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Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system
Energy Conversion and Management 50 (2009) 723–729
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system Chen Liang *, Xu Lingling, Shang Hongbo, Zhang Zhibin College of Material Science and Engineering, Nanjing University of Technology, Nanjing 210009, China
a r t i c l e
i n f o
Article history: Received 6 March 2008 Accepted 30 September 2008 Available online 17 November 2008 Keywords: Phase change material Butyl stearate Polyurea Interfacial polycondensation Microcapsule
a b s t r a c t For the last 20 years, microencapsulated phase change materials (MicroPCMs), which combine microencapsulation technology and phase change material, have been attracted more and more interest. By overcoming some limitations of the PCMs, the MicroPCMs improve the efficiency of PCMs and make it possible to apply PCMs in many areas. In this experiment, polyurea microcapsules containing phase change 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 core material. The MicroPCMs’ properties have been characterized by dry weight analysis, differential scanning calorimetry, Fourier transform IR spectra analysis and optical microscopy. The results show that the MicroPCMs were synthesized successfully and that, the phase change temperature was about 29 °C, the latent heat of fusion was about 80 J g 1, the particle diameter was 20–35 lm. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Today the world consumes energy at a rate of approximately 4.1 1020 J/yr. With the increase in population and the growth of economy, the demand for energy will be more than double of what is now in 2050, and more than triple by the end of the century [1]. But with the current rates of consumption, the reserves of fossil fuels in the form of oil and natural gas will fall short of this demand in nearly 50 years. With the aggravation of energy crisis, more and more attention is being paid to energy saving. Because of the optimum use of renewable energies [2], thermal energy storage (TES), the temporary storage system of high or low temperature energy for later use [3], has been attracting more and more interest. It is expected to be used in many areas, such as solar energy storage, cool storage, spacecraft thermal systems, and building temperature fluctuations control [2,3]. Phase change materials (PCMs) are one of the most promising areas of thermal energy storage. These can store/release energy from/to the surroundings during phase change. The quantity of energy per weight of the material stored/released is so large that less volume is required by the system to store the energy. In addition, during the phase change, the temperature remains nearly constant, which is beneficial for the control of the temperature of the surroundings [4]. As a phase change material, the main characteristics required are suitable phase change temperature, high
* Corresponding author. Tel./fax: +86 025 83587239. E-mail addresses: [email protected], [email protected] (C. Liang). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.09.044
change enthalpy, high thermal conductivity in both liquid and solid phases, good thermal stability, high phase change speed, small volume change, compatibility with container materials, and should be non-toxic, non-flammable, non-polluting, cheap and abundant [2,5]. However, this kind of heat storage system has also caused some problems. Many PCMs can corrode the container. This means that they have short service lives, and the packing and maintenance costs are high [3]. Also the PCMs may freeze on the heat exchanger surface. This will cause a poor heat transfer rate because of the low thermal 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 storage has been developed. Microencapsulated PCMs (MicroPCMs) are colloidal particles composed of a protective shell and one or more PCMs (core substance) [7,8]. Because these can overcome several limitations of the PCMs, the MicroPCMs improve the efficiency of PCMs and make it possible to apply PCMs more widely. Microcapsulated PCMs have many advantages such as reducing the reactivity of the PCMs with the outside environment, increasing the heat transfer areas of PCMs and permitting the core material to withstand frequent changes in the volume of the storage material during the phase change [6]. Nowadays many researchers are studying on this area. Shulkin and Stover [9] prepared the microcapsules by interfacial polyaddition between styrene-maleic anhydride (SMA) copolymers and polyamines. Cho [7] microencapsulated octadecane as a phase change material by interfacial polymerization with toluene-2,4-diisocyanate (TDI) and diethylenetriamine (DETA) used as monomers in an emulsion system. In China, Guanglong
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Fig. 1. The sketch map of interfacial polycondensation. Monomer: X, Y. Product: (X Y)n or (X)n.
et al. [10] microencapsulated n-hexadecane as a phase change material in polyurea, and the microencapsulated PCMs showed a good potential as thermal energy storage materials. Li-xin et al. [11] prepared a kind of heat energy storage microcapsule using melamine formaldehyde resin as a shell material and a phase change material, which melts at 24 °C as a core material. Also, Fan et al. [12] not only prepared microencapsulated phase change materials with melamine and formaldehyde as shell-forming monomers and N-octadecane as a core material, but also studied the effects of the nucleating agents (which can prevent the super-cooling), including sodium chloride, 1-octadecanol and pararffin, on the melting and crystallization behavior, morphology and dispersibility of microcapsules.
Fig. 2. FT-IR spectra of butyl stearate (a), microcapsulated PCMs (b) and empty microcapsules (c).
