Microencapsulated phase change material suspensions for cool thermal energy storage

Microencapsulated phase change material suspensions for cool thermal energy storage

Journal Pre-proof Microencapsulated phase change material suspensions for cool thermal energy storage G.V.N. Trivedi, R. Parameshwaran PII: S0254-05...

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Journal Pre-proof Microencapsulated phase change material suspensions for cool thermal energy storage

G.V.N. Trivedi, R. Parameshwaran PII:

S0254-0584(19)31329-X

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122519

Reference:

MAC 122519

To appear in:

Materials Chemistry and Physics

Received Date:

09 September 2019

Accepted Date:

02 December 2019

Please cite this article as: G.V.N. Trivedi, R. Parameshwaran, Microencapsulated phase change material suspensions for cool thermal energy storage, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122519

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof

Microencapsulated phase change material suspensions for cool thermal energy storage G. V. N. Trivedi, R. Parameshwaran* Department of Mechanical Engineering, Birla Institute of Technology and Science-Pilani, Hyderabad Campus, Hyderabad – 500 078, India *Corresponding author at: Tel.: +914066303665, fax: +91 4066303998 E-mail addresses [email protected] (G. V. N. Trivedi), [email protected], [email protected] (R. Parameshwaran) ABSTRACT In this study, dimethyl adipate, an organic ester, as the phase change material (core) was microencapsulated into melamine-formaldehyde (shell) in different shell-to-core ratios using in-situ polymerization technique. The microencapsulated phase change material suspensions (MPCMS) were prepared through the dispersion of microcapsules in appropriate proportions into the carrier fluid. The surface morphology of the prepared microcapsules were observed to be almost spherical. The microcapsules with low crystallinity offered greater resistance towards crack, which was attributed to the flexible structure of the shell. Surface structure studies has confirmed the chemical stability between the core and shell. In addition, the microcapsules exhibited good latent heat enthalpy of around 53 kJ/kg and 70 kJ/kg and they were thermally stable up to 160 °C. Furthermore, the viscosity of MPCMS was found to be very low, which enabled them to exhibit Newtonian flow behaviour. Thus, the test results have signified the MPCMS to be considered as a viable candidate for cool thermal energy storage application. Keywords: Phase change material; Microencapsulation; Thermal energy storage; Thermal properties; Viscosity.

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Journal Pre-proof 1. Introduction The global energy utilization has increased drastically because of economic growth, technological advancements, and industrial developments. Development of energy-efficient systems and new technologies that can conserve energy and improve energy utilization is of current research interest. Latent thermal energy storage (LTES) utilizing phase change materials (PCM) has gained a lot of attention due to its potential in energy savings. The LTES systems in general, store and release thermal energy through the phase transition of material at near isothermal conditions by virtue of phase transitions from solid to liquid or vice versa [1,2]. The organic PCMs by virtue of their high energy storage density, large specific heat capacity during phase change, low degree of supercooling, low toxicity and strong intermolecular bonding have gained appreciation in a variety of applications including thermal storage in building, solar energy storage, electronics, biomedical, textiles and so on [3,4]. However, the utilization of the PCMs in the form of emulsions for thermal energy storage has limitations such as, their incompatibility with the surrounding materials and difficulty in handling volume changes during phase change when PCM emulsions are subjected to flow. Furthermore, emulsion instabilities arise due to coalescence caused by the merging of two or more PCM droplets to form a single droplet, and Ostwald ripening arises due to the solubility differences of the dispersed phase contained in droplets of different sizes. These instabilities were found to be a concern in PCM emulsions [5]. In order to overcome these limitations, the microencapsulation techniques are used for coating a thin layer of a shell over the particles or droplet. This technique would increase the heat transfer area, reduce the PCM interaction with the surrounding environment, and enable to handle the PCMs in liquid state conveniently [6]. 2

Journal Pre-proof In recent years, numerous studies have been carried out in encapsulation of particles or droplets within polymer shells for various applications [7,8]. The research on encapsulation of PCMs was mostly focused on n-alkanes [9–11] and paraffins [12,13] due to their large latent heat and availability over a range of temperatures. On the other hand, dispersion of the microcapsules into a carrier (base) fluid results in the formation of the microencapsulated phase change material suspensions (MPCMS). The MPCMS prepared using n-hexdecane [14], n-eicosane [15], n-octadecane [16] and paraffin [17] have shown merits of large heat storage capacity, higher heat transfer rates and effective specific heats than the conventional fluids. However, these paraffin-based PCMs possess limitations like incongruent phase change properties and high cost, limiting their utilization for cooling applications. In this context, non-paraffin based compounds like fatty alcohols [18,19], fatty acid ester [20,21] were developed as PCMs for achieving suitable thermal properties. Fatty acid esters prepared by direct esterification of fatty acids with alcohol can be utilized as PCMs, as the carbon chains of fatty acids play a significant role in achieving good thermal storage capabilities. Due to the existence of carboxylic groups, these compounds are chemically stable, undergo small volumetric changes during phase change, low corrosion activity, non-toxic, odour free and exhibit large thermal energy storage capacity per unit mass [22]. In one of the author’s previous works [23,24], the dimethyl adipate (DMA) as the phase change material being embedded with hybrid nanocomposite particles (HiNPCM) was utilized for cooling application. The HiNPCM was enclosed in high-density polyethylene (HDPE) spherical modules, which were then placed inside a static packed-bed thermal energy storage tank. The heat transfer fluid (chilled water) was allowed to flow over the static packed bed thermal energy storage tank for energy interactions.

