Preparation, Properties, and Supercooling Prevention of Phase

Article pubs.acs.org/IECR

Preparation, Properties, and Supercooling Prevention of Phase Change Material n‑Octadecane Microcapsules with Peppermint Fragrance Scent Qian Wu,† Di Zhao,† Xin Jiao,† Yao Zhang,† Kenneth J. Shea,§ Xihua Lu,*,†,‡ and Gao Qiu*,‡ †

College of Chemistry, Chemical Engineering and Biotechnology, and ‡State Key Laboratory For Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China § Department of Chemistry and Materials Science, University of California, Irvine, California 92697, United States S Supporting Information *

ABSTRACT: A new kind of phase change material n-octadecane microcapsules containing peppermint fragrance scent has been synthesized through the interfacial polymerization of isophorone diisocyanate (IPDI) and hexamethylenediamine (HMDA). The average size, morphology, and chemical components of the microcapsules were characterized by laser particle size analyzer, scanning electron microscope (SEM), and Fourier transform infrared (FTIR), respectively. The thermal characterizations of the microcapsules were studied by differential scanning calorimetry (DSC) and thermal gravity analysis (TGA). The results show that the average size of the microcapsules was in the range of 4.0 to 7.0 μm and decreased with increasing emulsifier. The microcapsulated phase change materials (MicroPCMs) are spherical, and their surface is dented. The MicroPCMs without nucleating agents demonstrated high heat storage capacity but had supercooling phenomena. It was found that the addition of approximately 8.3 wt % nucleating agents 1-tetradecanol or paraffin in core materials can suppress MicroPCMs from supercooling. The MicroPCMs maintained a good thermal stability below 150 °C. MicroPCMs.11−15 But this method inevitably results in formaldehyde residue in microcapsules.16 Polyurethane and polyurea are two commonly used polymers to encapsulate PCMs and form MicroPCMs. The polyurea shell is usually synthesized by aromatic isocyanates such as diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI) and amine.17−20 Aromatic isocyanates can hydrolyze into highly toxic phenylamine. Aliphatic isophorone diisocyanate (IPDI) as monomer to replace aromatic isocyanates can form polyurea microcapsules without any phenylamine. In the preparation of polyurea MicroPCMs, the toxic cosolvent tetrahydrofuran, acetone, and cyclohexane are usually chosen to form the miscibility of n-alkane PCMs and isocyanate.19,20 Besides, supercooling is still a big obstacle to the industrial application of MicroPCMs. Supercooling leads to the decrease of crystallization temperature, and thus the latent heat will be released at a lower temperature, or in a wider temperature range.21 Yamagishi et al.21 found that the supercooling temperature of MicroPCMs (n-tetradecane and n-dodecane) decreased with the decrease of their diameters in the range of 5−100 μm. To solve these issues, researches have added nucleation agents, such as solid nanoparticles, alcohol, and high-melting point paraffin to promote heterogeneous nucleation during melt crystallization.22−27 In this paper, toxic-free dementholized peppermint oil (DPO) was used to replace highly toxic cosolvent. The main

1. INTRODUCTION Phase change materials (PCMs) are able to store and release thermal energy around the phase transition temperature region efficiently and repeatedly. They can store energy and regulate the surrounding temperature due to their high latent heat. Therefore, they have attracted attention as a kind of renewable thermal energy source in the era of energy crises.1−3 Microcapsulated phase change materials (MicroPCMs) were studied in the late 1970s because of their large storage capacity and isothermal nature of the storage process.4−7 The main advantages of MicroPCMs are as follows:8,9 (1) avoid the leakage of PCMs and isolate PCMs from matrix; (2) increase the heat transfer area of PCMs and improve their performance of thermal conductivity; (3) alter the decentralized state of PCMs and solve the problem that PCMs are incompatible with the surrounding medium in thermodynamics. The solid−liquid phase change materials are encapsulated by stable and high thermoplastic polymers using membrane-forming technique. The general techniques for preparing microcapsules are classified as physicomechanical, physic-chemical, and chemical methods.10 Research showed that the microcapsules prepared by the physicomechanical method had larger particle size, rougher surface, and lower thermal storage capacity.11 MicroPCMs prepared by physicomechanical method usually get high heat capacities of about 145−240 kJ·kg−1. The main limitation of this approach is the difficulty in scale-up of the process.12 The chemical methods, especially in situ polymerization and interfacial polymerization, are widely used in the preparation of microcapsules. In-situ polymerization of amino resins such as melamine formaldehyde (MF) and urea formaldehyde (UF) resin is often selected as the method of © XXXX American Chemical Society

