|M.Sc Student||Rosen Natalie|
|Subject||Encapsulation of Hydrated Salt Phase Change Materials|
within Polymer Monoliths through Emulasion -
|Department||Department of Materials Science and Engineering||Supervisor||Professor Michael Silverstein|
Phase change materials (PCMs) store and release thermal energy through their relatively high latent heats of melting and crystallization. Salt hydrates PCMs have several advantages over organic PCMs (such as paraffins and fatty acids): relatively high latent heats per volume, high thermal conductivities, non-flammability, and the availability of a wide range of transition temperatures. Since salt hydrates exhibit significant supercooling and tend to melt incongruently, resulting in irreversible melt-freeze processes, their utilization in thermal energy-storage applications has been limited.
The objective of this research was to develop an innovative method of encapsulating an inorganic salt hydrate PCM within a polymer monolith, templating within molten inorganic salt-in-oil ((is)l/o) high internal phase emulsions (HIPEs). This work focused on calcium chloride hexahydrate (CaCl2•6H2O, CC‑HH), with a melting point of 30 °C and a latent heat of fusion of 176 J/g, that was encapsulated within an elastomeric polyacrylate based on 2-ethylexyl acrylate (EHA). The effects of HIPE stabilization, locus of initiation, nucleating agent, crosslinking strategy, and polymerization procedures (such as pre-polymerization) were investigated. The emulsion-templated structures were characterized using scanning electron microscopy and the thermal properties were characterized using differential scanning calorimetry.
Based on the results of thermal cycling tests, the most promising encapsulation system exhibited a CC‑HH content of 75% dispersed within droplets of 100 to 300 µm, with melting and crystallization heats of ~120 J/gsample (~160 J/gCC‑HH). The addition of 3 wt% SrCl2•6H2O (SC-HH) as a nucleating agent reduced the extent of supercooling to 14 °C, in contrast to 43 °C for pure CC‑HH. The resulting average melting and crystallization temperatures were 34 °C and 20 °C, respectively. Unfortunately, the relatively interconnected droplet structure had a negative effect on the long-term thermal stability of the encapsulated CC‑HH, with irreversible phase separation and CaCl2•4H2O (CC-TH) formation producing a 40% reduction in latent heat. Several attempts were conducted to synthesize more closed droplet monolith by using interfacial polymerization with water-soluble thermal- and UV-initiators instead of external phase initiation using oil-soluble initiators. Although HIPEs were formed successfully, they did not undergo polymerization. The possible reasons are the opaque nature of HIPEs and the chlorine atoms found in CC-HH which seem to scavenge the free radicals.
The connection between the polyHIPE morphology and the thermal properties of the encapsulated CC-HH is clear from the polyHIPEs with smaller droplet size whose CC-TH formation rate is slower. Pre-polymerization of the monomers prior to HIPE formation and crosslinking using a polycaprolactone dimethacrylate (PCL-DMA, Mn=2000) produce more stable HIPEs with smaller droplets size, 100 to 300 µm, compared to crosslinking using ethylene glycol dimethacrylate (EGDMA) which results in droplets size of 300 to 500 µm.
This work demonstrated the potential of encapsulating a relatively large volume (75%) of inorganic PCM while preserving the thermal properties of the PCM. Further research finding conditions for interfacial initiation to enhance the encapsulation is crucial for a successful integration of the encapsulated PCM in thermal energy-storage applications.