Preparation and characterization of macrocapsules containing microencapsulated PCMs (phase change materials) for thermal energy storage

Preparation and characterization of macrocapsules containing microencapsulated PCMs (phase change materials) for thermal energy storage

Energy 91 (2015) 531e539 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Preparation and characte...

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Energy 91 (2015) 531e539

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Preparation and characterization of macrocapsules containing microencapsulated PCMs (phase change materials) for thermal energy storage Pengju Han a, Lixin Lu a, b, *, Xiaolin Qiu a, b, **, Yali Tang a, b, Jun Wang a, b a b

Department of Packaging Engineering, Jiangnan University, Wuxi, China Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Wuxi, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2015 Received in revised form 24 June 2015 Accepted 2 August 2015 Available online xxx

This paper was aimed to prepare, characterize and determine the comprehensive evaluation of promising composite macrocapsules containing microencapsulated PCMs (phase change materials) with calcium alginate gels as the matrix material. Macrocapsules containing microcapsules were fabricated by piercing-solidifying incuber method. Two kinds of microcapsules with n-tetradecane as core material, UF (urea-formaldehyde) and PMMA (poly(methyl methacrylate)) respectively as shell materials were prepared initially. For application concerns, thermal durability and mechanical property of macrocapsules were investigated by TGA (thermal gravimetric analysis) and Texture Analyser for the first time, respectively. The results showed excellent thermal stability and the compressive resistance of macrocapsules was sufficient for common application. The morphology and chemical structure of the prepared microcapsules and macrocapsules were characterized by SEM (scanning electron microscopy) and FT-IR (fourier transform infrared) spectroscopy method. Phase change behaviors and thermal durability of microcapsules and macrocapsules were investigated by DSC (differential scanning calorimetry). In order to improve latent heat of composite microcapsules, the core-shell weight ratio of tetradecane/UF shell microcapsules was chosen as 5.5:1 which obtained the phase change enthalpy of 194.1 J g1 determined by DSC. In conclusion, these properties make it a feasible composite in applications of textile, building and cold-chain transportation. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Microcapsule Macrocapsule PMMA (poly(methyl methacrylate)) UF (urea-formaldehyde) Thermal energy storage Cold chain

1. Introduction PCMs (phase change materials) are substances which are capable of storing or releasing large amounts of energy during phase change process [1]. With that physical peculiarity, PCMs have attracted huge interest in thermal energy storage field [2,3]. Generally, PCMs were mainly concentrated on two applications: (1) Using as stored energy when there is no energy production (i.e. renewable energies); (2) Reducing energy peaks and economical costs by shifting energy demand to off-peak hours [4]. However, the practical use of PCMs suffers from problems such as leakage and

* Corresponding author. Department of Packaging Engineering, Jiangnan University, Wuxi, China. ** Corresponding author. Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Wuxi, China. E-mail addresses: [email protected] (L. Lu), [email protected] (X. Qiu). http://dx.doi.org/10.1016/j.energy.2015.08.001 0360-5442/© 2015 Elsevier Ltd. All rights reserved.

corrosion [5,6], low thermal conductivity values [7,8], compatibility concerns [9e11], etc. Consequently, microencapsulation method attracted a renewed attention, which now has been widely used in thermal regulating fabrics [12,13], foams [14,15], heating and cooling of buildings [16,17], heat transfer fluid [18] and solar energy storage utilization [19,20]. The microencapsulation method is to hold the PCMs in liquid or solid phase and isolate it with different shell materials including styrene-methyl methacrylate copolymer [17], methyl methacrylate [21,22],gelatin-arabic gum [23], polyurea [24] and styrene [25,26], etc. Besides, UF (urea-formaldehyde) is commonly used as shell material in view of their reasonable cost, adequate strength and long shelf-life [27]. However, microencapsulated PCMs are prepared in micro even nano sizes which is difficult to use or gather. Conversely, Composite (shape-stabilized) PCMs are suitable for industrial production and practical applications [28], which are usually fabricated by embedding PCMs into shape stabilization supports such as HDPE (high density polyethylene), styrene,