Received: March 21, 2015 Revised: July 30, 2015 Accepted: August 5, 2015

A

DOI: 10.1021/acs.iecr.5b01074 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

formulas for making microcapsules containing 1-tetradecanol and paraffin in core materials.

components of DPO are L-menthol, menthyl acetate, menthone, and so on.28 It contains both hydrophobic alkyl groups and hydrophilic hydroxyl, ketonic and ester groups. So on the basis of the principle of dissolution in similar material structure, IPDI, DPO, and n-octadecane were miscible at a certain proportion. The addition of DPO not only solves the problem of the monomer being immiscible with phase change materials, but also avoids the malodor in traditional MicroPCMs. Gum Arabic is an emulsifier without charge and is well dispersed in deionized water through steric repulsion forces.29 The supercooling of the MicroPCMs was suppressed by adding the 1-tetradecanol and paraffin as nucleation agents in the microcapsules.

Table 2. Contents of 1-Tetradecanol in Core Materials n-octadecane/g DPO/g 1-tetradecanol/g 1-tetradecanol contents/wt %

n-octadecane/g DPO/g paraffin/g paraffin contents/wt %

Gum Arabic (20 wt %)/g Gum Arabic contents/wt % water/g HMDA (40 wt %)/g IPDI/g n-octadecane/g DPO/g total/g

A3

A4

A5

2 2 10.64

4 4 8.64

6 6 6.64 0.72 0.64 4.64 1.36 20

8 8 4.64

10 10 2.64

B3

B4

5.14 0.43 0.43 7.1

5.0 0.5 0.5 8.3

4.8 0.6 0.6 10.0

4.5 0.75 0.75 12.5

C1

C2

C3

C4

3.7

4.0

4.2

4.4

0.8 13.3

0.5 8.3

0.3 5.0

0.1 1.7

1.5

2.3. Characterization of MicroPCMs. The morphology of MicroPCMs was characterized by scanning electron microscope (SEM, Quanta-250, FEI, Czech). The composition of the MicroPCMs was determined by Fourier transform infrared (FTIR, Nicolet6700, Thermo Fisher Scientific Inc., USA) in the range of 250−4000 cm−1. All the FTIR samples were prepared by drying the microcapsules in air and thus the residue monomer was left in the sample after drying the microcapsule. The particle diameter of MicroPCMs was analyzed by laser particle size analyzer (LS13320, Beckman Coulter Inc.). The samples were diluted and dispersed by deionized water and then dropped into analyzer to obtain the data of them. Phase change properties of MicroPCMs and PCMs such as melting, crystallizing points, and phase change enthalpies were measured by using differential scanning calorimeter (DSC, 204F1, NETZSCH Scientific Instruments Trading Ltd.). The DSC measurements were performed at a heating rate of 5 °C/ min and in a temperature range of −10 to 50 °C in N2 conditions. The accuracy of temperature (−10 to 50 °C) is ±0.1 °C, and the accuracy of enthalpy is ±0.1%, which is calibrated by pure indium. The DSC curves were analyzed by NETZSCH Proteus Thermal Analysis. Thermal stabilities of pure n-octadecane and MicroPCMs were examined by using thermal gravimetric analysis (TGA, TG209F1, NETZSCH Scientific Instruments Trading Ltd.) in an N2 atmosphere from 30 to 600 °C at a heating rate of 5 °C/min.