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Journal Pre-proof The present research work is distinct in such a way that, the same DMA PCM was utilized in the preparation of the MPCMS which can store and release thermal energy when the MPCMS is subjected to flow. The microcapsules made up of the core DMA PCM and the melamine-formaldehyde shell were dispersed into the carrier fluid (chilled water) and the thermal properties of the MPCMS were explored for the intended cool TES application. Herein, the MPCM flows along with the carrier fluid, whereas the HiNPCM remained static and only chilled water was allowed to flow over the HiNPCM as reported in [23, 24]. However, to the best of our knowledge, there are no studies reported so far on microencapsulation of the DMA PCM with organic or inorganic shells being intended for the preparation of the MPCMS. Therefore in this study, an organic fatty acid ester-based PCM was microencapsulated into a polymer shell using in-situ polymerization technique. The prepared microcapsules were dispersed into a carrier fluid for achieving better thermal energy storage capabilities. Surface morphology and crystal structure of prepared microcapsules were studied using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) technique. The chemical structure, phase change properties, and thermal stability of the microcapsules were characterized using Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA), respectively. Thermal properties of the microcapsules were experimentally determined using hot disk analyzer. The resistance of microcapsule shell towards rupture under mechanical loads was also studied. Furthermore, the chemical stability of the MPCMS was studied using the FTIR analysis. The viscosity measurements were carried out to understand the flow behaviour of the MPCMS with respect to the carrier fluid and the test results are presented and discussed.

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Journal Pre-proof 2. Experimental 2.1 Materials Dimethyl adipate (C8H14O4) with a purity 99+% was procured from Alfa Aesar, and used as PCM. Melamine (C3H6N6) was purchased from Finar Limited, and formaldehyde (HCHO) (37-38 % w/w) procured from Loba Chemie were used in the preparation of shell monomer. The non-ionic surfactants tween 20 and span 60 as emulsifiers for preparation of oil-in-water (O/W) emulsion were purchased from SRL chemicals. Sodium hydroxide (NaOH) and ammonium chloride (NH4Cl) were obtained from SD Fine-Chem Limited and they were used as pH buffer and nucleating agent, respectively. The de-ionized double distilled water (DDW) obtained from Millipore distiller was used as solvent and carrier (base) fluid throughout the experiments. All chemicals obtained were utilized without any further purification for experiments. 2.2. Preparation of MPCM and MPCMS Microencapsulation of the dimethyl adipate (PCM) in the melamine-formaldehyde shell was carried using in-situ polymerization technique as illustrated in Fig. 1. Polymerization was carried out in three stages that involve preparation of O/W emulsion, the preparation of prepolymer solution, and the fabrication of microcapsules. In the first stage, non-ionic surfactants span 60 (4.8 gms) and tween 20 (4.8 mL) are mixed with DDW of 500 mL and heated to 60 °C. The aqueous surfactant mixture was homogenized under a low shear rate of 1500 rpm using homogenizer. PCM of 50 mL was added drop by drop to the mixture, and the speed was increased to 7000 rpm maintaining the temperature constant. The surfactant reduces the interfacial tension between oil and water media thereby; the oil droplets were attached to hydrophobic tails of the surfactant represented in Fig. 1. The creaming phenomenon, one of the instabilities of the emulsion was avoided during the preparation of O/W emulsion.

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Fig. 1 Schematic diagram showing preparation of microcapsules

In the second stage, DDW was mixed with melamine and formaldehyde solution for preparation of the pre-polymer solution. The solution was stirred at 350 rpm at a temperature of 65 °C. The quantities of the constituent materials required for the preparation of prepolymer solution are presented in Table 1. The 0.05M concentration NaOH solution was added to the mixture to adjust the pH to 11-12. The solution was stirred continuously until the solution became transparent. Table 1. Summary of constituent materials for preparation of pre-polymer solution Description

PCM

Melamine

Formaldehyde

DDW

Shell-to-core

(mL)

(gms)

(mL)