Many methods are developed for microcapsulation including interfacial polymerization, in situ polycondensation, and complex coacervation. The interfacial polycondensation is a kind of popular method. It has several advantages, such as high reaction speed,
Fig. 3. DSC curves of microcapsulated PCMs prepared with various ratios by weight of core and shell, 2:1(a), 3:1(b), 4:1(c).
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C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729 Table 1 Thermal characteristics of microcapsulated PCMs prepared with various ratios by weight of core and shell. No.
TDI (g)
EDA (g)
Butyl stearate (g)
Core/ shell
OP-10 (g)
DHfus (J g 1)
Tonset (°C)
1 2 3
4.35 4.35 4.35
1.50 1.50 1.50
12.00 17.55 23.40
2:1 3:1 4:1
1.00 1.00 1.00
76.31 85.92 76.58
28.60 29.37 28.49
Table 2 The capacity of microcapsulated PCM after different thermal cycles. Thermal cycles
DHfus (J g 1)
Tonset (°C)
Tpeak (°C)
1 20 400
85.92 84.81 88.20
29.37 29.33 29.05
33.80 34.01 34.16
2. Materials and methods mild reaction course and also its products have low penetrability. So it seems to be more feasible than others. The sketch map of interfacial polycondensation is shown in Fig. 1 [13]. The scheme of interfacial polycondensation can be explained as follows. The core material is made into droplet. The capsule shell reactive monomers polymerize on the surface of the droplets. When the initially formed oligomers are insoluble at the interface of the droplets, they grow, and a thin monolayer membrane forms around the droplets. The polycondensation leads the monolayer membrane to be a shell, and leads to the formation of a microscopic shell around the droplets at last [7]. Polyurea, polyurethane, polyester, polyamide and amine resin can be used as the shell monomers in the interfacial polycondensation process. In this paper, butyl stearate as a phase change material was microencapsulated by interfacial polymerization with toluene-2,4-diisocyanate (TDI) and ethylenediamine (EDA) used as monomers in an emulsion system.
2.1. Materials TDI (toluene-2,4-diisocyanate, C.P.) and EDA (Ethylenediamine, A.R), used as shell-forming monomers, were obtained from Shanghai Lingfeng Chemical Reagent Co. Ltd. Butyl stearate (A.R., Sinopharm Group Chemical Reagent Co., Ltd.) was employed as a core material. Nonionic surfactant, OP-10 (C.P., China National Medicines Group Shanghai Chemical Reagents Co.), was used as an emulsifier. Cyclohexane (A.R.), used as an assistant reagent (solvent media), was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. 2.2. Preparation of microcapsules The microcapsulation was carried out in a three-neck round-bottomed flask equipped with a mechanical stirrer. Before encapsulation, the core material (butyl stearate) was dissolved in
Fig. 4. DSC curves of microcapsulated PCMs after different thermal cycles, 1 cycle (a), 20 cycles (b), 400 cycles (c).
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an organic solution containing cyclohexane and TDI (oil soluble monomers), and an aqueous solution of OP-10 was prepared. Then the organic solution was added to the aqueous surfactant solution and the mixture was emulsified mechanically at a stirring rate of 500 r/m to form an O/W emulsion. After stirring for few minutes, the aqueous soluble monomers (EDA), which were diluted in distilled water before, were added into the emulsion system slowly and the mixture was heated to 65 °C. Then the interfacial polycondensation reaction took place between TDI and EDA at the oil–water interface. The reaction lasted for about 2–3 h. The resultant microcapsules were filtered, washed and dried.
by weight of core and shell have relatively high heat of fusion (near 80 J g 1) and appropriate melting point (near 28.7 °C). When the core–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 microcapsulated PCMs is 4:1. As an important property of microencapsulated PCMs, the thermal stabilities of samples were measured. The samples were subjected to cycles of melting and solidification in the experiment.
2.3. Analysis of the microcapsules
No.
TDI (g)
EDA (g)
Butyl stearate (g)
OP10 (g)
Dry weight of microcapsules (g)
Calculated dry weight (g)
1 2 3 4 5 6
4.35 4.35 4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50 1.50 1.50
12.00 12.00 12.00 12.00 12.00 12.00
0.00 0.50 1.00 1.50 2.00 2.50
16.22 16.16 16.37 16.98 16.56 16.50
17.85 17.85 17.85 17.85 17.85 17.85
The Fourier transform IR spectra (FT-IR) of the microcapsules were obtained to identify the structure of the shell polymer. The thermal properties of the microcapsules containing phase change materials (such as heat of fusion and melting point) were evaluated by Differential Scanning Calorimetry. The dry weight of samples was measured by BL-220H electronic balance. The shapes of the microcapsules were observed by optical microscope. The particle size and the distribution of microcapsules were measured by laser particle analyzer. The thermal stability of microcapsules was evaluated by testing through cycles of alternative heating and cooling.
