show the thermal characteristics of micro-capsulated PCMs prepared with various ratios by weight of coreand shell. It is found that all samples prepared with different ratios

Table 3Dry weight of MicroPCMs prepared at different stirring rates.

No. Shellmaterial(g)

Butylstearate(g)

Stirringrate (r/m)

Dry weight ofmicrocapsules (g)

Calculated dryweight (g)

1 5.85 12.00 300 15.88 17.852 5.85 12.00 400 13.34 17.853 5.85 12.00 500 16.08 17.854 5.85 12.00 600 15.69 17.855 5.85 12.00 700 15.66 17.85

by weight of core and shell have relatively high heat of fusion (near80 J g�1) and appropriate melting point (near 28.7 �C). When thecore–shell mass ratio is 4:1, the heat of fusion is higher than others.So the more appropriate ratio by weight of core and shell of micro-capsulated PCMs is 4:1.

As an important property of microencapsulated PCMs, the ther-mal stabilities of samples were measured. The samples were sub-jected to cycles of melting and solidification in the experiment.

Table 8Dry weight of microcapsules prepared with different core–shell mass III (OP-10,2.0 g).

No. TDI(g)

EDA(g)

ButylStearate(g)

Core/shell

Dry weight ofmicrocapsules (g)

Increased dryweight (g)

1 4.35 1.50 0.00 Empty 4.76 0.002 4.35 1.50 12.00 2:1 16.56 11.803 4.35 1.50 17.55 3:1 18.90 14.144 4.35 1.50 23.40 4:1 23.89 19.13

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The microcapsules were sealed in a glass tube, then repeatedlyheated in 50–60 �C water bath for 15 min, and then cooled in anice-water bath to 0–5 �C for 15 min. After 1, 20, 400 cycles of alter-

Fig. 5. Particle size distribution of microcapsule, 0 g OP-10 (

native heating and cooling, the energy storage of microcapsuleswas measured by DSC, and the results are shown in Fig. 4 andTable 2. After thermal cycles, even after 400 cycles of alternative

a), 0.5 g OP-10 (b), 1.5 g OP-10 (c) and 2.5 g OP-10 (d).

728 C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729

heating and cooling, the phase change temperature and the phasechange enthalpy of encapsulated butyl stearate maintained nearlythe same value. The microencapsulated PCMs have a good thermalstability.

3.3. Dry weight of microcapsules

The dry weight of microcapsulated PCMs versus different exper-imental parameters was measured by electronic balance. Becausedry weight indicates the outcome of microencapsulated PCMs ob-tained from the experiment, several useful information can be ob-tained by analyzing the dry weight of the samples.

The stirring rate of the mechanical overhead stirrer is an impor-tant synthesis parameter. So a series of stirring rates, 300 r/m,400 r/m, 500 r/m, 600 r/m and 700 r/m, respectively, were chosento prepare the microcapsules. Table 3 shows the relationship be-tween the stirring rate of the mechanical stirrer and dry weightof microcapsules. From Table 3, it can be concluded that the stir-ring rate of the mechanical stirrer has little influence on dryweight, because of the nearly similar value of dry weight of Micro-PCMs prepared at different stirring rates. Yet in fact, two low or toohigh stirring rate of the mechanical stirrer may cause some prob-lems, such as reuniting and spattering. So the suitable stirring rateof the mechanical stirrer is 500 r/m.

Emulsifier can diminish the droplet of core material and cancause the uniform size distribution of microcapsules. It is animportant reagent for microcapsule preparation. Among severalkinds of emulsifiers, OP-10 is a good one. However, the hydroxylgroups of OP-10 can react with TDI slightly. In order to clearwhether OP-10 influences the microcapsules formed or not, weprepared a series of PCMs samples with various emulsifier dosages,measured the dry weight, and listed it in Table 4. From the data, itcould be found that the emulsifier dosage had little effect on thedry weight of the microcapsules. It elucidates that the dosage ofOP-10 that reacted with TDI is little. The usage of emulsifier hasno prominent influence on the microcapsules formed.

Cyclohexane is an important assistant reagent. It is a kind of sol-vent medium. A series of samples with different cyclohexane dos-

Table 9D50 of microcapsules prepared with different emulsifier dosages.

No. TDI (g) EDA (g) Butyl stearate (g) OP-10 (g) D50 (lm)

1 4.35 1.50 12.00 0.50 30.912 4.35 1.50 12.00 1.00 30.713 4.35 1.50 12.00 1.50 31.454 4.35 1.50 12.00 2.00 6.645 4.35 1.50 12.00 2.50 29.67

Fig. 6. Optical microscopy photo

ages were perpared to estimate the influence of the cyclohexanedosage on microcapsules formed. The data are shown in Table 5.The dry weight of microcapsules prepared with different cyclohex-ane dosage is nearly the same, it can be found that the usage ofcyclohexane also had no great influence on the microcapsulesformed.

The package rate of MicroPCMs is an important parameter, andthis parameter can be obtained by analyzing the dry weight ofMicroPCMs. The microcapsules were prepared with various core–shell mass ratios and their dry weight was measured. Data areshown in Table 6–8. From sample 1 (empty microcapsules) tosample 4 (the core–shell mass ratio is 4:1), the core material in-creased, while the dry weight increased. Also, the increased dryweight and increased core material (butyl stearate) are generallyequal. So, it can prove indirectly that the package rate of Micro-PCMs is good.

3.4. Particle size distribution of microcapsules

The particle size distributions of microcapsule are shown in theFig. 5, when the amount emulsifier (OP-10) used was 0 g, 0.5 g,1.5 g and 2.5 g. The sizes of microcapsules are about 20–35 lm.As shown in Fig. 5, the size distribution of the microcapsules issharper when the emulsifiers were used during the reaction. Thiselucidates that the emulsifier has an important function in the uni-form size distribution of the microcapsules.

In order to clear the influence of the emulsifier dosage on thesize of microcapsules, we analyzed the D50 of microcapsules ofdifferent emulsifier dosages. From the data which are shown inTable 9, it is possible to know that although the emulsifier hasan important influence on the uniform size distribution ofmicrocapsules, it has little influence on the size value of themicrocapsules.

3.5. Shape of the microcapsules

Optical microphotographs of microencapsulated PCMs areshown in Fig. 6. The optical micorphotograph was taken after poly-merization, and it shows many small particles of about 20–35 lmdiameter. When they were dried, the surface of most of the micro-capsules was very smooth and the shape was very regular. How-ever, powdery clusters of small particles could be seen.

4. Conclusions

Polyurea microcapsules containing phase change materialswere prepared successfully by using the interfacial polycondensa-tion. The testing results show that MicroPCMs’ phase change tem-

graph of the microcapsules.

C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729 729

perature is about 29 �C, the latent heat of fusion is about 80 J g�1,the particle diameter is 20–35 lm, and that the MicroPCMs havea good property of thermal periodicity. Also with the dry weightanalysis, it is possible to know that the packing rate of MicroPCMsis good.

Acknowledgements

The authors would like to thank Liu Li, Liu Fang, Yang Jin et al.for their helpful advice and discussions.

Appendix A

DHfus

Latent heat of phase change materials (kJ/kg) Tonset The initial temperature of phase change (�C) Tpeak The peak temperature of phase change stage (�C) D50 Median particle diameter (lm)

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