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poly(ethylene-co-acrylic acid), butadiene, polymethacrylic acid, polystyrene resin, etc. [29]. Nevertheless, the composite PCMs are not encapsulated, and polymers covered outside acted as restricting or supporting materials [30,31]. As a result, leakage of PCMs will happen during the phase change process [32]. In fact, the two forms of PCMs can be joined together to fabricate composite microcapsules. Melone et al. [33] fabricated micro-encapsulated PCM-cellulose composite which was prepared by paper matrix containing 50 wt% commercially available micro-encapsulated PCM. Besides, the heat transfer evaluation for further application was also discussed. Nevertheless, compatibility concerns and mechanical property were not further investigated. Cabeza et al. [34] mixed a commercial microencapsulated PCM into concrete and built walls with them. The concrete contained about 5 wt% microencapsulated PCM had a phase change enthalpy of 110 J g1. Comparison between PCM modified walls and conventional walls of temperature and the mechanical property was discussed. However, this kind of composite microcapsules was not suitable for other application except buildings. In contrast, alginate hydrogels especially calcium alginate gels are capable of assembly in various sizes. Li et al. [35] fabricated millimeter-sized macrocapsules containing microcapsules with PMMA (poly(methyl methacrylate)) as shell material and calcium alginate as matrix material. The microstructure, micromechanism, sealness and phase change properties of the macrocapsules were investigated. However, the comprehensive evaluation of the mass influence of calcium chloride and sodium alginate were not discussed further. Furthermore, as millimeter-sized macrocapsules, mechanical property of macrocapsules should be concerned before the practical application. Qiu et al. [36] investigated the mechanical behaviors of individual microcapsules under continuous loading conditions by employing a micro/nanohardness tester. Besides, Pan et al. [37] determined the rupture force, displacement at rupture of single microcapsules through a micromanipulation technique containing a micro-sized glass probe and a force transducer. Giro-Paloma et al. [38] characterized the mechanical performance of microcapsules at different temperatures using Atomic Force Microscope. In view of increasingly need of microcapsule application in cold chain [39e41], the millimeter-sized macrocapsules have broad prospects to apply and recycle with recycle cases and insulating packages. However, there were several essential evaluations of the macrocapsules that Li et al. [35] or other published reports did not mention for further application. First, the mass influence of calcium chloride and sodium alginate was not investigated further, which is integrant for the comprehensive evaluation of the macrocapsules. Second, thermal durability and mechanical property of the macrocapsules were not discussed. The two properties are essential evaluations for materials in application. Third, the phase change enthalpy can be improved to a higher level, which provides higher efficiency for application. Besides, research on macrocapsules containing phase change microcapsules with UF as the shell material has not been reported in present literature. PMMA and UF are two common shell materials for microcapsules in cold chain. On the one hand, the comprehensive evaluation of the macrocapsules with PMMA shell microcapsules was necessary. On the other hand, macrocapsules with PMMA shell microcapsules can be a contrast to macrocapsules with UF shell microcapsules. Based on the above concerns, Macrocapsules containing two kinds of microcapsules were fabricated with different concentration of calcium chloride and sodium alginate. Thermal durability and compression forces under the same deformation were also investigated using TGA (thermal-gravimetric analysis) and Texture Analyser. In order to increase the phase change enthalpy, the coreeshell ratio of UF shell microcapsules was improved to a high level compared to PMMA shell microcapsules. Besides, characterizations