3. Results and discussion 3.1. FT-IR spectra FT-IR spectra of butyl stearate (a), empty microcapsules (b) and microcapsulated PCMs (c) are shown in Fig. 2. In spectra (b) and (c), an absorption band near 3,320 cm 1 is assigned to NH, and a band near 1640 cm 1 is assigned to C@O. These prove that the amine group (CONH) is formed. Also, in the two spectra there is no absorption band at 2200–2280 cm 1, which is assigned to an isocyanate group that indicates there is almost no unreacted isocyanate group in the microcapsule core. In spectra (a) and (b), we can find the absorption band near 1168 cm 1 of C–O–C of butyl stearate. This proves that chemical structure of butyl stearate, used as a kind of core material, has almost no change. From this discussion, it is possible to know that butyl stearate can be microencapsulated well by using polyurethane as a phase change material.
Table 4 Dry weight of MicroPCMs prepared with different emulsifier dosages.
Table 5 Dry weight of microcapsules prepared with different cyclohexane dosages (OP-10, 1.0 g). No.
TDI (g)
EDA (g)
Cyclohexane (g)
Dry weight of microcapsules (g)
Calculated dry weight (g)
1 2 3 4
4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50
9.00 12.00 15.00 18.00
4.30 4.40 4.52 4.80
5.85 5.85 5.85 5.85
Table 6 Dry weight of microcapsules prepared with different core–shell mass I (OP-10, 1.0 g). No.
TDI (g)
EDA (g)
Butyl stearate (g)
Core/ shell
Dry weight of microcapsules (g)
Increased dry weight (g)
1 2 3
4.35 4.35 4.35
1.50 1.50 1.50
0.00 12.00 17.55
Empty 2:1 3:1
4.40 16.37 21.89
0.00 11.87 17.49
Table 7 Dry weight of microcapsules prepared with different core–shell mass II (OP-10, 1.5 g).
3.2. Thermal characteristics of the microcapsules
No.
TDI (g)
EDA (g)
Core/ shell
Dry weight of microcapsules (g)
Increased dry weight (g)
The thermal properties of the microcapsules containing phase change materials were tested using Differential Scanning Calorimeter. Fig. 3 and Table 1 show the thermal characteristics of microcapsulated PCMs prepared with various ratios by weight of core and shell. It is found that all samples prepared with different ratios
Butyl stearate (g)
1 2 3 4
4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50
0.00 12.00 17.55 23.40
Empty 2:1 3:1 4:1
4.70 16.58 21.17 24.95
0.00 11.88 16.47 20.25
Table 3 Dry weight of MicroPCMs prepared at different stirring rates. No.
1 2 3 4 5
Shell material (g)
Butyl stearate (g)
Stirring rate (r/m)
Dry weight of microcapsules (g)
Calculated dry weight (g)
5.85 5.85 5.85 5.85 5.85
12.00 12.00 12.00 12.00 12.00
300 400 500 600 700
15.88 13.34 16.08 15.69 15.66
17.85 17.85 17.85 17.85 17.85
Table 8 Dry weight of microcapsules prepared with different core–shell mass III (OP-10, 2.0 g). No.
TDI (g)
EDA (g)
Butyl Stearate (g)
Core/ shell
Dry weight of microcapsules (g)
Increased dry weight (g)
1 2 3 4
4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50
0.00 12.00 17.55 23.40
Empty 2:1 3:1 4:1
4.76 16.56 18.90 23.89
0.00 11.80 14.14 19.13
C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729
The microcapsules were sealed in a glass tube, then repeatedly heated in 50–60 °C water bath for 15 min, and then cooled in an ice-water bath to 0–5 °C for 15 min. After 1, 20, 400 cycles of alter-
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native heating and cooling, the energy storage of microcapsules was measured by DSC, and the results are shown in Fig. 4 and Table 2. After thermal cycles, even after 400 cycles of alternative
Fig. 5. Particle size distribution of microcapsule, 0 g OP-10 (a), 0.5 g OP-10 (b), 1.5 g OP-10 (c) and 2.5 g OP-10 (d).
