Fabrication and Characterization of Bio-based PCM Microcapsules for Thermal Energy Storage Maryam Fashandi and Siu N. Leung, Lassonde School of Engineering, Department of Mechanical Engineering, York University, Toronto, ON, Canada Abstract Bio-based phase change microcapsules (MicroPCM) consist of polylactic acid (PLA) shell and butyl stearate core were fabricated by emulsion evaporation method. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimeter (DSC) were employed to characterize the morphology, the chemical structures, and the thermal properties of the fabricated MicroPCM. The results indicated that higher energy input during the emulsion step, utilized a sonicator, is critical to fabricate microPCM with smaller size (i.e., 10-12 µm) and narrower size distribution. In short, the experimental results demonstrated the possibility to fabricate 100% bio-based MicroPCM with enhanced environmental sustainability for thermal energy storage applications.

Introduction Thermal energy storage can be achieved by sensible heat or latent heat. Latent heat energy storage systems, including phase change materials (PCM), have the advantage of high energy storage density with extremely small temperature variation. This is realized by the large amount of latent heat being absorbed and released when the PCM undergoes phase change [1,2]. The environmental sustainable technology has a wide spectrum of applications, including waste heat recovery, solar energy storage, zero-energy building, temperature regulating textile, as well as thermal management in electronics and battery. The performance of these latent heat energy storage systems depend on the properties of PCM, which can be classified into organic and inorganic PCM. Examples of inorganic PCM include salt hydrates, salt, metals and alloys. While their high thermal conductivity, high heat of fusion, low cost, and low flammability are beneficial, their supercooling, corrosiveness and phase decomposition have limited their uses in various applications. Organic PCM demonstrated good thermal stability; however, their limitations include low thermal conductivity and potential leakage during phase transition. Among the two key families of organic PCM (i.e., paraffin-based and nonparaffinic-based), paraffinic PCM has been used for a long time, but its high flammability and cost have led to restricted uses in building materials. In contrast, nonparaffinic PCM (also called bio-based PCM), which are

esters of organic fatty acid made from vegetable oils, are cheaper than their paraffinic counterparts. Moreover, its fully hydrogenated structure also makes it stable against oxidation [3]. During solid-liquid phase transition, PCM undergoes volume change. In this context, PCM can be encapsulated by natural or synthetic polymers to form a micro-or-nanosphere with its diameter ranging from 1 to 1000 µm. The polymeric shell prevents the PCM from being exposed to the environment. Several polymers have been used as shell materials for the encapsulated PCM. Some examples include polystyrene (PS), polymethyl methacrylate (PMMA) [4], polyurea (PU) [5], and polylactic acid (PLA) [1]. Researchers have reported the fabrication of phase change microcapsules (MicroPCM) either with a biobased polymeric shell (i.e., PLA) [1] or with a bio-based PCM core (e.g., butyl stearate). Being a biodegradable and bio-based material [5] as well as possessing physical and mechanical properties comparable to conventional commodity polymers (e.g., polystyrene) [6], PLA is an attractive option to be used for the shell to promote MicroPCM’s environmental sustainability. For example, it was used to prepare MicroPCM with paraffin wax core using emulsion evaporation method [1]. On the other hand, bio-based PCM such as butyl stearate has been applied as a composite component with carbon nanotube [3], gypsum [7], and montmorillonite [8] to prepare MicroPCM with non-bio-based polymeric shells. Nevertheless, to the best knowledge of the authors, the fabrication of 100% bio-based MicroPCM, which consists of bio-based shell and core materials, has yet been reported In this work, the fabrication of 100% bio-based MicroPCM encapsulated butyl stearate by PLA shell is reported. Parametric studies, based on solvent evaporation method [9], were conducted to investigate the effect of agitation and filtration methods on the morphology of the fabricated MicroPCM. The solvent evaporation method is accompanied by oil-in-water emulsion, which is commonly used to encapsulated core materials that are either insoluble or have poor solubility in water [10]. The morphology, chemical structures, and thermal properties of the fabricated MicroPCM were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC).