of morphology, microstructure, spectroscopic analysis and phase change properties were determined through SEM (scanning electron microscope), fourier transform infrared spectroscopy (FT-IR) and DSC (differential scanning calorimeter). 2. Experimental 2.1. Materials Tetradecane (98.0%, Aladdin) was used as a core material. Urea (98.0%, A.R, Sinopharm Chemical Reagent Co., Ltd), melamine (99.0%, C.P, Sinopharm Chemical Reagent Co., Ltd) and formaldehyde (37%, A.R Sinopharm Chemical Reagent Co., Ltd) were used as monomers of the shell polymer of UF (urea-formaldehyde). Methyl methacrylate (MMA, 98.0%, C.P, Sinopharm Chemical Reagent Co., Ltd) was used as monomer of the shell polymer of poly(methyl methacrylate) (PMMA). And the MMA monomer was washed three times with 10% NaOH (A.R, Sinopharm Chemical Reagent Co., Ltd) followed by washing with deionized water to remove the inhibitor before use. 2,2-azobisisobutyronitrile (AIBN, C.P, Sinopharm Chemical Reagent Co., Ltd) was used as an oil soluble initiator. Polyethylene glycol octylphenol ether (Triton X-100, C.P, Sinopharm Chemical Reagent Co., Ltd) and sodium dodecyl benzene sulfonate (SDBS, 88%, A.R, Sinopharm Chemical Reagent Co., Ltd) were employed as surfactants. Triethanolamine (98.0%, A.R, Sinopharm Chemical Reagent Co., Ltd) and Citric acid monohydrate (40wt%, A.R, Sinopharm Chemical Reagent Co., Ltd) were used to control the pH during polymerization. Deionized water was used throughout the experiment. Nitrogen gas was of high-purify grade. 2.2. Fabrication of microcapsules with different shell materials 2.2.1. Fabrication of microcapsules with PMMA shell 5 g monomer, 15 g n-tetradecane and 0.1 g AIBN were mixed into a 50 ml beaker at 35  C for 20 min to build oil phase with a magnetic stirrer. 0.5 g SDBS, 0.5 g Triton X-100 and 80 ml deionized water were mixed into a 150 ml beaker at 45  C for 20 min to build aqueous phase with another magnetic stirrer. In the next step, the oil phase was dropwised into the aqueous phase with the magnetic stirrer working on. Then the mixture was homogenized using a homogenizer at 8000 rpm for 10 min to form a stable oil-in-water emulsion system. In reaction stage, the above emulsion system was transferred into a glass reactor and stirred using a mechanical stirrer with 600 rpm at 70  C for 5 h with N2 as the protective gas. After the reaction, the product was filtered and washed 4 times with deionized water. The obtained product was then dried at 40  C for 24 h. 2.2.2. Fabrication of microcapsules with UF shell In order to reduce the amount of formaldehyde and improve latent heat of microcapsules, the weight ratio of C14/UF shell was selected as 5.5:1 for the preparation of UF shell microcapsules. Preparation of UF precondensate should be the first step. Thus, 2.5 g urea, 0.5 g melamine and 6.5 ml 37 wt% formaldehyde solution were mixed with 10 ml deionized water in a glass reactor and pH of the mixture was adjusted to 8.5 with triethanolamine. The reaction for UF precondensate last for 1 h at 70  C. During the reaction, 30 g n-tetradecane, 0.6 g SDBS, 0.6 g Triton X-100 and 120 ml deionized water were mixed in a 250 ml beaker at 45  C for 20 min with a magnetic stirrer. Then the mixture was homogenized using a homogenizer at 8000 rpm for 10 min to form a stable oil-in-water emulsion system. After that, the solution of UF precondensate was dropwised into the oil-in-water emulsion system of which pH was then adjusted to 2.6 with citric acid solution. In reaction stage, the above emulsion system was transferred into a glass reactor and stirred using a mechanical stirrer with 600 rpm at 70  C for 5 h.

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After the reaction, the product was filtered and washed 4 times with deionized water. The obtained product was then dried at 40  C for 24 h.

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Table 2 Component parameters of macrocapsules.

2.3. Fabrication of macrocapsules containing microcapsules In the early research, thermal and mechanical property was found to vary with the quantitative change of sodium alginate and concentration of calcium chloride solution. In order to determine the optimal quantity of sodium alginate and concentration of calcium chloride solution, different quantity of raw materials were investigated. The component parameters of microcapsules and macrocapsules are listed in Tables 1 and 2. A typical macro-encapsulation procedure was carried out as follow: 0.4 g Sodium alginate was dissolved in 50 g deionized water, and then was stirred with 400 rpm at 50  C for 2 h to form a stable solution. 12 g microcapsules powder was dispersed uniformly into the solution mentioned at a stirring rate of 200 rpm. The obtained suspension solution was then dropped into 2.0 wt% calcium chloride solution by a piercing-solidifying incuber method using a disposable pipet, and the macrocapsules containing microcapsules were obtained subsequently. The resultant macrocapsules were then washed with deionized water 4 times and dried at 40  C for 24 h.