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C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729
heating and cooling, the phase change temperature and the phase change enthalpy of encapsulated butyl stearate maintained nearly the same value. The microencapsulated PCMs have a good thermal stability. 3.3. Dry weight of microcapsules The dry weight of microcapsulated PCMs versus different experimental parameters was measured by electronic balance. Because dry weight indicates the outcome of microencapsulated PCMs obtained from the experiment, several useful information can be obtained by analyzing the dry weight of the samples. The stirring rate of the mechanical overhead stirrer is an important 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 chosen to prepare the microcapsules. Table 3 shows the relationship between the stirring rate of the mechanical stirrer and dry weight of microcapsules. From Table 3, it can be concluded that the stirring rate of the mechanical stirrer has little influence on dry weight, because of the nearly similar value of dry weight of MicroPCMs prepared at different stirring rates. Yet in fact, two low or too high stirring rate of the mechanical stirrer may cause some problems, such as reuniting and spattering. So the suitable stirring rate of the mechanical stirrer is 500 r/m. Emulsifier can diminish the droplet of core material and can cause the uniform size distribution of microcapsules. It is an important reagent for microcapsule preparation. Among several kinds of emulsifiers, OP-10 is a good one. However, the hydroxyl groups of OP-10 can react with TDI slightly. In order to clear whether OP-10 influences the microcapsules formed or not, we prepared a series of PCMs samples with various emulsifier dosages, measured the dry weight, and listed it in Table 4. From the data, it could be found that the emulsifier dosage had little effect on the dry weight of the microcapsules. It elucidates that the dosage of OP-10 that reacted with TDI is little. The usage of emulsifier has no prominent influence on the microcapsules formed. Cyclohexane is an important assistant reagent. It is a kind of solvent medium. A series of samples with different cyclohexane dos-
Table 9 D50 of microcapsules prepared with different emulsifier dosages. No.
TDI (g)
EDA (g)
Butyl stearate (g)
OP-10 (g)
D50 (lm)
1 2 3 4 5
4.35 4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50 1.50
12.00 12.00 12.00 12.00 12.00
0.50 1.00 1.50 2.00 2.50
30.91 30.71 31.45 6.64 29.67
ages were perpared to estimate the influence of the cyclohexane dosage on microcapsules formed. The data are shown in Table 5. The dry weight of microcapsules prepared with different cyclohexane dosage is nearly the same, it can be found that the usage of cyclohexane also had no great influence on the microcapsules formed. The package rate of MicroPCMs is an important parameter, and this parameter can be obtained by analyzing the dry weight of MicroPCMs. The microcapsules were prepared with various core– shell mass ratios and their dry weight was measured. Data are shown in Table 6–8. From sample 1 (empty microcapsules) to sample 4 (the core–shell mass ratio is 4:1), the core material increased, while the dry weight increased. Also, the increased dry weight and increased core material (butyl stearate) are generally equal. So, it can prove indirectly that the package rate of MicroPCMs is good. 3.4. Particle size distribution of microcapsules The particle size distributions of microcapsule are shown in the Fig. 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 is sharper when the emulsifiers were used during the reaction. This elucidates that the emulsifier has an important function in the uniform size distribution of the microcapsules. In order to clear the influence of the emulsifier dosage on the size of microcapsules, we analyzed the D50 of microcapsules of different emulsifier dosages. From the data which are shown in Table 9, it is possible to know that although the emulsifier has an important influence on the uniform size distribution of microcapsules, it has little influence on the size value of the microcapsules. 3.5. Shape of the microcapsules Optical microphotographs of microencapsulated PCMs are shown in Fig. 6. The optical micorphotograph was taken after polymerization, and it shows many small particles of about 20–35 lm diameter. When they were dried, the surface of most of the microcapsules was very smooth and the shape was very regular. However, powdery clusters of small particles could be seen. 4. Conclusions Polyurea microcapsules containing phase change materials were prepared successfully by using the interfacial polycondensation. The testing results show that MicroPCMs’ phase change tem-
Fig. 6. Optical microscopy photograph of the microcapsules.
C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729
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 have a good property of thermal periodicity. Also with the dry weight analysis, it is possible to know that the packing rate of MicroPCMs is 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 Tonset Tpeak D50
Latent heat of phase change materials (kJ/kg) The initial temperature of phase change (°C) The peak temperature of phase change stage (°C) Median particle diameter (lm)
References [1] Lewis Nathan S, Crabtree George. Basic research needs for solar energy utilization. Chicago: Renée M. Nault Argonne National Laboratory; 2005.