3. RESULT AND DISCUSSION 3.1. Effects of Emulsifier Gum Arabic Mass Percentage on Particle Diameter, Morphology, and Dispersibility of MicroPCMs. The amount of emulsifier has great influence on the formation of MicroPCMS as shown in Figure 1. The results showed that the diameter of MicroPCMs decreased with an increasing amount of emulsifier Gum Arabic and had little change as the concentration of the emulsifier was raised to 8.0 wt %. Gum Arabic is a natural organic dispersant with no charge, and its chemical compositions contain gum aldose, galactose, and glucuronic acid, etc. The intermolecular steric repulsion force makes Gum Arabic a good dispersive effect. SEM images of very small part of whole MicroPCMs are presented in Figure 2. The shape of the MicroPCMs is spherical and the surface is dent. Their average size decreased as the amount of Gum Arabic was increased from 1.0 wt % to

Table 1. Contents of Gum Arabic in Raw Materials A2

B2

Table 3. Contents of Paraffin in Core Materials

2. EXPERIMENTAL SECTION 2.1. Materials. n-Octadecane (purity of 99 wt %, melting point of 27.0−29.0 °C) as phase change materials was purchased from Sinopharm Chemical Reagent Co., Ltd. Isophorone diisocyanate (IPDI, purity of 99 wt %; Adamas Reagent Co., Ltd.) and hexamethylenediamine (HMDA, purity of 70 wt %; Sigma-Aldrich, Co.) were used as monomers. Gum Arabic (viscosity of 60.0−130.0 cPa·s; Sinopharm Chemical Reagent Co., Ltd.) was used as dispersant. Dementholized peppermint oil (DPO, CP, miscible in ethanol, ether and trichloromethane, but insoluble in water) as a cosolvent was purchased from Hefei Fengle Fragrance Co. Ltd. Nucleation agents 1-tetradecanol (purity of 98.0 wt %, melting point 36.5− 39.5 °C) and paraffin (melting point 62.0−64.0 °C) were purchased from Sinopharm Chemical Reagent Co. Ltd. 2.2. Preparation of MicroPCMs. The microcapsules were synthesized by the interfacial polymerization. An appropriate amount of Gum Arabic was dispersed into deionized water and formed the aqueous phase. The miscible mixture of a certain ratio of IPDI, DPO, and n-octadecane at 30 °C was used as oil phase. Then the aqueous phase was mixed with oil phase by a high-shear dispersion homogenizer at the speed of 6600 rpm for 3 min to form an emulsion in a beaker. After an appropriate amount of HMDA was slowly dropped into the emulsion at a stirring rate of 500 rpm for 30 min with a magnetic stirrer while the temperature of the solution was controlled at 35 °C, then the temperature was raised to 55 °C rapidly, and the emulsion was cured under 500 rpm of agitation for 2 h to form a microcapsule slurry. After being washed by hot distilled water, toluene, and hot distilled water, the prepared MicroPCMs were then handled by filtration and drying. Table 1 lists a formula for preparation of microcapsules and the contents of Gum Arabic in raw materials. 1-Tetradecanol and paraffin as nucleating agents were soluble with the oil phase at 35 and 65 °C, respectively. Other processes were the same as those of microcapsules without nucleation agents. Tables 2 and 3 list the

A1

B1

B


Article


Figure 1. Curve of volume average particle diameter of MicroPCMs change with emulsifier Gum Arabic mass percentage.

5.0 wt %. If the gum Arabic contents were low, the steric repulsion force between gum Arabic molecules and the disperse effect on microcapsules would decrease. When its contents increased, the free gum Arabic molecules and the adsorbed long-chain molecules were intertwined to make the viscosity of microcapsules increased. 3.2. FTIR of MicroPCMs. The FTIR spectra of monomer IPDI is shown in Figure 3a. An absorption band at 2260 cm−1 in Figure 3a is assigned to the isocyanate group -NCO. As monomer IPDI with HMDA was polymerized to form polyurea, the absorption band at 2260 cm−1 disappear in spectra A1, B2, and C2 (Figure 3b), indicating that there is no isocyanate residue left in the shell and core of microcapsules. There are several reasons for that.17,30 First, the reaction rate of -NCO and -NH is high. When HMDA is added into the emulsion, the isocyanate group in the IPDI reacted fast with the amine group in the HMDA. Second, HMDA is compatible with the mixture of n-octadecane and DPO, so it can diffuse into the interface of the emulsion and ensure the -NCO in IPDI is reacted completely. The peaks at 1637 and 1562 cm−1 in spectra A1 are respectively assigned to −CO and −NH in the urethane group. As shown in Tables 1, 2, and 3, MicroPCMs A1 contains no nucleating agents, MicroPCMs B2 contains 8.3 wt % 1-tetradecanol, and MicroPCMs C2 contains 8.3 wt %

