Novel method for microencapsulation of eutectic hydrated salt

. Phase change materials (PCMs), as latent thermal energy storage, have received plenty of attention in recent decades due to their excellent performance in energy conservation. Inorganic hydrated salts have many advantages over organic phase change materials, such as higher thermal storage density, low cost, nontoxicity and nonflammability. This work presents a simple and environmentally friendly method for the preparation of microcapsules with inorganic hydrated salts as core materials. Microcapsule using EHS composed of SSD and DHPD as core material and EC/ABS as shell material has been prepared and investigated by various characterization techniques. The scanning electron microscope (SEM) results indicate that the microcapsules are basically spherical with a core-shell structure. In addition, Fourier-transform infrared spectroscopy (FT-IR) analysis results suggest that the microcapsules are only the physical coating of composite shell material to core material. Furthermore, the phase transition enthalpy is 168.3 J/g according to the differential scanning calorimeter (DSC) analysis. Besides, the method of microencapsulation may be extended to other liquid hydrates salts at room temperature.


Introduction
The demand of human society for fossil energy has been rapidly increasing accompanied by the growth of population and technological advancements in the world, and a series of environmental problems have also emerged [1][2][3] .Therefore, improving energy utilization and developing and utilizing new energy, such as solar and wind energy, are important ways to alleviate energy shortage and environmental pollution [1,4] .Phase change energy storage technology came into being and has developed rapidly in recent years.Phase change materials are the core and basis of phase change energy storage technology, which can store or release heat through the transformation of their own phase states within a negligible temperature fluctuation.Therefore, phase change energy storage materials can not only improve the utilization efficiency of traditional fossil energy by transferring and reusing hightemperature waste heat, such as industrial waste heat, but also solve the discontinuity and spatial inconsistency between the supply and demand of new energy such as solar energy and wind energy [5,6] .Phase change materials are usually classified into two major categories: inorganic and organic compounds.Organic PCMs include paraffin, fatty acids, sugar alcohols and dicarboxylic acids, whereas inorganic PCMs comprise metals, alloys, salts and hydrate salts.Undoubtedly, compared to organic compounds, inorganic phase change materials possess the conspicuous advantages of low cost, higher thermal storage density, thermal conductivity, non-toxicity and non-flammability.
Sodium carbonate decahydrate (Na 2 CO 3 •10H 2 O), also known as crystalline sodium carbonate, has a high latent heat of phase transition(208J/g) and a suitable phase transition temperature (30.4 °C), which is a potential inorganic phase transition heat storage material.However, Na 2 CO 3 •10H 2 O cannot be used as a phase change material directly.Na 2 CO 3 •10H 2 O, the same as most inorganic hydrated salts, has a serious phase separation phenomenon and a high degree of supercooling (above 20 °C), which are seriously affected its thermal storage performance [7,8] .
In this work, a novel eutectic hydrate salt (Na 2 CO 3 •10H 2 O-Na 2 HPO 4 •12H 2 O) eutectic hydrate salt was synthesized successfully and was used as the core material to prepare phase change material microcapsules.Besides, most of the organic solvents can be recovered in the process of microencapsulation fabrication, which greatly reduces the manufacturing cost.The heat storage properties and thermal stability of the phase change material microcapsules were investigated by DSC and TG, and the microcapsules were observed by scanning electron microscopy (SEM).In addition, the chemical structure and crystal characteristics of the microcapsules were also determined by FT-IR and XRD tests.

Preparation of binary eutectic hydrated salt (EHS)
A series of SCD-DHPD eutectic mixtures with different weight ratios were fabricated by the heating mixing method.SCD and DHPD were uniformly mixed in the beaker(total mass is about 20g), then placed in a water bath kept at 50 o C and completely melted.The solution was stirred for 20 min at 400 r/min until it was uniform.During the process of solidification, the real-time temperature of the hydrated salt mixtures was recorded by the multichannel temperature measuring system.

Preparation of EHS@EC&ABS microcapsules
Firstly, the composite shell materials (EC&ABS) were dissolved in 50 ml methylene dichloride at room temperature, in a round-bottomed flask with three necks.Secondly, the melted core material (SCD-DHPD) was suspended and dispersed in the shell material solution, and then formed into an emulsion state under stirring at 1000 r/min for 20min.Subsequently, coacervation of the composite shell material was induced by dropwise addition of PDMS, the mixture was transferred to a large beaker containing 500 ml of n-heptane.
Finally, microcapsule particles appeared at the bottom of the solution.The supernatant in the beaker was transferred to a rotary evaporator, and the recovered organic solvent was obtained by vacuum distillation.The remaining solid particles were washed twice with n-heptane and water, respectively to obtain the microcapsule product.

Characterization
The thermal properties of the samples were obtained by differential scanning calorimeter (DSC 200F1) with 50 mL/min of N 2 as the sample purge gas and protective gas, the test temperature range is 0~60 o C, and the heating/cooling rate is 5 o C /min.The scanning electron microscope (SEM, SU-8010) was employed to observe the micromorphology of samples after gold coating treatment.The chemical structures were evaluated by Fourier transform infrared spectroscopy (FT-IR).The crystallization behaviors of SSD, DPHD, and EHS were characterized using XRD analysis over the 2θ range 5-70 o .