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Experimental Materials Polylactic acid (PLA, Ingeo 8052D NatureWorks) was used to fabricate the shell of the MicroPCM. Butyl stearate (Alfa Aeser Company) was used as the PCM core. Polyvinyl alcohol (PVA, Sigma Aldrich) with an average molecular weight of 146000-186,000 g·mol-1 and 87-89% hydrolyzed was used as the emulsifier. Dichloromethane (DCM, Caledon Laboratory Chemical), with the density of 1320 kg·m-3 was used as the solvent for the oil medium. Deionized water was used as aqueous medium. The physical properties of polylactic acid and butyl stearate are summarized in Tables 1 and 2, respectively. Table 1. Physical Properties of Polylactic Acid Property Value Unit Density Crystalline Melting Temperature Glass Transition Temperature

1240

kg·m-3

145-160

°C

55-60

°C

Table 2. Physical Properties of Butyl Stearate Property Value Unit Density Melting Temperature Boiling Temperature Latent Heat of Fusion

861

kg·m-3

17-22

ͦC

343

ͦC

107.67

J·g-1

morphology of MicroPCM, a subset of samples were also prepared by sonicating the water-oil system at different levels of amplitude (i.e., 40% and 50%) for 5 minutes using a sonicator probe (QSonica Q700) before the evaporation step. Two filtration schemes (i.e., namely one-step and two-step filtration) were used to obtain the fabricated MicroPCM. In the one-step filtration, the final emulsion was filtered by quantitative filter paper and subsequently rinsed several times by deionized water. In the two-step filtration, the final emulsion was poured into the quantitative filter paper in a funnel and allowed it to settle for 3 h. After that, the precipitates were removed and remaining emulsion was transferred to another quantitative filter paper to be obtained the remaining MicroPCM. After the extraction of MicroPCM, the microcapsules were placed in an oven at 50°C for 12 h. Eventually, the MicroPCM clumps were gently ground in order to separate them from the agglomerates. The different agitation and/or filtration methods of different MicroPCM samples are summarized in Table 3. Table 3. Summary of sample preparation of MicroPCM Filtration Sample Agitation Method Method PCM1 PCM2 PCM3 PCM4

Mechanical stirring at 1600 rpm for 1 hr Sonication at 40% amplitude for 5 min + Mechanical stirring at 1600 rpm for 1 hr Sonication at 50% amplitude for 5 min + Mechanical stirring at 1600 rpm for 1 hr Sonication at 50% amplitude for 5 min + Mechanical stirring at 1600 rpm for 1 hr

One-step One-step One-step Two-step

Preparation of PLA-butyl stearate microcapsules PLA-butyl stearate capsules have been prepared using the emulsion evaporation method. For the aqueous medium, a 3% PVA solution was prepared by dissolving three grams of PVA in 97 mL of deionized water. The solution was cured for 30 minutes at room temperature to let it swell. After that, it was heated up to 80°C for 2 hours to let the PVA dissolve completely. For the oil medium, 1.2 g of PLA and 0.2 g of butyl stearate was added to 14.9 mL of DCM. The mixture was cured for 2 h to obtain a PLA-butyl stearate solution. After the preparation of both the aqueous phase and oil phase solutions, 10 g of PLA-butyl stearate solution was added to 60-80 g of PVA solution. After the water-oil system had been cured for 3 h, the temperature of the mixture was elevated to 45°C while undergoing different stirring and agitation methods to evaporate the DCM. In order to investigate the effect of agitation energy on the

Characterization of PLA-butyl stearate microcapsules The morphology, chemical structures, and thermal properties of the MicroPCM were analyzed to evaluate their characteristics. Scanning Electron microscopy (FEI Company, Quanta 3D FEG) was used to investigate the morphology and size of the microcapsules. The dried MicroPCM were sputter coated with gold (Denton Vacuum, Desk V Sputter Coater). The particle size distribution was obtained by analyzing the SEM micrographs using ImageJ. The chemical structures of the microcapsules and the each of its components (i.e., PLA and butyl stearate) were analyzed using Fourier transform infrared spectroscopy (Bruker Alpha-P FT-IR Spectrophotometer). The spectra were collected by averaging signals from 32 scans at a resolution of 4cm-1 in the range of 4000-400 cm-1. The thermal properties of the microspheres were evaluated using differential scanning

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calorimetry (TA Instrument, DSC Q2000). The samples underwent a heat-cool cycle from -60 to 80 ͦ C at a heating or cooling rate of 5 °C/min.