(mL)

mass ratio

MPCM1

50ml

28

50

100

1.287

MPCM2

50ml

21

37.5

75

0.967

In the third stage, O/W emulsion was transferred on to a magnetic stirrer and pre-polymer solution was added drop by drop into the O/W emulsion stirring at 1200 rpm at the temperature of 65 °C. Simultaneously, 1 gm NH4Cl diluted in 25 ml of DDW was added as a nucleating agent to the solution and stirred for 45 minutes. The resultant microcapsules were then collected and dried in the hot air oven for 12 hours to obtain them in powder form. In the preparation of MPCMS, the as-prepared microcapsules of appropriate volume fractions ranging from 0.01 % to 0.2 % were dispersed into the carrier fluid. The microcapsules and the MPCMS were then investigated using the respective characterization techniques as presented in Table 2. 6

Journal Pre-proof Table 2. Summary of specifications of equipment employed in characterization Instrument for characterization FESEM+EDS

Model & make APERO S, FEI

Nature of test sample Powder

Specifications

Method of testing

Beam intensity : 0.2- 30 kV(STEM) Detector: ETD

Electron beam using field emission gun & X-ray excitation of powder

XRD

ULTIMA IV, RIGAKU

Powder

Intensity : 40kV, 30mA

Cu Kα (λ=1.54060 Ao)

FTIR

FTIR-4200, JASCO

Powder, Liquid

Infrared rays

DSC

TA-60, SHIMADZU

Solid, Liquid

Infrared wavelength: 400 to 4000 cm-1 Max resolution : 0.5 cm-1 Temperature range : -50 °C to 400 °C

TGA

DTG-60, SHIMADZU

Solid, Liquid

Temperature range: RT to 1100 °C,

Heating and cooling

Thermal properties

TPS 500S, Hot disk

Solid, Liquid, Powder

Thermal conductivity: 0.03 to 200 W/mK Accuracy : ±2 %

Transient plane source (TPS)

Viscosity

MCR 302, Anton Paar

Fluid

Max torque : 200 mNm Min torque rotation: 1nNm

Concentric cylinders

Heating and cooling source

2.3 Instrumentation and characterization methods The surface morphology and elemental analysis of the microcapsules were carried out using FESEM (Apero S, FEI) working in high vacuum mode equipped with EverhartThornley SE detector (ETD). The prepared microcapsules were non-conductive and were sputter-coated with a gold-palladium alloy of 10 nm using Lecia EM ACE200 to avoid the accumulation of electrostatic charge. The surface morphology of microcapsules was obtained in accelerating voltage of 20 kV. It was accompanied by an elemental composition analysis using coplanar elemental dispersive X-ray spectroscopy (EDS) detector. The sputter of gold-palladium alloy suppressed while carrying the elemental analysis, as role of the coating was to provide a conductive layer to prevent charging of microcapsules. The average particle size of prepared microcapsules was obtained based on the measurement of particles in the FESEM images [25]. The ImageJ software was used to measure the size of more than 100 particles. 7

Journal Pre-proof 𝑛

Mean (𝐷)=∑𝑖 = 1𝐷𝑖/𝑛

(1) 𝑛

2 Standard deviation (σ) = ∑𝑖 = 1(𝐷𝑖 ― 𝐷) /𝑛

(2)

where Di represents the diameter of the particle, and n represents the number of particles Crystalline structure of the microcapsules was recorded using ULTIMA IV, RIGAKU Xray diffractometer (XRD). The diffractometer with Cu Kα (λ=1.54060 Ao) at 40 kV and 30 mA was employed as a source of radiation. The scanning rate was kept at 2 °/min in a range of 5-90 °. The chemical structure and chemical stability of microcapsules and MPCMS were performed using FTIR (JASCO, FT/IR-4200). The infrared transmittance spectra were recorded using KBr pellets over a frequency range of 4000 cm-1 to 400 cm-1. Microcapsules and MPCMS were mixed with KBr and the transmittance was recorded with respect to wavenumber. Thermal stability of the prepared microcapsules was carried out using TGA (SHIMADZU). Microcapsules of mass 4 mg to 5 mg approximately, were loaded in the platinum crucible and tested by heating it from 30 °C to 600 °C at a rate of 10 °C/min under nitrogen atmosphere. The mass loss of microcapsules was measured against the increase in temperature. The phase change characteristics of microcapsules and PCM were evaluated using the DSC (SHIMADZU). The samples were crimped into aluminium pans and, subjected to cyclic heating and cooling processes from -30 °C to +30 °C at a rate of 5 °C/min under nitrogen atmosphere. Thermal properties of microcapsules and MPCMS were performed using thermal constant analyzer (Hot disk TPS 500S) using transient plane source method. The measurements for microcapsules were performed at room temperature (25 °C) using Kapton 5501. The required electric current, supplied to the Kapton sensor being positioned within a slightly rammed 8