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[2] Zalba Belen, Marin Jose M, Cabeza Luisa F, Mehling Harald. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251–83. [3] He Bo, Setterwall Fredrik. Technical grade paraffin waxes as phase change materials for cool thermal storage and cool storage systems capital cost estimation. Energy Convers Manage 2002;43:1709–23. [4] Hong Ye, Xinshi Ge. Preparation of polyethylene–paraffin compound as a formstable solid–liquid phase change material. Sol Energy Mater Sol Cells 2000;64:37–44. [5] Khudhair Amar M, Farid Mohammed M. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Convers Manage 2002;45:236–75. [6] Hawlader MNA, Uddin MS, Khin Mya Mya. Microencapsulated PCM thermalenergy storage system. Appl Energy 2003;74:195–202. [7] Cho Jeong-Sook, Kwon Aehwa, Cho Chang-Gi. Microencapsulation of octadecane as a phase-change material by interfacial polymerization in an emulsion system. Colloid Poly Sci 2002;280:260–6. [8] Kuanjun Fang, Youquan Wang, Yuqing Cai, Yun Shao, Jusheng Wang. Influence of emulsification on the preparation and permeability of microcapsule. J Qingdao Univ 1998;13(4):9–12. [9] Shulkin Anna, Stove Harald DH. Polymer microcapsules by interfacial polyaddition between styrene-maleic anhydride copolymers and amines. J Membr Sci 2002;209:421–32. [10] Guanglong Zou, Xiaozheng Lan, Zhicheng Tan, Lixian Sun, Tao Zhang. Microencapsulation of n-hexadecane as a phase change material in polyurea. Wuli Huaxue Xuebao 2004;20(1):90–3. [11] Li-Xin Wang, Junfeng Su, Li Ren. Study of compatibility of thermal energy storage microcapsule with melamine formaldehyde resin as shell. Fine Chem 2003;20(12):705–8. [12] Fan YF, Zhang XX, Wang XC, Li J, Zhu QB. Super-cooling prevention of microcapsulated phase change material. Thermochim Acta 2004;413:1–6. [13] Jian Song, Lei Chen, Xiaojun Li. Technology and application of microencapsulation. Beijing: Chemical Industry Press; 2001.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system Chen Liang *, Xu Lingling, Shang Hongbo, Zhang Zhibin College of Material Science and Engineering, Nanjing University of Technology, Nanjing 210009, China
a r t i c l e
i n f o
Article history: Received 6 March 2008 Accepted 30 September 2008 Available online 17 November 2008 Keywords: Phase change material Butyl stearate Polyurea Interfacial polycondensation Microcapsule
a b s t r a c t For the last 20 years, microencapsulated phase change materials (MicroPCMs), which combine microencapsulation technology and phase change material, have been attracted more and more interest. By overcoming some limitations of the PCMs, the MicroPCMs improve the efficiency of PCMs and make it possible to apply PCMs in many areas. In this experiment, polyurea microcapsules containing phase change 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 core material. The MicroPCMs’ properties have been characterized by dry weight analysis, differential scanning calorimetry, Fourier transform IR spectra analysis and optical microscopy. The results show that the MicroPCMs were synthesized successfully and that, the phase change temperature was about 29 °C, the latent heat of fusion was about 80 J g 1, the particle diameter was 20–35 lm. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Today the world consumes energy at a rate of approximately 4.1 1020 J/yr. With the increase in population and the growth of economy, the demand for energy will be more than double of what is now in 2050, and more than triple by the end of the century [1]. But with the current rates of consumption, the reserves of fossil fuels in the form of oil and natural gas will fall short of this demand in nearly 50 years. With the aggravation of energy crisis, more and more attention is being paid to energy saving. Because of the optimum use of renewable energies [2], thermal energy storage (TES), the temporary storage system of high or low temperature energy for later use [3], has been attracting more and more interest. It is expected to be used in many areas, such as solar energy storage, cool storage, spacecraft thermal systems, and building temperature fluctuations control [2,3]. Phase change materials (PCMs) are one of the most promising areas of thermal energy storage. These can store/release energy from/to the surroundings during phase change. The quantity of energy per weight of the material stored/released is so large that less volume is required by the system to store the energy. In addition, during the phase change, the temperature remains nearly constant, which is beneficial for the control of the temperature of the surroundings [4]. As a phase change material, the main characteristics required are suitable phase change temperature, high
* Corresponding author. Tel./fax: +86 025 83587239. E-mail addresses: [email protected], [email protected] (C. Liang). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.09.044
change enthalpy, high thermal conductivity in both liquid and solid phases, good thermal stability, high phase change speed, small volume change, compatibility with container materials, and should be non-toxic, non-flammable, non-polluting, cheap and abundant [2,5]. However, this kind of heat storage system has also caused some problems. Many PCMs can corrode the container. This means that they have short service lives, and the packing and maintenance costs are high [3]. Also the PCMs may freeze on the heat exchanger surface. This will cause a poor heat transfer rate because of the low thermal 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 storage has been developed. Microencapsulated PCMs (MicroPCMs) are colloidal particles composed of a protective shell and one or more PCMs (core substance) [7,8]. Because these can overcome several limitations of the PCMs, the MicroPCMs improve the efficiency of PCMs and make it possible to apply PCMs more widely. Microcapsulated PCMs have many advantages such as reducing the reactivity of the PCMs with the outside environment, increasing the heat transfer areas of PCMs and permitting the core material to withstand frequent changes in the volume of the storage material during the phase change [6]. Nowadays many researchers are studying on this area. Shulkin and Stover [9] prepared the microcapsules by interfacial polyaddition between styrene-maleic anhydride (SMA) copolymers and polyamines. Cho [7] microencapsulated octadecane as a phase change material by interfacial polymerization with toluene-2,4-diisocyanate (TDI) and diethylenetriamine (DETA) used as monomers in an emulsion system. In China, Guanglong
724
C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729
Fig. 1. The sketch map of interfacial polycondensation. Monomer: X, Y. Product: (X Y)n or (X)n.