Results and Discussion

and the two-step filtration (i.e., PCM4) had significantly smaller microcapsule sizes as well as a knowingly narrower size distribution. The average sizes and standard deviations of MicroPCM prepared by different agitation and/or filtration methods are summarized in Table 4.

Morphology of PLA-butyl stearate microcapsules Figure 1 illustrates the microscopic morphology of MicroPCM fabricated by different agitation and/or filtration schemes. The micrographs reveal that all MicroPCM were spherical in shape and had considerably smooth surface. Some pores were observed to be on their surface, and were believed to be caused by the evaporation of DCM from the inner region of the emulsion. It is apparent that sonication at 50% amplitude with two-step filtration (i.e., PCM4) resulted in smaller MicroPCM size than the other three cases. This suggested that the agitation energy during the DSM evaporation step as well as the filtration method were critical to control the sizes of MicroPCM.

(a)

(b) Figure 1. SEM micrographs of PLA-butyl stearate microcapules prepared by: (a) PCM1; (b) PCM2; (c) PCM3; and (d) PCM4 Size distributions of PLA-butyl stearate microcapsules The size distributions of MicroPCM prepared by different agitation and/or filtration methods are plotted in Figures 2(a) through (d). For MicroPCM obtained by the one-step filtration method (i.e., PCM1, PCM2, and PCM3), the size distribution curves revealed that inserting a sonication step would result in a narrower size distribution although the average size of microcapsules were only reduced very slightly. Moreover, increasing the sonication amplitude from 40% to 50% also enhanced the uniformity of the MicroPCM size. Most importantly, MicroPCM prepared with sonication of 50% amplitude

(c)

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the peak related to CH2 group comparing to the FTIR spectrum of pure butyl stearate revealed that both PLA and butyl stearate existed in the MicroPCM. In conclusion, butyl stearate was well encapsulated by PLA resin.

(d) Figure 2. Particle size distribution of the microspheres Table 4. Average sizes of MicroPCM prepared by different agitation and/or filtration approach Standard Sample Average Size [µm] Deviation [µm] PCM1 PCM2 PCM3 PCM4

22.9 22.8 22.6 11.2

Chemical structures microcapsules

11.6 10.4 10.4 4.18 of

PLA-butyl

stearate

FTIR spectra of PLA, butyl stearate, and PLA-butyl stearate microcapsules are illustrated in Figures 3(a) through (c). It can be observed from Figure 3(a) that PLA had obvious absorption peaks at 2996 and 2946 cm-1, which correspond to the CH3 bond in the molecular structure. The sharp peak around 1747 cm-1 resulted from the stretching vibrations of C=O groups. The absorption peaks at around 1081-1181 cm-1 were related to the C-O group in the ester functional group. Figure 3(b) shows the FTIR spectrum of butyl stearate. The two sharp peaks at around 2853 and 2922 cm-1 belong to the CH2 group, which is the main group in the butyl stearate’s backbone. As a result, the CH2 peaks were sharp and intense. The sharp peak around 1737 cm-1 was related to the C=O groups. The FTIR spectrum of the PLA-butyl stearate microcapsules is shown in Figure 3(c). The characteristic peaks in 2853 and 2922 cm-1, attributed to CH2 group, belonged to the butyl stearate core. The absorption peaks at 2996 and 2922 cm-1 were related to CH3 bond, which were related to both the PLA shell and the butyl stearate core. The sharp peak at 1747 was related to C=O group, which could also be found in both the shell and the core materials. However, the significantly lower intensity in

Figure 3. FTIR spectra of: (a) PLA; (b) butyl stearate; and (c) PLA-butyl stearate microcapsules