Sample

Sodium alginate/%

Calcium chloride/%

1 2 3 4 5 6 7 8 9 10 11 12

0.80% 0.80% 0.80% 0.80% 1.00% 1.00% 1.00% 1.00% 1.20% 1.20% 1.20% 1.20%

2% 4% 6% 8% 2% 4% 6% 8% 2% 4% 6% 8%

Microcapsules were spheres ranging from about 0.2 to 0.7 mm. The surfaces of microcapsules were smooth and compact, which was different with n-octadecane microcapsules [42] with dimples on surfaces. The difference was concerned with phase change temperature of n-octadecane (28.2  C) and n-tetradecane (5.6  C). Thus, the phase of n-octadecane changed into solid state in room temperature (25  C) accompanied with shrinkage and caused the dimples on the shell surfaces. In contrast, n-tetradecane was still liquid state in room temperature leading to the smooth and compact surfaces of the prepared microcapsules. U-Microcapsules were prepared with UF shell and the coreeshell ratio was 5.5:1. As can be seen in Fig. 1(b), there were spheres and tubular or reticular structures with diameters about 100 nm. The tubular or reticular structure might be explained by the extreme coreeshell ratio. During the condensation polymerization of UF shell, there was not enough shell material to deposit around the core material to finish the closure. Consequently, the uncovered part of adjacent liquid drops of core material gradually merged together under agitation, and then the shell material of the adjacent liquid drops bonding together to form the tubular or reticular structure under the condensation polymerization.

2.4. Characterization Field emission scanning electron microscope (FESEM, HITACHI S4800) was used to investigate the morphology, size and microstructure of capsules. The prepared microcapsules and macrocapsules were dispersed on conductive carbon adhesive tapes to attach to a FESEM stub, and then gold-coated. FT-IR spectra of samples were recorded in the wavelength range 4000e400 cm1 using a Thermo Fisher FTIR (NICOLET IS10) analyzer. Texture Analyser (FTC TMS-Pro) was used to investigate the mechanical property of macrocapsules. Mode: Measure force in compression; Test speed: 3 mm/s; Distance: 1.5 mm. The enthalpy and phase change properties of microcapsules and macrocapsules were investigated by a differential scanning calorimeter (DSC, TA Q2000). The determination temperature varied from 20 to 25  C at a heating rate of 10  C/min under nitrogen atmosphere. The thermal stability of microcapsules and macrocapsules were investigated using a thermalgravimetric analysis (TGA, TA Q500). The determination temperature varied from 30 to 500  C at a heating rate of 10  C/min under nitrogen atmosphere.

3.2. Morphology and microstructure of macrocapsules containing microcapsules The millimeter-sized macrocapsules filled with microcapsules of different shell were fabricated using piercing-solidifying incuber method. As shown in Fig. 2, the diameters of the composite globule were approximately between 2 and 3 mm, which were similar to the macrocapsules prepared by Li et al. [35]. Fig. 3 and Fig. 4 showed the microstructure of macrocapsules containing microcapsules of UF shell and PMMA shell, respectively. As can be seen in Figs. 3 and 4 (a), wrinkles and fluctuations were found on surfaces of the macapsules, which should be calcium alginate used as matrix material. It appeared that some microcapsules were distributed outside the matrix, which should be covered by a thin layer matrix, since the microcapsules was a little hazy

3. Results and discussion 3.1. Morphology and microstructure of microcapsules The morphological analysis of the prepared microcapsules was carried out using SEM methods. As shown in Fig. 1(a), the PTable 1 Component parameters of microcapsules with different shell materials. Sample e

U -Microcapsules Pf-Microcapsules a b c d e f

MMAa (g)

AIBNb (g)

37wt% Formaldehyde solution (g)

Urea (g)

Melamine (g)

Triton X-100c (g)

SDBSd (g)

H2O (g)

n-tetradecane (g)

5 0

0.1 0

0 6.5

0 2

0 0.5

0.4 0.6

0.4 0.6

80 120

15 30

Methyl methacrylate. 2,20 -Azobisisobutyronitrile. Polyethylene glycol tert-octylphenyl ether. Sodium dodecylbenzene sulfonate. Urea-formaldehyde shell. Poly(methyl methacrylate) shell.