Property characterization of SCD-DHPD (EHS)
Fig. 1A is the photographs of different proportions of SCD-DHPD hydrated salts in the melted state.Obviously, when the content of SCD is less than 40%, the melted SCD-DHPD is homogeneous and transparent, and there is no solid precipitation at the bottom of the sample tubes, i.e., no phase separation occurs; while when the mass ratio of SCD is more than 40%, there are different degrees of phase separation phenomenon.The phase transition temperature of the binary mixed salt system shows a "V" shaped trend of decreasing and then increasing with the increase of SCD content.It can be seen that when the mass fraction of SCD in the mixed salt is 40%, SCD and DHPD form a eutectic hydrated salt.The eutectic salt has a phase transition temperature of 21.8 o C and a subcooling degree of 2.5 o C, and can be directly used as the core material of phase change material microcapsules.Fig. 1C shows the XRD patterns of the hydrated salts SCD, DHPD and their eutectic salt SCD-DHPD (the mass fraction of SCD is 40%).It can be seen from the spectrum that the diffraction peaks of SCD appear at 17.2 o , 22.6 o , 31.3 o , and 35.7 o , and the main diffraction peaks of DHPD appear at 16.8 o , 20.4 o , and 30.9 o .All these characteristic diffraction peaks emerge in the spectrum of the eutectic salt SCD-DHPD and no new diffraction peaks appear.Comparison of the microscopic morphology shows that the eutectic salt SCD-DHPD (Fig. 1F) contains granular SCD crystals (Fig. 1D) and rod-shaped DHPD crystals (Fig. 1E).Combined with SEM and XRD analysis, SCD and DHPD are physically mixed to form the eutectic salt SCD-DHPD, and no chemical reaction occurs.
Fig. 2 illustrates the micromorphology of microcapsules.As shown in the figure, the microcapsules are roughly spherical with good encapsulation and no obvious pores and cracks on the surface, but the microcapsules mostly adhere together and the particle sizes of microcapsules are quite different.This may be due to the non-uniform emulsification of the molten state eutectic salt in the shell material solution.Fig. 2C shows a broken microcapsule, and the phase change material can be seen through the cracks in the shell material.Fig. 2D shows an enlarged view of Fig. 2C, and the shell material thickness of the microcapsule is approximately 2.42 μm.The IR spectra of the core material eutectic salt SCD-DHPD, the composite shell material ABS plastic/ethyl cellulose and the phase change material microcapsules are shown in Fig. 3.The broad and strong absorption peaks located between 3600-2750 cm -1 are caused by the stretching vibration of -OH in the hydrated salt crystalline water; the stretching vibration absorption peaks of ν as (P-O), ν s (P-O) and ν as (P-OH) are at 1067.28, 954.31 and 863.23 cm - 1 ; the absorption peaks at 1709.28 cm -1 are caused by the stretching vibration of C=O, and the characteristic absorption peaks of asymmetric and symmetric stretching vibration of CO 3 2-appear at 1458.16 cm -1 and 1012.78cm -1 , respectively, which are also the characteristic peaks of CO 3 2-.Fig. 3c shows the characteristic absorption peaks of the composite shell material, the broad peak at 3471.21 cm -1 corresponds to the stretching vibration of -OH; the absorption peaks due to the symmetric C-H stretching of the -CH 2 and -CH 3 groups appear at 2971.49 and 2920.82cm -1 , while the peaks at 1449.88 and 1376.15cm -1 are caused by the out-of-plane C-H bending vibration.The characteristic absorption peak of -CN of the acrylonitrile unit in ABS is observed at 2239.01 cm -1 .The four adjacent peaks between 2000 and 1660 cm -1 are caused by the pan absorption of C-H and C=C in-plane deformation vibrations, while the peak at 699.93 cm -1 is caused by the C-H out of plane stretching vibration in the monosubstituted benzene ring, so these two peaks prove that the presence of the monosubstituted benzene ring in the composite shell material.In addition, the strong absorption peak at 1098.65 cm -1 is attributed to the C-O-C stretching vibration in the EC.It can be found that the characteristic absorption peaks of both core material and shell material can be found in the IR spectra of microcapsules, and no new characteristic peaks appear except these characteristic peaks, which indicates that the microcapsules prepared by the organic phase separation method are only the physical coating of composite shell material to the core material, and no chemical reaction appears during the preparation process.Fig. 4 demonstrates the melting and freezing curves of SCD-DHPD, MEPCM and microcapsule after 50 heating-cooling cycles.The latent heat value of phase transition after microcapsule encapsulation is reduced, which is due to the fact that the microcapsule shell material only plays the role of protection and support, and does not contribute to the heat storage enthalpy value of the microcapsule.And after 50 cycles, the latent heats of the microcapsules in the melting stage and crystallization stage are 117.7 J/g and 108.3 J/g, which decrease by 10.4% and 9.7%, respectively, indicating that the prepared microcapsules have good cycling stability.

Conclusion
In this work, the microcapsule is prepared by coacervation method using EHS composed of SCD and DHPD as core material and EC/ABS as shell material.The eutectic hydrate salt(40wt% Na 2 CO 3 •10H 2 O-60wt% Na 2 HPO 4 •12H 2 O) has a phase transition temperature of 21.8 °C and a melting enthalpy of 168.3 J/g.SEM results display that the microcapsules are basically spherical, and the composite shell material forms a protective film on the surface of the eutectic salt, that is, the microcapsules have a core-shell structure.The FT-IR results show that there is no chemical reaction between the core material and the shell material of the phase change material microcapsule.The DSC analysis demonstrates that the melting enthalpy and crystallization enthalpy of the eutectic salt is 168.3J/g and 152.2 J/g, respectively, and that of the microcapsules are 131.4J/g and 119.9 J/g, separately.The latent heat of the microcapsules in the melting and crystallization stages, after 50 freeze-thaw cycles, decreased by 10.4% and 9.7%, which indicates that the prepared microcapsules have certain practical applications.