Journal Pre-proof powder sample, raises the temperature of the sensor, and the resistance was recorded with a function of time. The thermal conductivity of the MPCMS was measured using a liquid sample holder equipped with a small cell to fill the MPCMS. The Kapton sensor dipped in the MPCMS, records the resistance upon the temperature rise for a given heat power. A thermostatic electric cooler bath was utilized for maintaining the temperature as 14 °C for all the measurements. Furthermore, the thermal conductivity of minimum and maximum volume fractions of the MPCMS were measured above the phase change temperature of the microcapsules. This was done in order to avoid any abrupt change in the heat capacity of the MPCMS due to the temperature variations in the region of phase change [17]. All measurements were carried in accordance with ISO 220077-2 The mechanical compression testing of the microcapsules was carried out similar to the method described in [19], using the apparatus shown in Fig. 2. To maintain uniformity, microcapsules of uniform weight were subjected to compression held for 3 minutes. The resulted microcapsules were dried in the oven for 24 hours before testing the latent heat.

Fig. 2. Schematic showing apparatus for mechanical compression testing

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Journal Pre-proof The viscosity measurements of MPCMS were carried using the rheometer (Anton Paar MCR 302). The rheometer consists of concentric-cylinder arrangement, and the MPCM colloidal suspension was filled in a disposable cylinder. Under the torque of spindle, the dynamic viscosity was measured at a temperature of 14 °C. All measurements were carried in accordance with ISO 3219. 3. Results and discussion Microcapsules with melamine-formaldehyde as the shell and dimethyl adipate as the core were prepared through in-situ polymerization. The characterization studies pertaining to microstructure, chemical structure, thermal stability, phase change characteristics and thermal properties for the microcapsules are discussed. Viscosity measurements for MPCM colloid suspensions for various volume fractions are presented in the following sections. 3.1. Microstructural and elemental composition The microstructural analysis and elemental composition analysis of the as-prepared microcapsules were investigated using FESEM coupled with EDS detector. As presented in Table 1., the microcapsules were synthesized with two different ratios and their microstructures observed using FESEM are shown in Fig. 3. The observed images reveal that the microcapsules obtained under controlled synthesis conditions for both the reactions were almost spherical, and the average particle size measured using FESEM was 5.6 µm and 5.2 µm for MPCM1 and MPCM2, respectively. However, a few of the microcapsule shell surfaces have adhered to each other similar to the phenomenon observed in [26,27]. The adhered surfaces or small particle clusters represented in Fig. 3D occur due to the uncontrolled drying process [28]. During the drying process, as the liquid from shell surface evaporates, the particle concentration in a layer increases with a decrease in the liquid concentration. This resulted in the development of

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Journal Pre-proof compressive capillary forces by the formation of the liquid menisci of two microcapsules (particle bridges). These forces tend to shrink the particles towards each other followed by a further resubmerge into the liquid. The process of evaporation, shrinkage, and submerging continues, based on particle interaction forces [29]. As the shell surface layer attained strong enough strength to overcome compressive capillary forces, the shrinkage process stopped, and the particles tend to become dry solid crust as represented in Fig. 3D.

Fig. 3. (A) Surface morphology of MPCM1, (B) EDS and elemental composition of MPCM1, (C) Particle size distribution of MPCM1, (D) Surface morphology of MPCM2, (E) EDS and elemental composition of MPCM2, (F) Particle size distribution of MPCM2

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Journal Pre-proof The difference in the surface morphology of microcapsules as observed in Fig. 3A and Fig. 3D was attributed to the amount of the shell precursor (methylamine) used for preparing microcapsules. Hence, by limiting the quantity of methylamine in the reaction can result in obtaining smooth shell surfaces. Elemental compositions of the microcapsules studied using the EDS, and their respective elemental spectra are presented in Fig. 3B and Fig. 3E. The results infer that, both the samples (MPCM1 and MPCM2) are composed of carbon, nitrogen, and oxygen as major constituents. The presence of carbon and oxygen elements confirmed the presence of organic PCM, while the presence of nitrogen confirmed the formation of amine structure around the PCM droplet [25]. Amine shell formation around the PCM droplets can be described in two stages - methylolation and condensation. In the first stage of methylolation, reaction of melamine with formaldehyde solution occurs in the aqueous phase under alkaline conditions. In the second stage of condensation, formation of oligomers containing methylene and methylene ether bridges takes place [30]. The formed oligomers collapse on the surface of the oil droplets attached to the hydrophobic tails of surfactant. Due to thermal or acidic/alkaline conditions, chain formation and crosslinking take place on the surface and this leads to formation of solid shell material around the droplet. Thus, EDS analysis confirmed the formation of microcapsules with melamineformaldehyde as the shell and DMA as the core. 3.2. Crystal structure of microcapsules The broad characteristic diffraction peaks of microcapsules prepared in two combinations of the shell-to-core ratios are shown in Fig. 4. The shell strength and encapsulation properties of microcapsules were influenced by the crystal structure and crystallinity of the shell materials [31]. Based on the XRD results, the MPCM2 has exhibited reasonably higher peak

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Journal Pre-proof intensity when compared to MPCM1. This high crystallinity increased the chain rigidity of polymer network that increased the cracking tendency of the shells [32]. Accordingly, the encapsulation properties of MPCM1 and MPCM2 varied with respect to their peak intensities. For instance, MPCM1 showed low diffraction peak intensity, which corresponds to its higher resistance towards cracking at the cost of reduced encapsulation properties. Similarly, MPCM2 resulted in better encapsulation properties, but with reduced resistance towards cracking [32].