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Fig. 1. SEM micrograph of Microcapsules with different shells: (a): PMMA shell (b): UF shell.

Fig. 3. Microstructure of macrocapsules containing microcapsules of UF shell: (a): surface morphology, (b): internal morphology.

adhered to the surface of macrocapsule by matrix material in Fig. 4(a), which might due to the impact by microcapsules solution drops and calcium chloride solution. It is obvious that the matrix material mainly concentrated on the surface of macrocapsule. The reasonable explanation was that the reaction between calcium ion and sodium alginate was rapid, and the formed complex prevented the continued penetration of calcium chloride solution into internal section of macrocapsule.

3.3. Spectroscopic analysis of PCM/microcapsules/macrocapsules

Fig. 2. Morphology of macrocapsules containing microcapsules.

compared with microcapsules shown in Fig. 1. As shown in Figs. 3 and 4 (b), microcapsules uniformly distributed in the internal area of macrocapsules, which indicated that microcapsules were successfully encapsulated by macrocapsules. Moreover, the microcapsules inside the macrocapsules should also be covered by matrix material, as the clear spheres and tubular or reticular structure of UF shell microcapsules turned into blocky structure from Fig. 1(b) to Fig. 3(b). Meanwhile, single microcapsules also

The FT-IR spectra obtained for PCM, microcapsules and macrocapsules with PMMA shell are displayed in Fig. 5(aec). It can be seen from Fig. 5(aec) that the peaks at 2960, 2920 and 2850 cm1 are the characteristic bands of C14 regarding its stretching vibrations of CeH of eCH3 and eCH2 groups, respectively. Furthermore, the bending vibration of CeH of eCH2 bands were observed at 1380 and 1470 cm1, and the swing vibration of CeH were observed at 719 cm1. From Fig. 5(b) and (c), the peaks of 1730 and 1270e1150 cm1 correspond respectively to stretching vibrations of C]O and CeO which belong to shell material of PMMA. The peaks of 1630 and 544 cm1 exit in Fig. 5(c) correspond respectively to stretching vibrations of eCOO and eCl which reveal the existence of matrix material of calcium alginate. Besides, the broad peak at 3430 cm1 was associated with eOH group of calcium alginate.

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belong to UF shell material. For matrix material, the peak of eCl has the identical position at 544 cm1 while peak correspond to eCOO overlaps the peak of C]O at 1660 cm1. Moreover, the broad peak observed in Fig. 6(bec) at 3430 cm1 was associated with eOH and NeH groups.

3.4. Phase change behaviors of microcapsules/macrocapsules

Fig. 4. Microstructure of macrocapsules containing microcapsules of PMMA shell: (a): surface morphology, (b): internal morphology.

The phase change behaviors of microcapsules/macrocapsules were characterized with DSC at a heating rate of 10  C min1 under a nitrogen atmosphere. Phase change data of microcapsules and macrocapsules were detailedly shown in Table 3. The samples from U-1 to U-12 and P-1 to P-12 ranked according to Table 2. In Table 3, average enthalpy DH was obtained from jDHmj and jDHcj. U-microcapsules with coreeshell ratio of 5.5:1 had the highest phase change enthalpy of 194.1 J g1. The PCM content calculated by phase change enthalpy was 92.4%, which was higher than 84.6% of PCM content calculated through coreeshell ratio. There were two possible reasons about the difference of phase change enthalpy. One should be that a part of shell material did not participate in the condensation polymerization but lost in the process of preparation of UF precondensate or other process. The other might be that some PCM uncovered by shell material still adhered to the surface of microcapsule after the washing process. Fig. 7(a) and (b) show the heating curves of macrocapsules and microcapsules with different shells compared with PCM. Samples UMicrocapsules, P-Microcapsules, U-1 and P-1 were used as the typical cases to compare with PCM. As can be seen in Fig. 7, the endothermic peak of the four samples were wider than the pure PCM, which means they need more time to finish the melting process. Therefore, thermal conductivity of shell and matrix material needed improvement in later researches. As shown in Table 3, phase change enthalpy of macrocapsules slightly declined with the increase of the concentration of sodium alginate and calcium chloride, which can be recognized from U-1 to U-5, U-5 to U-9 and U-9 to U-12. That can be partially attributed to the fact that more matrix material was manufactured. Another reason can be discovered by the comparison of DH reduction between the concentration of sodium alginate and calcium chloride. As can be seen in Table 3, the DH reduction due to the increase of the concentration of calcium chloride was higher, which can be found through that the DH reduction was 36.8 J g1 from sample U-1 to U-4, 25.7 J g1 from U-5 to U-8 and 27.8 J g1 from U-9 to U-12 while the DH reduction was 11.3 J g1 from U-1 to U-5, 7.2 J g1 from U-5 to U-9. Thus, the concentration of calcium chloride solution was the main factor which caused the DH reduction. And the reason mentioned