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Thermal analysis of PLA-butyl stearate microcapsules Thermal analyses of PLA-butyl stearate MicroPCM were conducted using DSC. The DSC thermogram of all four samples of MicroPCM were very similar to each other, and the thermogram for PLA-butyl stearate prepared without the sonication step and with the one-step filtration method is shown in Figure 4. The multiple endothermic peaks in the heating cycle of the DSC curve show that butyl stearate has two phase transitions. The one at lower temperature was related to the transformation of butyl stearate crystals from one form into another. The sharp endothermic peak observed between 17 and 22°C was related to the solid-liquid phase transition of butyl stearate. A second order transition was observed between 55 and 60°C. This was attributed to glass transition of PLA shell.

microcapsules (MicroPCM). Fourier transform infrared analyses confirmed that encapsulation of butyl stearate by PLA shell was successfully achieved. Parametric studies were conducted to investigate the effects of agitation and filtration methods on the characteristics of MicroPCM. The smaller microcapsules had a mean diameter of 11.2 µm. Thermal analyses using differential scanning calorimetry also revealed the effects of agitation and filtration methods on the encapsulation efficiency of MicroPCM. The lower than expected encapsulation efficiency of all MicroPCM fabricated in this study suggests that future studies will be needed to optimize the composition of different components in order to enhance the encapsulation efficiency. Nevertheless, the successful fabrication of 100% MicroPCM is expected to further promote the environmental sustainability of the already environmental friendly latent heat energy storage technology. Table 5. Average sizes of MicroPCM prepared by different agitation and/or filtration approaches Latent Heat of Encapsulation Sample PCM [J/g] Efficiency [%] PCM1 PCM2 PCM3 PCM4

13.28 15.55 12.55 12.68

12.34 14.45 11.66 11.78

Acknowledgement Figure 4. DSC thermogram of PLA-butyl stearate MicroPCM (PCM1) The phase change latent heat of MicroPCM can be determined by calculating the area under the endothermic peak. The encapsulate efficiency of PLA-butyl stearate microcapsules can be calculated by Equation (1): ΔH (1) Encapsule efficiency = × 100% Δ H PCM where ∆H and ∆HPCM are the latent heat of fusion of MicroPCM and butyl stearate, respectively. Table 5 summarizes the encapsulate efficiencies of the four MicroPCM samples. It can be observed that the encapsulate efficiency was not influenced by the agitation and filtration methods, and therefore was independent of the size of MicroPCM.

Conclusion Microencapsulation of a bio-based PCM (i.e., butyl stearate) with a bio-based polymeric shell (i.e., polylactic acid (PLA) was conducted by emulsion evaporation method to fabricate 100% bio-based phase change

The authors are grateful about the financial support by the Natural Sciences and Engineering Research Council of Canada.

References [1]

J. Wang, K. Sun, J. Wang, and Y. Guo, Polym. Plast. Technol. Eng., 52, 1235 (2013). [2] S.-G. Jeong, O. Chung, S. Yu, S. Kim, and S. Kim, Sol. Energy Mater. Sol. Cells, 117, 87 (2013). [3] S. Yu, S.-G. Jeong, O. Chung, and S. Kim, Sol. Energy Mater. Sol. Cells, 120, 549 (2014). [4] P. Han, L. Lu, X. Qiu, Y. Tang, J. Wang, Energy, 91, 531 (2015). [5] M. Li, M. Chen, and Z. Wu, Appl. Energy, 127, 166 (2014). [6] J.-M. Raquez, Y. Habibi, M. Murariu, and P. Dubois, Prog. Polym. Sci., 38, 1504 (2013). [7] T. Shi, W. Sun, and Y. L. Yang, Mater. Struct., 47, 533 (2014). [8] X. Fang, Z. Zhang, and Z. Chen, Energy Convers. Manag., 49, 718 (2008). [9] A. Jamekhorshid, S.M. Sadrameli, and M. Farid, Renew. Sustain. Energy Rev., 31, 531 (2014). [10] M. Li, O. Rouaud, and D. Poncelet, Int. J. Pharm., 363, 26, (2008).

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