et al. [10] microencapsulated n-hexadecane as a phase change material in polyurea, and the microencapsulated PCMs showed a good potential as thermal energy storage materials. Li-xin et al. [11] prepared a kind of heat energy storage microcapsule using melamine formaldehyde resin as a shell material and a phase change material, which melts at 24 °C as a core material. Also, Fan et al. [12] not only prepared microencapsulated phase change materials with melamine and formaldehyde as shell-forming monomers and N-octadecane as a core material, but also studied the effects of the nucleating agents (which can prevent the super-cooling), including sodium chloride, 1-octadecanol and pararffin, on the melting and crystallization behavior, morphology and dispersibility of microcapsules.
Fig. 2. FT-IR spectra of butyl stearate (a), microcapsulated PCMs (b) and empty microcapsules (c).
Many methods are developed for microcapsulation including interfacial polymerization, in situ polycondensation, and complex coacervation. The interfacial polycondensation is a kind of popular method. It has several advantages, such as high reaction speed,
Fig. 3. DSC curves of microcapsulated PCMs prepared with various ratios by weight of core and shell, 2:1(a), 3:1(b), 4:1(c).
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C. Liang et al. / Energy Conversion and Management 50 (2009) 723–729 Table 1 Thermal characteristics of microcapsulated PCMs prepared with various ratios by weight of core and shell. No.
TDI (g)
EDA (g)
Butyl stearate (g)
Core/ shell
OP-10 (g)
DHfus (J g 1)
Tonset (°C)
1 2 3
4.35 4.35 4.35
1.50 1.50 1.50
12.00 17.55 23.40
2:1 3:1 4:1
1.00 1.00 1.00
76.31 85.92 76.58
28.60 29.37 28.49
Table 2 The capacity of microcapsulated PCM after different thermal cycles. Thermal cycles
DHfus (J g 1)
Tonset (°C)
Tpeak (°C)
1 20 400
85.92 84.81 88.20
29.37 29.33 29.05
33.80 34.01 34.16
2. Materials and methods mild reaction course and also its products have low penetrability. So it seems to be more feasible than others. The sketch map of interfacial polycondensation is shown in Fig. 1 [13]. The scheme of interfacial polycondensation can be explained as follows. The core material is made into droplet. The capsule shell reactive monomers polymerize on the surface of the droplets. When the initially formed oligomers are insoluble at the interface of the droplets, they grow, and a thin monolayer membrane forms around the droplets. The polycondensation leads the monolayer membrane to be a shell, and leads to the formation of a microscopic shell around the droplets at last [7]. Polyurea, polyurethane, polyester, polyamide and amine resin can be used as the shell monomers in the interfacial polycondensation process. In this paper, butyl stearate as a phase change material was microencapsulated by interfacial polymerization with toluene-2,4-diisocyanate (TDI) and ethylenediamine (EDA) used as monomers in an emulsion system.
2.1. Materials TDI (toluene-2,4-diisocyanate, C.P.) and EDA (Ethylenediamine, A.R), used as shell-forming monomers, were obtained from Shanghai Lingfeng Chemical Reagent Co. Ltd. Butyl stearate (A.R., Sinopharm Group Chemical Reagent Co., Ltd.) was employed as a core material. Nonionic surfactant, OP-10 (C.P., China National Medicines Group Shanghai Chemical Reagents Co.), was used as an emulsifier. Cyclohexane (A.R.), used as an assistant reagent (solvent media), was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. 2.2. Preparation of microcapsules The microcapsulation was carried out in a three-neck round-bottomed flask equipped with a mechanical stirrer. Before encapsulation, the core material (butyl stearate) was dissolved in
Fig. 4. DSC curves of microcapsulated PCMs after different thermal cycles, 1 cycle (a), 20 cycles (b), 400 cycles (c).
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an organic solution containing cyclohexane and TDI (oil soluble monomers), and an aqueous solution of OP-10 was prepared. Then the organic solution was added to the aqueous surfactant solution and the mixture was emulsified mechanically at a stirring rate of 500 r/m to form an O/W emulsion. After stirring for few minutes, the aqueous soluble monomers (EDA), which were diluted in distilled water before, were added into the emulsion system slowly and the mixture was heated to 65 °C. Then the interfacial polycondensation reaction took place between TDI and EDA at the oil–water interface. The reaction lasted for about 2–3 h. The resultant microcapsules were filtered, washed and dried.
by weight of core and shell have relatively high heat of fusion (near 80 J g 1) and appropriate melting point (near 28.7 °C). When the core–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 microcapsulated PCMs is 4:1. As an important property of microencapsulated PCMs, the thermal stabilities of samples were measured. The samples were subjected to cycles of melting and solidification in the experiment.