Fig. 5. FT-IR spectra of PCM, microcapsules with PMMA shell and the corresponding macrocapsules.

The characteristic patterns of the FT-IR spectra of PCM, microcapsules and macrocapsules with UF shell are shown in Fig. 6(aec). The characteristic bands of C14 shown in Fig. 6(aec) are of the identical position within Fig. 5(aec). However, the peaks at 1660 and 1560 cm1 in Fig. 6(bec) correspond respectively to stretching vibrations of C]O and CeN, and peaks of 1030 cm1 correspond to bending vibrations of NeH. And all these characteristic bands

Fig. 6. FT-IR spectra of PCM, microcapsules with UF shell and the corresponding macrocapsules.

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Table 3 Phase change properties of microcapsules/macrocapsules. Sample

Toma ( C)

Tpmb ( C)

DHmc (J g1)

PCM U-microcapsules U-1i U-2 U-3 U-4 U-5 U-6 U-7 U-8 U-9 U-10 U-11 U-12 P-microcapsules P-1j P-2 P-3 P-4 P-5 P-6 P-7 P-8 P-9 P-10 P-11 P-12

5.6 5.3 5.6 5.8 5.9 5.7 5.8 5.7 5.7 5.8 5.7 5.8 5.7 5.6 4.6 4.1 3.9 5.1 4.8 4.5 4.7 4.9 5.1 4.1 4.9 4.6 4.9

7.3 12.0 10.9 8.1 10.2 7.0 10.8 10.1 9.4 10.8 11.3 6.9 9.8 6.2 9.4 9.3 8.2 8.6 8.9 9.6 9.9 9.3 8.9 11.8 11.4 9.2 11.2

213.0 200.8 166.8 130.0 139.1 127.8 158.4 147.4 136.4 129.6 148.3 143.2 135.0 117.2 162.7 110.3 93.6 105.9 113.7 112.9 106.4 95.8 98.7 106.9 107.6 98.2 100.9

a b c d e f g h i j

Tocd ( C)

Tpce ( C)

DHcf (J g1)

DHg (J g1)

Measured PCM contenth (wt %)

1.64 1.3 2.0 2.2

4.2 0.5 3.5 3.7 3.7 4.4 3.9 4.3 3.8 3.9 3.9 4.6 4.3 4.6 2.9 1.8 2.3 2.4 1.5 0.4 1.0 0.2 1.8 4.8 4.4 2.9 4.9

208.9 187.4 159.5 124.2 132.3 125.0 145.4 140.2 131.7 122.7 141 134.7 130.7 116.7 159.7 105.4 90.2 105.5 111.7 108.5 102.6 94.9 99.7 103.9 95.3 96.7 98.7

211.0 194.1 163.2 127.1 135.7 126.4 151.9 143.8 134.1 126.2 144.7 138.9 132.9 116.9 161.2 107.9 91.9 105.7 112.7 110.7 104.5 95.4 99.2 105.4 101.5 97.5 99.8

92.4 77.7 60.5 64.6 60.2 72.0 68.5 63.8 60.1 68.9 66.2 63.3 55.7 76.8 51.4 43.8 50.3 53.7 52.7 49.8 45.4 47.3 50.2 48.3 46.4 47.5