Figure 3. FT-IR spectra of IPDI and MicroPCMs: (a) IPDI and MicroPCMs A1 (without nucleating agents); (b) MicroPCMs A1 without nucleating agents, B2 with 8.3 wt % 1-tetradecanol, and C2 with 8.3 wt % paraffin in core materials.

paraffin in core materials. The FTIR spectra for MicroPCMs A1, B2, and C2 are compared in Figure 3. Although the core compositions of microcapsules are different, the spectra A1, B2, and C2 are almost identical in Figure 3b. The results indicate that the addition of the nucleation agents has little effect on the copolymerization of −NH2 of HMDA with −NCO of IPDI. 3.3. Melting and Crystallization Behaviors of MicroPCMs. The DSC curves of MicroPCMs A1 without nucleating agents and pure n-octadecane are shown in Figure 4. MicroPCMs A1 have only one endothermic peak at 28.6 °C and their latent heat is 173.2 J/g. Three peaks which are labeled

Figure 2. SEM micrographs of MicroPCMs with various concentrations of Gum Arabic in raw materials: (S1) 1.0 wt %; (S2) 2.0 wt %; (S3) 3.0 wt %; (S4) 4.0 wt %; (S5) 5.0 wt % (×3000). C


Article


A1 at 4.2 °C is attributed to homogeneous nucleation which is the transition from liquid to the thermodynamically stable triclinic crystal phase, while the homogeneous nucleation peak of pure n-octadecane occurs at 24.1 °C. So the supercooling appears in MicroPCMs A1. The supercooling is caused by lacking nucleating agents in microscopic space, and the crystallization temperature reduces with the decrease of particle diameter. Because of reducing crystallization temperature the latent heat is released at a lower temperature and in a wider temperature range. So the application of MicroPCMs is limited. To solve the problem, nucleation agents 1-tetradecanol or paraffin were added into the oil phase and the MicroPCMs were prepared (Table 2 and 3). Table 4 lists the melting and crystallization behavior of MicroPCMs with different concentrations of 1-tetradecanol and paraffin or without nucleating agents. The melting and crystallization enthalpies of MicroPCMs with nucleating agents range from 154.0 to 165.0 J/g which is lower than that of MicroPCMs without nucleating agents. First, from Table 3, the amount of noctadecane in the core material of MicroPCMs with paraffin was less than that of MicroPCMs without nucleating agents. Second, the enthalpies of 1-tetradecanol (211.4 J/g) and paraffin (174.3 J/g) are lower than that of n-octadecane. Figure 5 shows the DSC curves of MicroPCMs with various concentrations of 1-tetradecanol in core materials and

Figure 4. DSC curve of MicroPCMs A1 without nucleating agents and pure n-octadecane.

as α, β, and γ22,25 from high to low temperature are observed on the DSC cooling curve of MicroPCMs A1, and their latent heat is 170.4 J/g. The encapsulation ratios can be calculated using η% =

ΔHMicroPCMs ΔHC ×

MC

× 100 (1)

MMicroPCMs

where ΔH MicroPCMs represents the melting enthalpy of MicroPCMs, and ΔHC (259.6 J/g) represents the melting enthalpy of n-octadecane as measured by the DSC. MMicroPCMs and MC are the mass of n-octadecane in the microcapsules and bulk n-octadecane. So the encapsulation ratio of MicroPCMs A1 is 99.6%. The contents of n-octadecane in MicroPCMs are calculated using the following eq 2: η′% =

ΔHMicroPCMs × 100 ΔHC

(2)

On the basis of eq 2, the contents of n-octadecane are 66.7% in MicroPCMs A1, which is similar to that of the n-octadecane microcapsules preparaed by Zhang et al.31 Peaks α and β of MicroPCMs A1 imply that the encapsulated n-octadecane follows a two-step phase transition mechanism, liquid-rotator phase transition (peak α, 19.4 °C) and rotatortriclinic phase transition (peak β, 12.9 °C).23 The peak α is attributed to the heterogeneous nucleation of n-octadecane at the shell. And the heterogeneous nucleation of n-octadecane leads to the peak β.22 The rotator phase of n-octadecane is a weak-ordered crystalline phase, which is observed in confined geometry such as microcapsules.25 The peak γ of MicroPCMs

Figure 5. DSC curves of MicroPCMs with various concentrations of 1tetradecanol in core materials (the contents increased from B1 to B4) and MicroPCMs A1 without nucleating agents.