2.3. Analysis of the microcapsules
No.
TDI (g)
EDA (g)
Butyl stearate (g)
OP10 (g)
Dry weight of microcapsules (g)
Calculated dry weight (g)
1 2 3 4 5 6
4.35 4.35 4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50 1.50 1.50
12.00 12.00 12.00 12.00 12.00 12.00
0.00 0.50 1.00 1.50 2.00 2.50
16.22 16.16 16.37 16.98 16.56 16.50
17.85 17.85 17.85 17.85 17.85 17.85
The Fourier transform IR spectra (FT-IR) of the microcapsules were obtained to identify the structure of the shell polymer. The thermal properties of the microcapsules containing phase change materials (such as heat of fusion and melting point) were evaluated by Differential Scanning Calorimetry. The dry weight of samples was measured by BL-220H electronic balance. The shapes of the microcapsules were observed by optical microscope. The particle size and the distribution of microcapsules were measured by laser particle analyzer. The thermal stability of microcapsules was evaluated by testing through cycles of alternative heating and cooling.
3. Results and discussion 3.1. FT-IR spectra FT-IR spectra of butyl stearate (a), empty microcapsules (b) and microcapsulated PCMs (c) are shown in Fig. 2. In spectra (b) and (c), an absorption band near 3,320 cm 1 is assigned to NH, and a band near 1640 cm 1 is assigned to C@O. These prove that the amine group (CONH) is formed. Also, in the two spectra there is no absorption band at 2200–2280 cm 1, which is assigned to an isocyanate group that indicates there is almost no unreacted isocyanate group in the microcapsule core. In spectra (a) and (b), we can find the absorption band near 1168 cm 1 of C–O–C of butyl stearate. This proves that chemical structure of butyl stearate, used as a kind of core material, has almost no change. From this discussion, it is possible to know that butyl stearate can be microencapsulated well by using polyurethane as a phase change material.
Table 4 Dry weight of MicroPCMs prepared with different emulsifier dosages.
Table 5 Dry weight of microcapsules prepared with different cyclohexane dosages (OP-10, 1.0 g). No.
TDI (g)
EDA (g)
Cyclohexane (g)
Dry weight of microcapsules (g)
Calculated dry weight (g)
1 2 3 4
4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50
9.00 12.00 15.00 18.00
4.30 4.40 4.52 4.80
5.85 5.85 5.85 5.85
Table 6 Dry weight of microcapsules prepared with different core–shell mass I (OP-10, 1.0 g). No.
TDI (g)
EDA (g)
Butyl stearate (g)
Core/ shell
Dry weight of microcapsules (g)
Increased dry weight (g)
1 2 3
4.35 4.35 4.35
1.50 1.50 1.50
0.00 12.00 17.55
Empty 2:1 3:1
4.40 16.37 21.89
0.00 11.87 17.49
Table 7 Dry weight of microcapsules prepared with different core–shell mass II (OP-10, 1.5 g).
3.2. Thermal characteristics of the microcapsules
No.
TDI (g)
EDA (g)
Core/ shell
Dry weight of microcapsules (g)
Increased dry weight (g)
The thermal properties of the microcapsules containing phase change materials were tested using Differential Scanning Calorimeter. Fig. 3 and Table 1 show the thermal characteristics of microcapsulated PCMs prepared with various ratios by weight of core and shell. It is found that all samples prepared with different ratios
Butyl stearate (g)
1 2 3 4
4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50
0.00 12.00 17.55 23.40
Empty 2:1 3:1 4:1
4.70 16.58 21.17 24.95
0.00 11.88 16.47 20.25
Table 3 Dry weight of MicroPCMs prepared at different stirring rates. No.
1 2 3 4 5
Shell material (g)
Butyl stearate (g)
Stirring rate (r/m)
Dry weight of microcapsules (g)
Calculated dry weight (g)
5.85 5.85 5.85 5.85 5.85
12.00 12.00 12.00 12.00 12.00
300 400 500 600 700
15.88 13.34 16.08 15.69 15.66
17.85 17.85 17.85 17.85 17.85
Table 8 Dry weight of microcapsules prepared with different core–shell mass III (OP-10, 2.0 g). No.
TDI (g)
EDA (g)
Butyl Stearate (g)
Core/ shell
Dry weight of microcapsules (g)
Increased dry weight (g)
1 2 3 4
4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50
0.00 12.00 17.55 23.40
Empty 2:1 3:1 4:1
4.76 16.56 18.90 23.89
0.00 11.80 14.14 19.13
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The microcapsules were sealed in a glass tube, then repeatedly heated in 50–60 °C water bath for 15 min, and then cooled in an ice-water bath to 0–5 °C for 15 min. After 1, 20, 400 cycles of alter-
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native heating and cooling, the energy storage of microcapsules was measured by DSC, and the results are shown in Fig. 4 and Table 2. After thermal cycles, even after 400 cycles of alternative
Fig. 5. Particle size distribution of microcapsule, 0 g OP-10 (a), 0.5 g OP-10 (b), 1.5 g OP-10 (c) and 2.5 g OP-10 (d).