Fig. 4. XRD Peaks of the microcapsules

3.3. Chemical structure and chemical stability of microcapsules The FTIR results presented in Fig. 5 shows the chemical structure and chemical stability of the prepared microcapsules. The weak absorption peak in PCM spectra at 2954 cm-1 was attributed to the asymmetric stretching vibration of methyl (-CH3) group. The strong and sharp absorption at 1739 cm-1 corresponds to C=O stretching vibration of the saturated aliphatic ester. The absorption peaks at 1179 cm-1, 1076 cm-1, 1011 cm-1 were attributed to the C-O stretching vibration of the ester compound [23]. The shell material synthesized without core has shown a broad absorption band in the functional group region at 3140 cm-1, which corresponds to the N-H bending vibration of 13

Journal Pre-proof anime. The absorption peak at 1403 cm-1 signified the asymmetric stretching vibration of the nitro group (N=O). The broad absorption bands in the fingerprint region at 1161 cm-1 corresponds to the stretching vibration of aliphatic amines (C-N). The sharp absorption peaks at 811 cm-1 was due to the in-pane and out-of-plane bending vibration of the triazine ring [30,33,34]. The prepared microcapsules (MPCM1 and MPCM2) have exhibited similar absorption spectra. The absorption peaks near 1739 cm-1 and 1006 cm-1 corresponding to the ester functional groups of PCM. In addition to these characteristic vibrational peaks near 3140 cm-1, 1403 cm-1 and 812 cm-1 correspond to the functional group of the melamineformaldehyde shell.

Fig. 5. FTIR spectra of microcapsules

Thus, the FTIR results confirmed the formation of melamine-formaldehyde as the shell through polymerization on the droplets of DMA acting as the core, supporting the observations of elemental composition as presented in Fig. 3B and Fig. 3E. Furthermore, the existence of the characteristic peaks of core and shell materials without additional peaks has

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Journal Pre-proof justified that, the prepared microcapsules has no chemical interaction, and formation of shell and core happens only through physical interaction. 3.4. Thermal stability of microcapsules The thermal stability of PCM and microcapsules was investigated using TGA, and the results are presented in Fig. 6 and Table 3. The test results reveal that, the PCM (dimethyl adipate) has shown one-step mass loss with an increase in temperature from 30 °C to 600 °C. The single-step decomposition of the PCM was due to the presence of the saturated ester bonds and the presence of the ester bonds were also supported by the FTIR studies as depicted in Fig. 5 [35].

Fig. 6. TGA graphs of the PCM and the microcapsules.

The mass loss has started at a temperature of 130 °C, and the PCM decomposed completely at around 160 °C leaving aside no residue. Furthermore, there was no traces of erosion of the crucible being observed after the complete decomposition of the PCM. Thus, it confirmed that, the dimethyl adipate utilized as the PCM in this study was non-corrosive [36]. The as-prepared microcapsules (MPCM1 and MPCM2) have exhibited two-step decomposition in line with the results obtained in [19].

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Journal Pre-proof Table 3. Thermal stability of the microcapsules

PCM MPCM1

First step Onset temperature (°C) 130.8 113.5

End set temperature (°C) 156.8 129.6

MPCM2

107.5

135

Mass loss (%) 99.32 37.8

Second step Onset temperature (°C) 375.8

End set temperature (°C) 405.2

Mass loss (%) 58.7

Residue weight (% ) 0 3.43

38.2

375.6

402.3

51.6

10.2

The first step of decomposition for the microcapsules occurred between 30 °C and 160 °C. During this step, water vapour absorbed on the surface of the shell material was evaporated. This step was further followed by the vaporization of PCM encapsulated into the shell material due to the breakdown of the oxygenated hydrocarbon ester compounds into lower molecular hydrocarbons, carbon monoxide (CO) and carbon dioxide (CO2) [37]. The decomposition of PCM associated with the rupture of shell material together was attributed to the increased internal pressure created within the shell due to the expanding gases by virtue of increased temperature [19]. The inset of Fig. 6 shows the FESEM images of ruptured/cracked microcapsules after complete decomposition of the PCM. The second step decomposition that starts around 160 °C was ascribed to the shell material. During this step, continuous degradation of the shell from 160°C to 350 °C was observed and that was due to the conversation of ether bridges into methylene bridges through the elimination of formaldehyde under nitrogen atmosphere [30,33]. The continuous degradation followed by a sudden mass loss observed over 350 °C to 400 °C was due to the breakdown of methylene bridges [33,38]. Furthermore, the mass loss occurred beyond 400°C pertains to the degradation of triazine ring. Thus, the TGA results confirmed the thermal stability of the MPCM1 and MPCM2 up to 160 °C. However, the MPCMs operating temperature for the intended cool TES application is almost 16 times less than its decomposition temperature. Hence, the MPCMs have shown reasonable thermal stability and can be considered suitable for low-temperature TES application. 16