MicroPCMs A1 without nucleating agents. All the endothermic peaks of the microcapsules with 1-tetradecanol are similar to that of MicroPCMs A1, as illustrated in Figure 5. The figure indicates that the addition of 1-tetradecanol has little effect on

Table 4. Melting and Crystallization Behavior of MicroPCMs with Different Concentration of 1-Tetradecanol and Paraffina A1 concn/wt % Tm/°C ΔHm/(J/g) Tc/°C

ΔHc/(J/g) Ac/%

A β γ α β γ

0 28.6 170.4 19.4 12.9 4.2 173.2 37.08 30.06 32.86

1-tetradecanol 7.1 30.2 165.5 22.4 16.7 10.6 164.4 26.39 52.44 21.17

8.3 30.3 156.7 16.2

156.4

10.0 29.7 156.2 22.1 16.6 10.4 153.5 38.91 41.26 19.83

paraffin 12.5 30.1 155.8 22.7 17.2 10.9 154.8 30.13 47.68 22.18

1.7 28.1 161.9 17.5 9.7

5.0 27.3 159.5 16.0 11.4

8.3 27.7 159.7 14.9

13.3 27.0 154.1 13.7

159.9 82.70 17.30

159.5 83.68 16.32

157.4

153.9

Tm, the melting peak temperature of endothermic curves; Tc, the crystallization peak temperature of exothermic curves; ΔHm, the melting enthalpy of endothermic curves; ΔHC, the crystallization enthalpy of endothermic curves; Ac, the area (relative latent heat) percent of α, β, or γ peaks in all endothermic curves a

D


Article


Table 4, the peaks of endothermic curves and exothermic curves decreased slightly. The results show that with the increase of paraffin in core materials from 1.7 wt % to 8.3 wt % the supercooling of MicroPCMs was reduced, and 8.3 wt % of paraffin in core materials can greatly suppress the supercooling. The exothermic curve of MicroPCMs C2 containing 8.3 wt % paraffin in core materials has only one peak at 14.9 °C, and its onset point (22.5 °C) is close to that (21.2 °C) of the endothermic curve. Paraffin is composed of n-alkane with various carbon atoms and triclinic crystal exists in those nalkanes, including the thermodynamically stable phase of noctadecane is triclinic crystal. So the paraffin encapsulated into core materials was helpful for promoting the formation of triclinic crystal l in n-octadecane. 3.4. Thermal Stability of MicroPCMs. Thermal stability is an important property of encapsulated PCMs for their practical application. The thermal stability of n-octadecane, polyurea polymer, and MicroPCMs was evaluated by means of TGA as shown in Figure 7. The weight of n-octadecane, polyurea, and

the endothermic process of MicroPCMs. From the exothermic curves, the crystallization temperature of MicroPCMs containing 1-tetradecanol is higher than that of MicroPCMs without nucleating agents. From Table 4, as 1-tetradecanol in core materials was increased from 7.1 wt % to 8.3 wt %, the relative latent heat in γ region of exothermic curves decreased from 21.17% to 0, indicating that the supercooling was suppressed. However, as 1-tetradecanol was increased to 12.5 wt % continuously, the γ area ratio in all exothermic curves was increased to 22.18%. This indicates that the further increasing concentration of 1-tetradecanol negatively enhanced the supercooling behavior of MicroPCMs. Such phenomenon could be due to the reappearing homogeneous nucleation of PCM at a much lower temperature of about 10 °C as 12.5 wt % 1- tetradecanol was added into core materials. At 8.3 wt % of 1tetradecanol, the exothermic curve had only one wide peak at 16.2 °C. The onset point (23.2 °C) of the integrated exothermic curve is close to that (onset point 24.9 °C) of the endothermic curve, showing that 1-tetradecanol at 8.3 wt % in the core material can effectively suppress the supercooling of the MicroPCMs. This is because the melting points range of 1tetradecanol is 36.5−39.5 °C which is higher than that of noctadecane. As the temperature was decreased, the 1tetradecanol crystallized at first. With further decrease of temperature, the 1-tetradecanol crystal nucleus precipitated into the oil phase which provided n-octadecane with nucleation agents. However, the super-cooling degree further increased as 1-tetradecanol was increased from 8.3 wt % to 12.5 wt %. Figure 6 illustrates the DSC curves of MicroPCMs with different concentrations of paraffin in core material as

Figure 7. TGA curves of DPO, n-octadecane, polyurea polymer, and MicroPCMs A1 without nucleating agents, B2 with 8.3 wt % 1tetradecanol, and C2 with 8.3 wt % paraffin in core materials.

