Preparation, characterization and thermal properties of PMMA/n-heptadecane microcapsules as novel solid–liquid microPCM for thermal energy storage
Introduction
In recent years, the use of phase change material (PCM) for thermal energy storage has gained extensive attention owing to increasing energy consumption and environment pollution problems. PCMs have the ability to change their state with a certain temperature range and can store and release large amounts of energy during phase change process [1], [2], [3]. Therefore, there are several studies on thermal properties and energy storage performance of a large number of pure and composite compounds including salt hydrates, paraffin wax and non-paraffin organic have been investigated as PCM [4], [5], [6], [7], [8], [9]. Recently, microencapsulated PCM (microPCMs) technique has been developed to enlarge the utility fields of the PCMs. MicroPCM is a form of PCM encapsulated in natural and synthetic polymeric capsules, which range in size from less than 1 μm to more than 1000 μm [10], [11]. The advantages of microPCMs are (1) protecting the PCM against the influences of the outside environment, (2) increasing the heat transfer area, (3) permitting the core material, due to coating, to withstand changes in volume of the PCM, as the phase change occurs, and (4) allowing small and portable energy storage system [12], [13], [14], [15]. Therefore, these features make them more functional than the traditional PCMs in many applications such as functional fibers [16], [17], [18], [19], [20] thermal insulation, [21], [22] heat transfer, [23], [24] solar energy utilization, [25], [26] and building materials [27], [28], [29].
A number of studies have been carried out on microencapsulation of PCMs using different polymers as microcapsule shell materials. Microcapsules and nanocapsules containing n-octadecane with melamine-formaldehyde shell were fabricated by in situ polymerization [11], [30]. In that study, the effects of different parameters on diameters, morphologies, phase change properties and thermal stabilities of the capsules were investigated by using Fourier Transform Infrared (FT-IR), SEM, DSC and TGA analysis methods. Polyurea microcapsules containing n-eicosane and n-hexadecane as PCMs were synthesized by interfacial polymerization technique and characterized by using the FT-IR, SEM, DSC and TGA analysis methods [10], [12]. Different type microPCMs were prepared with a polymer shell of polystyrene by using suspension polymerization technique and investigated thermal properties, the morphology and the particle size distribution of the microPCMs [31]. A series of polyurethane microcapsules containing n-octadecane were synthesized as microPCMs [32]. The microPCMs containing paraffin as core material and gelatin/acacia as shell materials were prepared by complex coacervation and spray-drying methods [15]. The n-tetradecane was encapsulated by using polystyrene, polymethylmethacrylate, polyvinylacetate and polyethylmethacrylate as shell materials to form core–shell structure [33]. The microcapsules containing urea–formaldehyde polymer as shell material and n-pentadecane, n-eicosane and a paraffin wax as core material were synthesized [34].
As seen from the literature surveys above, a number of different polymers were used to prepare microPCMs but there is little study in literature on microencapsulation of PCMs with polymethyl methacrylate (PMMA). Polymethyl methacrylate (PMMA) is a thermoplastic and transparent acrylic polymer. It is often used in a wide range of fields and applications because of its moderate properties, easy handling and processing, and low cost. In addition, it has relatively good mechanical and good protection against outside environment [35], [36], [37]. From this point of view, PMMA is a versatile material and is the promising polymer as shell material in the preparation of microcapsules containing PCMs for thermal energy storage applications.
In this study, novel microPCMs containing n-heptadecane as core material and PMMA as shell materials were synthesized by emulsion polymerization. The microPCMs were characterized using SEM and particle size distribution (PSD) analysis and FT-IR spectroscopy. Thermal properties and thermal stability of the microPCMs were determined by DSC and TG analysis. In addition, in order to determine thermal reliability of the microPCMs, they were subjected to 5000 thermal cycling (melting and freezing) and their thermal properties were measured after the thermal cycling.
Section snippets
Materials
n-Heptadecane was used as core material and obtained from Fluka Company. Methylmethacrylate and allyl methacrylate were obtained from Fluka Company and double distilled before use. Tertbutylhydroperoxide and Triton X-100 were purchased from Merck Company and used as initiator and surfactant respectively. Ferrous sulphate (FeSO4⋅7H2O), ammonium persulphate, and sodium thiosulphate (Na2S2O7) were obtained from Sigma Aldrich Company and used without further purification.
Preparation of the microcapsules
PMMA/heptadecane
Morphology and PSD investigation of microPCMs
The morphologies of the microcapsules are crucial for achieving the desired properties and depend on the choice of the polymers that entrap the core material and the synthesis parameters. Fig. 1 shows the surface morphology of PMMA/heptadecane microcapsules studied by SEM analysis. As shown in Fig. 1, the PMMA/heptadecane microcapsules have relatively uniform sizes, spherical shape and smooth surface. The similar appearance was coincided in different microcapsules prepared by using different
Conclusions
PMMA/heptadecane microcapsules are prepared as novel solid–liquid microPCMs by emulsion polymerization technique at the emulsion stirring rate of 2000 rpm. The FT-IR results have confirmed that n-heptadecane had been successfully encapsulated inside the PMMA microcapsules. SEM and PSD analyses showed that the microcapsules have smooth, compact surface and spherical shape with average diameter of 0.26 μm. By using DSC analysis method, the temperatures of melting and freezing and latent heats of
Acknowledgments
We would like to thank The Scientific & Technical Research Council of Turkey (TUBITAK) for their financial support for this study (The Project Code: 107T607-TBAG- 197 HD/311). Authors also thank Dr Ahmet Karadağ for TGA analysis and thank Altınay Boyraz (Erciyes University Technology Research & Developing Center) for SEM and DSC analysis.
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