Journal Pre-proof 3.5. Phase change characteristics of microcapsules The phase change temperatures and latent heat of the microcapsules were evaluated using DSC, and the results obtained are plotted in Fig. 7. Results signify that, the PCM and the microcapsules have exhibited single endothermic peaks. The results summarized in Table 4 suggest that, microcapsules prepared with two different shell-to-core ratio have exhibited relatively low latent heat potential when compared to pure PCM. This observation is in well agreement with the XRD results obtained for MPCM1 and MPCM2.

Fig. 7. DSC graphs of PCM and microcapsules

Table 4. Phase change properties and thermal properties Sample

Phase change properties during melting

Thermal properties

PCM

Onset temperature (°C) 9.2

Peak temperature (°C) 13.4

Latent heat (J/g) 153

Thermal conductivity (W/m K) 0.2126

Volumetric specific heat (MJ/m3 K) 1.324

MPCM1

6.73

8.8

52.79

0.0946

0.9067

MPCM2

6.35

8.65

69.96

0.09

0.7305

In addition, the low enthalpy of latent heat observed could be due to the excess shell material pre-polymer utilization in preparation, which could have contained high amine content in the microcapsules. Therefore, the encapsulation ratio was calculated from DSC 17

Journal Pre-proof measurements to describe the effective encapsulation and performance of the PCM into the shell material for thermal energy storage application [41]. The encapsulation ratio (R) can be calculated according to Eq.(3) [41]. R= (ΔHm,mpcm / ΔHm,pcm)100

(3)

where, ΔHm,mpcm and ΔHm,pcm represent latent heat of fusion of microcapsules and PCM, respectively. The DSC results obtained in this study for MPCM1 and MPCM2 were compared with literature and are summarized in Table 5. It is pertinent to note that, the melting point and encapsulation ratios of microcapsules were appreciable for the cool thermal energy storage applications. Table 5. Comparison of present study-results with literature Shell

PCM

Melting point (°C)

Encapsulation ratio (%)

Reference

Paraffin

47-48

29-65

[12]

Lauric acid

44.9

46.2

[39]

1-Dodecanol

27.5

47-54

[19]

Eutectic mixture

4-5

51-53

[26]

Organo-Silica

n-Octadecane

28.5

47-53

[10]

Calcium carbonate

n-Tetradecane

5.88

26

[40]

Melamineformaldehyde

Dimethyl adipate

6.35-6.73

34-46

Present study

Melamineformaldehyde

3.6. Thermal properties Thermal conductivity of microcapsules and PCM were experimentally measured using the transient plane source (Hot disc) method at room temperature, and the average values are presented in Table 4. Measurements were conducted thrice to check the repeatability of obtained values, and it was found that the deviation was not more than 2 %. Thermal conductivities of the prepared MPCM1 and MPCM2 were measured to be 0.0946 W/m K and 0.09 W/m K, respectively. The difference in thermal conductivity can be related to the different shell-to-core ratios of the microcapsules. As the shell material exhibited higher thermal conductivity over the PCM [42], higher amounts of amine could 18

Journal Pre-proof have resulted in an increase in thermal conductivity for MPCM1. Thermal conductivity results obtained in this study were compared with the literature reported on the amine shell materials measured using TPS method as summarized in Table 6. However, the thermal conductivity and volumetric specific heat capacities of the microcapsules were found to be lower than that of the pure PCM, and this similar trend was observed in [43–45]. Table 6. Comparison of thermal conductivity with literature measured using TPS method Reference Shell material Thermal Conductivity (W/m K) [43] Urea-formaldehyde 0.0557 - 0.1231 Melamine-formaldehyde

0.0815±0.0002 - 0.1176±0.0004

[44]

Melamine-formaldehyde

0.09

[45]

Melamine-formaldehyde

0.09 – 0.0946

Present study

Furthermore, it is necessary to point out that, the presence of the air voids when performing the measurements could have increased the interfacial contact resistance for powder samples compared to the liquid, which could have resulted in lower experimental values for the powder samples [46]. Thus, it is proposed that, by incorporating thermally conductive materials into the shell or core structures, the thermal conductivity of the microcapsules can be expected to improve significantly. 3.7. Mechanical testing of the microcapsules The surface morphology of microcapsules (MPCM1 and MPCM2) subjected to two different compressive loads (2 MPa and 4 MPa) are shown in Fig. 8, and the enthalpy losses calculated using DSC measurements are presented in Fig. 9. The FESEM images illustrate that, due to the applied compressive loads, the microcapsules have exhibited cracks and fractures. However, the cracks and shell fractures noticed for MPCM1 were lower than that of MPCM2, which was attributed to their good shell flexibility. This in turn was in good agreement with the observations made from XRD results. Due to the constrained polymer network of the MPCM2 they have shown high rigidity and least