MicroPCMs decreased with increasing temperature. Pure DPO began to evaporate and lose weight at approximately 54.4 °C and lost weight completely at approximately 151.1 °C, and pure n-octadecane began to evaporate and lose weight at approximately 111.2 °C and lost weight completely at approximately 183.7 °C. The mass loss of polyurea at 310.6 °C is the thermal decomposition temperature of the polyurea shell. The thermal resistant temperature of MicroPCMs A1 is 148.1 °C, indicating that the encapsulated mixture of DPO and n-octadecane was released at a higher temperature owing to the protection of the microcapsule shell. For the A1 sample, the core materials include n-octadecane and DPO. On the basis of the A1 formulation, the n-octadecane wt %, the PCM wt % is about 4.64/(4.64 + 1.36) = 77.3 wt %. When this value is estimated from the direct measurement as TGA shown in Figure 7, it is about 75%; the theoretical PCM wt % is about 75 × 77.3% = 58 wt %, which is different from the calculated value of 66.7 wt %. This error could be the loss of DPO in core materials due to evaporation and unencapsulation of DPO during the process of preparation. Further, the thermal resistant temperatures of samples MicroPCMs B2 and C2 are 145.6 and 151.7 °C as shown in Figure 7. The thermal resistant temperatures of the MicroPCMs without nucleation agent and containing nucleation agents were close, indicating that the nucleation agents has little effect on the thermal resistant temperatures of MicroPCMs. MicroPCMs A1 still kept 20% weight at 200 °C and continuously lost weight until 350 °C at which point the polyurea polymer thermally degraded.

Figure 6. DSC curves of MicroPCMs with various concen trations of paraffin in core materials (the contents decreased from C1 to C4) and MicroPCMs A1 without nucleating agents.

compared to MicroPCMs A1 without nucleating agents. Similar to Figure 5, all the endothermic curves of the microcapsules with paraffin have one peak, and their peaks are similar to the endothermic peak of MicroPCMs A1. From the exothermic curves, the crystallization temperature of MicroPCMs with paraffin is higher than that of MicroPCMs without nucleating agents. With the decrease of paraffin, the exothermic curves transformed from unimodal to double, which was labeled as α and β from high to low temperature. The peak α is attributed to the heterogeneous and the peak β is attributed to the homogeneous.24 From Table 4, as paraffin in core material was increased from 1.7 wt % to 8.3 wt %, the area (relative latent heat) of peak β decreased from 17.30% to 0 and accordingly that of peak α increased. As paraffin in core material was increased from 8.3 wt % to 13.3 wt %, the exothermic curves have only one peak. From Figure 6 and E


