Effect of water soluble polymers on formation of Na2SO4 contained SiO2 microcapsules by W/O emulsion for latent heat storage

Abstract

Synthesis of microcapsulation of Na2SO4·10H2O for latent heat storage using water-in-oil (W/O) was proposed. Effect of water soluble polymers such as polyethyleneimie (PEI), poly (acrylic acid) (PAA), polyethylene glycol (PEG), polyvinyl alcohol (PVA), and polyvinylpyrrolidone (PVP) on formation of Na2SO4 contained SiO2 microcapsule was investigated. These polymers were added in the water phase with Na2SO4 aq. and Tween 20 as a surfactant. To increase stability of the W/O emulsion, Span 80 as a surfactant and hydroxypropyl cellulose (HPC) as thickner were also added in 1-octaol as oil phase. The stable W/O emulsion promotes SiO2 shell formation at a W/O interface. PEI and PAA, which have positively and negatively ionized groups, didn’t capsulate Na2SO4. They have strong interactions between Na+ or SO42− and COO− or N+, which prevent from stable W/O emulsion. PEG, PVA, and PVP successfully formed SiO2 microcapsules including Na2SO4, which were confirmed by an elemental analysis. Although there were no large difference in Na2SO4 content in the PEG, PVA, and PVP-derived microcapsules, the heat storage amount of the PEG-derived microcapsule was the best in the three. Heat resistance of the PEG-derived microcapsule was much higher than those of the PVA and PVP-derived ones, therefore, it seems that interactions between Na2SO4·10H2O and the polymers affect reactions between dehydration and hydration. The other difference of PEG with PVA and PVP seems to be affinity with Tween 20. PEG has high affinity with Tween 20 due to similar chemical structure compared to the other two. It is thought that PEG surrounds Na+ and SO42− including much amount of H2O, and Tween 20 can additionally cover PEG/Na+, SO42−, and H2O. Because of such designed structure, dehydrated Na2SO4 hydrates with H2O near itself to form quasi Na2SO4·10H2O during cooling process. That is why PEG-derived microcapsule exhibited high heat storage performance.