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Journal Pre-proof flexibility of the shell while compared to MPCM1, and this has paved way for easy formation of cracks as represented in Fig. 8C and Fig. 8D. The inset in Fig. 8D shows the MPCM2 microcapsule that has undergone fracture, leading to the leakage of the PCM from the shell.

Fig. 8. Surface morphology of microcapsules subjected to pressure A) MPCM1 2 MPa B) MPCM1 4 MPa C) MPCM2 2 MPa D) MPCM2 4 MPa

Fig. 9. Bar chart showing enthalpy loss of MPCM1 and MPCM2.

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Journal Pre-proof To estimate the amount of PCM leaked/dripped out under the applied load, enthalpy loss ratio was calculated for the microcapsules using DSC measurements, and the results are summarized in Table 7. The results reveal that, MPCM1 has exhibited reasonably lower enthalpy loss of PCM compared to that of MPCM2, due to good shell flexibility. The similar observations of large enthalpy loss due high rigidity of shell were also reported in [19]. Table 7. Enthalpy loss calculated from DSC Before compression At ambient pressure

After compression 2 MPa Latent heat (J/g) (B)

Pure PCM

Latent heat (J/g) (A) 153

MPCM1 MPCM2

Sample

4 MPa Latent heat (J/g) (C)

-

Enthalpy loss (%) [(A-B)/A]*100 -

-

Enthalpy loss (%) [(A-C)/A]*100 -

52.79

32.90

37.7

30.80

41.7

69.96

33.23

52.5

27.40

60.8

Thus, the MPCM1 has shown high resistance towards cracking and fractures by preventing the PCM from leaking out of the shell compared with MPCM2. These observations are in good agreement with the crystal structures studies for MPCM1 and MPCM2. 3.8. Chemical stability of the MPCMS The chemical stability of the prepared MPCMS and the carrier (base) fluid were studied using FTIR and results presented in Fig.10. The carrier fluid has shown a broad absorption peak at 3436 cm-1 and 1587 cm-1 corresponding to the stretching vibration and bending vibration of hydroxyl functional group [47]. The prepared MPCMS has exhibited the absorption bands of hydroxyl (O-H) group. In addition to these, characteristic absorption peaks exhibited at 1739 cm-1, 811 cm-1 corresponds to the C=O stretching and N=O asymmetric stretching vibrations of the microcapsules respectively. Thereby the FTIR results confirmed the existence of the characteristic absorption peaks for both microcapsules and base fluid and thus justify the microcapsules has shown no chemical interaction with the carrier (base) fluid. 21

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Fig. 10. FTIR spectra of the base fluid, MPCMS1 and MPCMS2

3.9 Thermal conductivity of the MPCMS The thermal conductivity of the MPCMS were carried using thermal constant analyzer and the obtained results are shown in the Fig.11.

Fig. 11. Thermal conductivity of carrier fluid, MPCMS1 and MPCMS2

The results infer that MPCMS1 and MPCMS2 have shown a marginal decrease in thermal conductivity values with increased volumetric dispersion of microcapsules. This

22

Journal Pre-proof behaviour can be ascribed due to the low thermal conductivity of the microcapsules when compared to that of the carrier fluid. Similar observations of the decrease in thermal conductivity were also reported in [17,48]. Furthermore, the decrease in thermal conductivity was around 1 % and 3 % for the volume fractions 0.01% and 0.2% respectively. 3.10. Viscosity The viscosity measurements of MPCMS were carried out above the melting temperature of the PCM and the results obtained are presented in Fig. 12 and Fig. 13. The results depicts the flow behaviour of MPCMS, wherein it was observed that, for all the volume fractions of MPCM1 and MPCM2 in the carrier fluid, the shear stress showed a linear relationship with the shear rate.

Fig. 12. Flow curves of MPCMS with various volume fractions of MPCM1

This trend as observed for MPCMS has enabled them to behave as the Newtonian fluid. The flow resistance of the MPCMS was not affected due to the phase change of

23

Journal Pre-proof microcapsules [49]. Due to the high sphericity of the prepared microcapsules as observed from FESEM images, the hydrodynamic interaction forces between the particles were smaller. Moreover, the microparticles offered less resistance against the applied shear rate, that resulted in minor variations in the viscosity of MPCMS, when compared to the carrier (base) fluid [50].