Article


(2) Farid, M. M.; Khudhair, A. M.; Khudhair, S. A. K.; Al-Hallaj, S. A review on phase change energy storage: materials and applications. Energy Convers. Manage. 2004, 45, 1597−1615. (3) Barreneche, C.; Gracia, A. D.; Serrano, S.; Navarro, M. E.; Borreguero, A. M.; Fernández, A. I.; Carmona, M.; Rodriguez, J. F.; Cabeza, L. F. Comparison of three different devices available in Spain to test thermal properties of building materials including phase change materials. Appl. Energy 2013, 109, 421−427. (4) Sánchez, L.; Sánchez, P.; Lucas, A. D.; Carmona, M.; Rodriguez, J. F. Using Neural Networks or Linear Models to Predict the Characteristics of Microcapsules Containing Phase Change Materials. Macromol. Symp. 2010, 287, 162−167. (5) Rodriguez Romero J. F.; Sanchez Silva M. L.; Sanches Paredes P.; Lucas Martinez A. D.; Torres Barreto M. L. Process for microencapsulation of phase change materials, microcapsules obtained and uses thereof. U.S. Patent 0,022,697 A1, Jan. 28, 2010. (6) Rodriguez Romero J. F.; Sanchez Silva M. L.; Sanches Paredes P.; Lucas Martinez A. D.; Torres Barreto M. L. Process for microencapsulation of phase change materials, microcapsules obtained and uses thereof. EP 1,994,063A1 Nov. 26, 2008. (7) Rodriguez Romero J. F.; Sanchez Silva M. L.; Sanches Paredes P.; Lucas Martinez A. D.; Torres Barreto M. L. Process for microencapsulation of phase change materials, microcapsules obtained and uses thereof. WO 107,171 A1, Sep. 27,2007. (8) Liu, Y. F.; Yang, R. J. Preparation and application of microcapsule. Mater. Rev. 1998, 12, 50−52. (9) Wu, Z. L.; Long, W. D. Application of microcapsules thermal storage by changing phase in architectural energy efficiency. Build. Energy Environ. 2006, 25, 20−23. (10) Khakzad, F.; Alinejad, Z.; Shirin-Abadi, A. R.; Ghasemi, M.; Mahdavian, A. R. Optimization of parameters in preparation of PCM microcapsules based on melamine formaldehyde through dispersion polymerization. Colloid Polym. Sci. 2014, 292, 355−368. (11) Hawlader, M. N. A.; Uddin, M. S.; Khin, M. M. Microencapsulated PCM thermal energy storage system. Appl. Energy 2003, 74, 195−202. (12) Sarier, N.; Onder, E. Organic phase change materials and their textile applications: An overview. Thermochim. Acta 2012, 540, 7−60. (13) Su, J. F.; Schlangen, E.; Qiu, J. Design and construction of microcapsules containing rejuvenator for asphalt. Powder Technol. 2013, 235, 563−571. (14) You, M.; Zhang, X. X.; Li, W.; Wang, X. C. Effects of MicroPCMs on the fabrication of MicroPCMs/polyurethane composite foams. Thermochim. Acta 2008, 472, 20−24. (15) Li, M. G.; Zhang, Y.; Xu, Y. H.; Zhang, D. Effect of different amounts of surfactant on characteristics of nanoencapsulated phasechange materials. Polym. Bull. 2011, 67, 541−552. (16) Li, W.; Wang, J. P.; Wang, X. C.; Wu, S. Z.; Zhang, X. X. Effects of ammonium chloride and heat treatment on residual formaldehyde contents of melamine- formaldehyde microcapsules. Colloid Polym. Sci. 2007, 285, 1691−1697. (17) Cho, J. S.; Kwon, A.; Cho, C. G. Microencapsulation of octadecane as a phase-change material by interfacial polymerization in an emulsion system. Colloid Polym. Sci. 2002, 280, 260−266. (18) Ma, Y. H.; Chu, X. D.; Tang, T. Y.; Yao, Y. W. Adjusting phase change temperature of microcapsules by regulating their core compositions. Mater. Lett. 2012, 82, 39−41. (19) Zou, G. L.; Lan, X. Z.; Tan, Z. C.; Sun, L. X.; Zhan, T. Microencapsulation of n- hexadecane as a phase change material in polyurea. Acta Phys.-Chim. Sin. 2004, 20, 90−93. (20) Wan, X.; Wang, Y.; Cheng, B.; Guo, B. H. Preparation and characterization of microcapsule containing phase change materials for functionally thermal fluid. Plastics 2006, 35 (11−14), 90. (21) Yamagishi, Y.; Sugeno, T.; Ishige, T. An evaluation of microencapsulated PCM for use in cold energy transportation medium. Proceedings of the 31st Intersociety Energy Conversion Engineering Conference, IECEC 96, Chetty, P R K, Jackson, W D.; IEEE: Washington, DC, 1996; Vol. 3, pp 2077−2080.

MicroPCMs B2 and C2 still remained about 60% weight at 200 °C. Therefore, the encapsulated nucleation agents can enhance the thermal stability of MicroPCMs. As temperature was further increased, the MicroPCMs B2 containing 1-tetradecanol demonstrated higher thermal stability than MicroPCMs C2 containing paraffin.