Onset temperature on DSC heating curve. Peak temperature on DSC heating curve. Enthalpy on DSC heating curve. Onset temperature on DSC cooling curve. Peak temperature on DSC cooling curve. Enthalpy on DSC cooling curve. Average enthalpy of jDHmj and jDHcj. Ratio of DH of Microcapsules and that of corresponding n-tetradecane bulk. Sample 1 containing Microcapsules with urea-formaldehyde shell. Sample 1 containing Microcapsules with Poly(methyl methacrylate) shell.

above should be water absorbed by redundant calcium chloride from air, which reduced the weight ratio of PCM, especially when the concentration of sodium alginate was low. Nevertheless, the DH reduction of P-Macrocapsules was only about 8 J g1 on average from P-5 to P-8 and from P-9 to P-12. And that was not as obvious as Usamples, which may due to the hydrophobic group on PMMA shell. 3.5. Thermal durability of microcapsules/macrocapsules Thermal durability was an important parameter influencing the practical application of microcapsules and macrocapsules, which can be measured by TGA analysis method. The TGA curves of microcapsules and macrocapsules with different shells compared with PCM were shown in Fig. 8. As can be seen from Fig. 8, the degradation processes of PCM were occurred at only one step which began at 85.7  C precisely calculated by tangent method. And the onset point of degradation processes of U-microcapsules and Pmicrocapsules were respectively 99.8  C and 98.9  C, which were higher than pure PCM. The degradation processes of the two kinds of microcapsules both had two separate thermal degradation steps which correspond to the PCM and shell materials, respectively. As shown in Fig. 8, it can be clearly noted that sample U-1 and P-1 revealed stronger thermal durability compared to U-Microcapsules and P-Microcapsules. Table 4 showed the staged onset temperature of sample U-1 and P-1 that shown in Fig. 8. The degradation process of sample U-1 had four steps with different slopes. The slope of the first and second steps were a little gentle, which might represent the evaporation of water and a small part of PCM in microcapsules that were not

perfectly covered by matrix material, respectively. The slope of the third step was sharper, which might correspond to the evaporation of the most part of PCM in microcapsules that was covered by matrix material. The fourth step was the sharpest, and this referred to the degradation of UF shell that began at 218.9  C on TGA curve of U-microcapsules. The delay temperature might be owing to that the macromolecules of matrix material and shell material had a hydrogen-bond interaction. The steps of thermal degradation may be more easily recognized by DTG (differential thermal gravity) curves which were first-order differential of TGA curves as shown in Fig. 9. As can be seen in Fig. 9, there was a small peak on DTG curve at about 160  C, whose onset point was the boundary of the first to second stage. Similarly, the onset point of the overlapping peaks at about 225  C and 260  C were the boundaries of the second to third and third to fourth stages, respectively. Similarly, the degradation process of sample P-1 also had four steps with different slopes as seen in Table 4. The four steps of P-1 had the similar degradation processes as U-1 but higher degradation temperatures which meant stronger thermal durability of the PMacrocapsules. Besides, the thermal durability of macrocapsules declined as the increase of the concentration of calcium chloride. And this circumstance was suitable for all the four steps of U-samples as can be seen in Fig. 10. The reasonable explanation might also be the water that mentioned in DH reduction of macrocapsules. Free water as micro-molecules penetrated into macromolecules of matrix material and UF shell, and lead to the decrease of intermolecular forces. Therefore, the thermal durability of macrocapsules was impaired at macro level.

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Table 4 Staged onset temperature of sample U-1 and P-1. Stage

1 2 3 4

Staged onset temperature U-1 ( C)

P-1 ( C)

50.0 148.3 211.7 250.0

50.0 150.2 255.8 365.5

Fig. 9. TGA and DTG curves of sample U-1. Fig. 7. DSC curves of PCM, microcapsules and macrocapsules of different shells: (a): PMMA shell (b): UF shell.