Fig. 13. Flow curves of MPCMS with various volume fractions of MPCM2

The relative viscosity, which is the ratio of viscosity of suspension to the viscosity of base fluid is a crucial factor, that represents the increase in the viscosity of suspension due to the dispersion of solid particles in the carrier (base) fluid. There are various correlations to predict the rheological behaviour of suspensions with an increase in volume fraction of dispersants. The first correlation proposed by Einstein [51] as represented in Eq. (4) based on the assumption that, the particles are non-interacting and

24

Journal Pre-proof spherical in nature, remains valid for the low fractions with a linear increase in viscosity over increase in the volume fraction. µrel = (1+2.5ϕ)

(4)

The Brinkman correlation [52], which can be considered to be an extension to the Einstein’s correlation is given by, µrel = 1/(1- ϕ)5/2

(5)

The Vand equation [53] was extensively used to predict the relative viscosity of the microencapsulated phase change material suspensions [54], and the equation is given by, µrel = (1-ϕ-A ϕ2)-2.5

(6)

The parameter ‘A’ depends on the diameter of particles, which vary from 3.4 to 4.45 [5]. In this context, the relative viscosity of the MPCMS with respect to the volume fraction of microcapsules and with standard classical correlations are presented in Fig. 14. It was observed that, at a volume fraction of 0.2 %, the MPCMS2 has shown a small deviation from the classical models. This sort of similar deviation was equally observed by the nanofluids due to their cluster formation [55], and this can also be justified in the case of MPCMs.

Fig. 14. Relative viscosity of MPCMS compared with standard correlations

25

Journal Pre-proof Because the clusters of microcapsules being formed by adhesion of particles during the drying process were clearly observed through FESEM images. Hence, the dispersion of the microcapsules with volume concentrations ranging from 0.01 % to 0.2 % has exhibited marginal variations in viscosity as well as Newtonian behaviour. In total, these attributes have enabled the MPCMS to be considered as a potential candidate for thermal energy storage. 4. Conclusions The microencapsulation with DMA as the core and with melamine-formaldehyde as the shell was carried out using in-situ polymerization. The as-prepared microcapsules were dispersed into the carrier fluid for thermal energy storage application. The following conclusions were made from the experimental investigation carried out on the microcapsules: 

The microcapsules under controlled synthesis achieved high sphericity, with an average particle size of 5.6 µm and 5.2 µm for MPCM1 and MPCM2, respectively.



Crystal structure studies justified that MPCM1 with low diffraction peak intensity exhibited higher resistance towards cracking.



The chemical structure studies carried out using FTIR confirmed that, the prepared microcapsules exhibited no chemical interaction, and the formation of core and shell was only by physical interaction.



The TGA measurements have shown that both the microcapsules (MPCM1 and MPCM2) were stable up to 160 °C, which is much higher than the operating temperature range of the PCM.



The latent heat of enthalpy measured using DSC was around 53 J/g and 70 J/g, and the phase change temperatures were measured to be 6.73 °C and 6.35 °C for MPCM1 and MPCM2, respectively. Thus, the DSC results indicate that microcapsules are suitable for cool thermal energy storage application. 26

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Mechanical testing of microcapsules carried out at 2 MPa and 4 MPa has revealed that, MPCM1 exhibited good shell flexibility and high resistance towards fractures compared to that of MPCM2. That is, the enthalpy loss ratio for MPCM1 and MPCM2 was ranging from 37.7 % to 41.7 % and 52.5 % to 60.8 %, respectively.



The FTIR studies carried out for the MPCMS has confirmed that the dispersed microcapsules (MPCM1 and MPCM2) has shown no chemical interaction with the carrier (base) fluid.



The as-prepared MPCMS with volume concentrations ranging from 0.01 % to 0.2 % has exhibited the Newtonian behaviour with a very marginal increase in viscosity.



In total, the high sphericity, good flexibility, high thermal stability, good thermal energy storage properties, relatively low viscosity and Newtonian behaviour were significant for both microcapsules and MPCMS. Hence, these properties have turned out to be good attributes for MPCMS and which enabled them to be considered as a potential candidate for the cool thermal energy storage application.

Acknowledgements The authors gratefully acknowledge DST, New Delhi for providing financial support to carry out this research under DST-ECR scheme (DST Sanction Order No. ECR/2017/001146). Authors are thankful to central analytical laboratory, BITS Pilani, Hyderabad campus for their technical assistance and support in materials characterization. Authors would like to acknowledge Mr. K Ramachandran Pillai for his assistance in proofreading the manuscript. References [1]

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Journal Pre-proof Highlights 

Microcapsules with organic ester PCM were prepared for cool TES application.



Microcapsules with low crystallinity offered higher resistance towards cracking.



Microcapsules have exhibited latent heat of enthalpy of 53 kJ/kg and 70 kJ/kg.



Encapsulation ratio of 46 % was achieved under controlled synthesis conditions.



MPCMS with varied volume fractions of microcapsules showed Newtonian behaviour.