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heating and cooling, the phase change temperature and the phase change enthalpy of encapsulated butyl stearate maintained nearly the same value. The microencapsulated PCMs have a good thermal stability. 3.3. Dry weight of microcapsules The dry weight of microcapsulated PCMs versus different experimental parameters was measured by electronic balance. Because dry weight indicates the outcome of microencapsulated PCMs obtained from the experiment, several useful information can be obtained by analyzing the dry weight of the samples. The stirring rate of the mechanical overhead stirrer is an important 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 chosen to prepare the microcapsules. Table 3 shows the relationship between the stirring rate of the mechanical stirrer and dry weight of microcapsules. From Table 3, it can be concluded that the stirring rate of the mechanical stirrer has little influence on dry weight, because of the nearly similar value of dry weight of MicroPCMs prepared at different stirring rates. Yet in fact, two low or too high stirring rate of the mechanical stirrer may cause some problems, such as reuniting and spattering. So the suitable stirring rate of the mechanical stirrer is 500 r/m. Emulsifier can diminish the droplet of core material and can cause the uniform size distribution of microcapsules. It is an important reagent for microcapsule preparation. Among several kinds of emulsifiers, OP-10 is a good one. However, the hydroxyl groups of OP-10 can react with TDI slightly. In order to clear whether OP-10 influences the microcapsules formed or not, we prepared a series of PCMs samples with various emulsifier dosages, measured the dry weight, and listed it in Table 4. From the data, it could be found that the emulsifier dosage had little effect on the dry weight of the microcapsules. It elucidates that the dosage of OP-10 that reacted with TDI is little. The usage of emulsifier has no prominent influence on the microcapsules formed. Cyclohexane is an important assistant reagent. It is a kind of solvent medium. A series of samples with different cyclohexane dos-
Table 9 D50 of microcapsules prepared with different emulsifier dosages. No.
TDI (g)
EDA (g)
Butyl stearate (g)
OP-10 (g)
D50 (lm)
1 2 3 4 5
4.35 4.35 4.35 4.35 4.35
1.50 1.50 1.50 1.50 1.50
12.00 12.00 12.00 12.00 12.00
0.50 1.00 1.50 2.00 2.50
30.91 30.71 31.45 6.64 29.67
ages were perpared to estimate the influence of the cyclohexane dosage on microcapsules formed. The data are shown in Table 5. The dry weight of microcapsules prepared with different cyclohexane dosage is nearly the same, it can be found that the usage of cyclohexane also had no great influence on the microcapsules formed. The package rate of MicroPCMs is an important parameter, and this parameter can be obtained by analyzing the dry weight of MicroPCMs. The microcapsules were prepared with various core– shell mass ratios and their dry weight was measured. Data are shown in Table 6–8. From sample 1 (empty microcapsules) to sample 4 (the core–shell mass ratio is 4:1), the core material increased, while the dry weight increased. Also, the increased dry weight and increased core material (butyl stearate) are generally equal. So, it can prove indirectly that the package rate of MicroPCMs is good. 3.4. Particle size distribution of microcapsules The particle size distributions of microcapsule are shown in the Fig. 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 is sharper when the emulsifiers were used during the reaction. This elucidates that the emulsifier has an important function in the uniform size distribution of the microcapsules. In order to clear the influence of the emulsifier dosage on the size of microcapsules, we analyzed the D50 of microcapsules of different emulsifier dosages. From the data which are shown in Table 9, it is possible to know that although the emulsifier has an important influence on the uniform size distribution of microcapsules, it has little influence on the size value of the microcapsules. 3.5. Shape of the microcapsules Optical microphotographs of microencapsulated PCMs are shown in Fig. 6. The optical micorphotograph was taken after polymerization, and it shows many small particles of about 20–35 lm diameter. When they were dried, the surface of most of the microcapsules was very smooth and the shape was very regular. However, powdery clusters of small particles could be seen. 4. Conclusions Polyurea microcapsules containing phase change materials were prepared successfully by using the interfacial polycondensation. The testing results show that MicroPCMs’ phase change tem-
Fig. 6. Optical microscopy photograph of the microcapsules.
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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 have a good property of thermal periodicity. Also with the dry weight analysis, it is possible to know that the packing rate of MicroPCMs is 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 Tonset Tpeak D50
Latent heat of phase change materials (kJ/kg) The initial temperature of phase change (°C) The peak temperature of phase change stage (°C) Median particle diameter (lm)
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