2.8 mm as possible to make sure the accuracy. As can be seen in Fig. 11, compression force of P-Macrocapsules changed periodically from sample P-1 to P-12. From P-1 to P-4, P-5 to P-8 and P-9 to P-12 which had the same concentration of sodium alginate solution, compression force decreased obviously. Meanwhile, from P-1 to P-5 to P-9 which had the same concentration of calcium chloride solution, compression force increased gentlely. In conclusion, compression force decreased with the increment of concentration of calcium chloride solution and improved with the increment of concentration of sodium alginate solution. The result was in accordance with that discussed in thermal analysis part. Water was a plasticizer as micromolecular inside the shell and matrix materials of macrocapsules, which decreased the rigidity of the matrix material and weaken the compression force of macrocapsules at macro level.

Fig. 8. TGA curves of PCM, microcapsules and macrocapsules of different shells: (a): PMMA shell (b): UF shell.

3.6. Mechanical property of macrocapsules The chosen test speed and the distance was to ensure there was no PCM leaked out. P-samples were chosen as the same size of

Fig. 10. TGA curves of U-macrocapsules with different concentration of calcium chloride solution.

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Fig. 11. Compressive resistance of P-Macrocapsules.

4. Conclusions This work is focused on the synthesis, characterization and determination of macrocapsules contain microcapsules with calcium alginate gels as the matrix material. Two kinds of microcapsules were prepared with n-tetradecane as core material, UF (ureaformaldehyde) and PMMA (poly(methyl methacrylate)) as shell materials, respectively. The FT-IR analysis confirmed the C14 as the core material, UF and PMMA as the shell materials and calcium alginate as the matrix material. The SEM analysis revealed that the surfaces of PMMA microcapsules were smooth and compact spheres compared to tubular or reticular structures of UF microcapsules with extreme coreeshell ratio. Besides, most of the microcapsules were covered with matrix material as the second shell material. The phase change enthalpy of 194.1 J g1 was obtained by C14/UF shell microcapsules with the extreme coreeshell ratio of 5.5:1, and the corresponding microcapsules possessed a phase change enthalpy of 163.15 J g1. TGA analysis confirmed that macrocapsules had much stronger thermal durability than microcapsules. Furthermore, the compressive resistance test showed that no PCM leaked out when deformation over half the size of 1.8 mm and the compressive resistance was 7 N on average, which was sufficient for practical applications on textile, building and coldchain transportation fields. References [1] Basa G, Deveci SS, Yalcin D, Bayraktar O. Properties of n-Eicosane-loaded silk fibroin-chitosan microcapsules. J Appl Polym Sci 2011;121:1885e9. [2] Chol JC, Kim SD. Heat-transfer characteristics of a latent heat storage system using MgCl2$6H2O. Energy 1992;17:1153e64. [3] Alkan C, Sar A, Karaipekli A. Preparation, thermal properties and thermal reliability of microencapsulated n-eicosane as novel phase change material for thermal energy storage. Energy Convers Manag 2011;52:687e92. [4] Lazaro A, Gunter E, Mehling H, Hiebler S, Marin JM, Zalba B. Verification of a Thistory installation to measure enthalpy versus temperature curves of phase change materials. Meas Sci Technol 2006;17:2168e74. [5] Tuncbilek K, Sari A, Tarhan S, Ergunes G, Kaygusuz K. Lauric and palmitic acids eutectic mixture as latent heat storage material for low temperature heating applications. Energy 2005;30:677e92. [6] Maruoka N, Akiyama T. Exergy recovery from steelmaking off-gas by latent heat storage for methanol production. Energy 2006;31:1632e42. [7] Medinaa MA, Kingb JB, Zhang M. On the heat transfer rate reduction of structural insulated panels (SIPs) outfitted with phase change materials (PCMs). Energy 2008;33:667e78. [8] Huang L, Petermann M, Doetsch C. Evaluation of paraffin/water emulsion as a phase change slurry for cooling applications. Energy 2009;34:1145e55. [9] Song GL, Ma SD, Tang GY, Yin ZS, Wang XW. Preparation and characterization of flame retardant form-stable phase change materials composed by EPDM, paraffin and nano magnesium hydroxide. Energy 2010;35:2179e83. [10] Xia L, Zhang P, Wang RZ. Numerical heat transfer analysis of the packed bed latent heat storage system based on an effective packed bed model. Energy 2010;35:2022e32.

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