4. CONCLUSIONS Phase change material n-octadecane dissolved in DPO was encapsulated by a polyurea shell through interfacial polymerization of IPDI with HMDA to prepare MicroPCMs with a peppermint fragrance scent. The polyurea shell was synthesized by interfacial polymerization using IPDI and HMDA as monomers with high homogenization speed. DPO was used as cosolvent for solving the problem that IPDI is immiscible with phase change materials, also avoiding the malodor and toxicity of traditional MicroPCMs. The average size of MicroPCMs is in the range of approximately 4.0−7.0 μm and decreased with increasing emulsifier, and had little change as the emulsifier Gum Arabic was increased up to 8.0 wt %. The shape of the MicroPCMs is spherical and their surface is dented. The MicroPCMs A1 without nucleation agents have only one endothermic peak at 28.6 °C close to the endothermic peak of pure n-octadecane and their latent heat is 133.2 J/g. There are three exothermic peaks α, β, and γ on the DSC cooling curve of MicroPCMs A1. The crystallization temperature of MicroPCMs A1 shifted to a lower temperature and in a wider temperature range as compared to that of pure n-octadecane. The supercooling of MicroPCMs can be suppressed by feeding about 8.3 wt % 1-tetradecanol or 8.3 wt % paraffin in core materials. The MicroPCMs without and with nucleation agents demonstrated much better thermal stability than pure noctadecane. The addition of nucleation agents further enhanced the thermal stability of MicroPCMs.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01074. The particle diameter distribution of the MicroPCMs with various contents of emulsifier (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-21-67792776. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (51473032), Shanghai Municipality research projects (13520720100) and “Shanghai Qianren Program” for financial supports.

■

REFERENCES

(1) Dutil, Y.; Rousse, D. R.; Salah, N. B.; Salah, N. B.; Lassue, S.; Zalewski, L. A review on phase-change materials: mathematical modeling and simulations. Renewable Sustainable Energy Rev. 2011, 15, 112−130. F


Article

Industrial & Engineering Chemistry Research (22) Zhang, X. X.; Fan, Y. F.; Tao, X. M.; Yick, K. L. Crystallization and prevention of supercooling of microencapsulated n- alkanes. J. Colloid Interface Sci. 2005, 281, 299−306. (23) Xie, B. Q.; Liu, G. M.; Jiang, S. C.; Zhao, Y.; Wang, D. J. Crystallization behaviors of n-octadecane in confined space: crossover of rotator phase from transient to metastable induced by surface freezing. J. Phys. Chem. B 2008, 112, 13310−13315. (24) Fan, Y. F.; Zhang, X. X.; Wang, X. C.; Li, J.; Zhu, B. Supercooling prevention of microencapsulated phase change material. Thermochim. Acta 2004, 413, 1−6. (25) Cao, F. Y.; Yang, B. Supercooling suppression of microencapsulated phase change materials by optimizing shell composition and structure. Appl. Energy 2014, 113, 1512−1518. (26) Wang, H.; Wang, J. P.; Wang, X. C.; Li, W.; Zhang, X. X. Preparation and properties of microencapsulated phase change materials containing two-phase core materials. Ind. Eng. Chem. Res. 2013, 52, 14706−14712. (27) Tang, X. F.; Li, W.; Zhang, X. X.; Shi, H. F. Fabrication and performances of microencapsulated n-alkanes with copolymers having n-octadecyl side chains as shells. Ind. Eng. Chem. Res. 2014, 53, 1678−1687. (28) Chen, Z. M. Study on specific chromatogram and determination of chemical components in peppermint oil. Chin. J. Pharm. Anal. 2011, 31, 1957−1960. (29) Yan, Y.; Liu, J.; Zhang, H. P. Effects of NaCl and dispersants on preparation of microencapsulated phase change paraffin. New Chem. Mater. 2009, 37, 56−59. (30) Zhang, H. Z.; Wang, X. D. Synthesis and properties of microencapsulated n- octadecane with polyurea shells containing different soft segments for heat energy storage and thermal regulation. Sol. Energy Mater. Sol. Cells 2009, 93, 1366−1376. (31) Zhang, X. X.; Fan, Y. F.; Tao, X. M.; Yick, K. L. Fabrication and properties of microcapsules and nanocapsules containing n-octadecane. Mater. Chem. Phys. 2004, 88, 300−307.

G


Preparation, Properties, and Supercooling Prevention of Phase

Recommend Documents